ADS1251U/2K5 [TI]
24-Bit, 20kHz, Low-Power ANALOG-TO-DIGITAL CONVERTER; 24位20kHz ，低功耗模拟数字转换器
| 型号： | ADS1251U/2K5 |
| 厂家： | 
TEXAS INSTRUMENTS |
| 描述： | 24-Bit, 20kHz, Low-Power ANALOG-TO-DIGITAL CONVERTER |
| 文件： | 总17页 (文件大小：322K) |
| 中文： | 中文翻译 | 下载： | 下载PDF数据表文档文件 |
ADS1251
A
D
S
1
2
5
1
SBAS184A – MARCH 2001 – REVISED SEPTEMBER 2003
24-Bit, 20kHz, Low-Power
ANALOG-TO-DIGITAL CONVERTER
DESCRIPTION
FEATURES
The ADS1251 is a precision, wide dynamic range, delta-
sigma, Analog-to-Digital (A/D) converter with 24-bit resolu-
tion operating from a single +5V supply. The delta-sigma
architecture features wide dynamic range, and 24 bits of no
missing code performance. Effective resolution of 19 bits
(1.5ppm of rms noise) is achieved at conversion rates up to
20kHz.
● 24 BITS—NO MISSING CODES
● 19 BITS EFFECTIVE RESOLUTION UP TO
20kHz DATA RATE
● LOW NOISE: 1.5ppm
● DIFFERENTIAL INPUTS
● INL: 15ppm (max)
The ADS1251 is designed for high-resolution measurement
applications in cardiac diagnostics, smart transmitters, indus-
trial process control, weigh scales, chromatography, and
portable instrumentation. The converter includes a flexible,
2-wire synchronous serial interface for low-cost isolation.
● EXTERNAL REFERENCE (0.5V to 5V)
● POWER-DOWN MODE
● SYNC MODE
● LOW POWER: 8mW at 20kHz
5mW at 10kHz
The ADS1251 is a single-channel converter and is offered in
an SO-8 package. It is pin-compatible with the faster ADS1252
(41.7kHz data rate).
APPLICATIONS
● CARDIAC DIAGNOSTICS
● DIRECT THERMOCOUPLE INTERFACES
● BLOOD ANALYSIS
● INFRARED PYROMETERS
● LIQUID/GAS CHROMATOGRAPHY
● PRECISION PROCESS CONTROL
ADS1251
VREF
CLK
+VIN
4th-Order
∆Σ
Modulator
SCLK
Serial
Digital
Filter
Interface
DOUT/DRDY
–VIN
+VDD
GND
Control
Please be aware that an important notice concerning availability, standard warranty, and use in critical applications of
Texas Instruments semiconductor products and disclaimers thereto appears at the end of this data sheet.
All trademarks are the property of their respective owners.
PRODUCTION DATA information is current as of publication date.
Products conform to specifications per the terms of Texas Instruments
standard warranty. Production processing does not necessarily include
testing of all parameters.
Copyright © 2001-2003, Texas Instruments Incorporated
www.ti.com
ABSOLUTE MAXIMUM RATINGS(1)
ELECTROSTATIC
DISCHARGE SENSITIVITY
Analog Input: Current ............................................... ±100mA, Momentary
±10mA, Continuous
Voltage .................................... GND – 0.3V to VDD + 0.3V
This integrated circuit can be damaged by ESD. Texas
Instruments recommends that all integrated circuits be handled
with appropriate precautions. Failure to observe proper han-
dling and installation procedures can cause damage.
VDD to GND ............................................................................ –0.3V to 6V
VREF Voltage to GND ............................................... –0.3V to VDD + 0.3V
Digital Input Voltage to GND ................................... –0.3V to VDD + 0.3V
Digital Output Voltage to GND ................................. –0.3V to VDD + 0.3V
Operating Temperature ...................................................... –40°C to 85°C
Lead Temperature (soldering, 10s) .............................................. +300°C
Power Dissipation .......................................................................... 500mW
ESD damage can range from subtle performance degrada-
tion to complete device failure. Precision integrated circuits
may be more susceptible to damage because very small
parametric changes could cause the device not to meet its
published specifications.
NOTE: (1) Stresses above those listed under “Absolute Maximum Ratings”
may cause permanent damage to the device. Exposure to absolute maximum
conditions for extended periods may affect device reliability.
PACKAGE/ORDERING INFORMATION
SPECIFIED
PACKAGE
DESIGNATOR(1)
TEMPERATURE
RANGE
PACKAGE
MARKING
ORDERING
NUMBER
TRANSPORT
MEDIA, QUANTITY
PRODUCT
PACKAGE-LEAD
ADS1251
SO-8
D
–40°C to +85°C
ADS1251U
ADS1251U
Rails, 100
"
"
"
"
"
ADS1251U/2K5
Tape and Reel, 2500
NOTE: (1) For the most current specifications and package information, refer to our web site at www.ti.com.
PRODUCT FAMILY
PRODUCT
# OF INPUTS
MAXIMUM DATA RATE
COMMENTS
Includes PGA from 1 to 8
ADS1250
ADS1251
ADS1252
ADS1253
ADS1254
1 Differential
1 Differential
1 Differential
4 Differential
4 Differential
25.0kHz
26.8kHz
41.7kHz
20.8kHz
20.8kHz
Includes Separate Analog and Digital Supplies
ELECTRICAL CHARACTERISTICS
All specifications at TMIN to TMAX, VDD = +5V, CLK = 8MHz, and VREF = 4.096, unless otherwise specified.
ADS1251U
TYP
PARAMETER
CONDITIONS
MIN
MAX
UNITS
ANALOG INPUT
Full-Scale Input Voltage
Absolute Input Voltage
Differential Input Impedance
+VIN – (–VIN
)
±VREF
V
V
+VIN or –VIN to GND
CLK = 3.84kHz
CLK = 1MHz
–0.3
VDD
430
1.7
210
6
MΩ
MΩ
kΩ
pF
pA
nA
CLK = 8MHz
Input Capacitance
Input Leakage
At +25°C
5
50
1
At TMIN to TMAX
DYNAMIC CHARACTERISTICS
Data Rate
20.8
kHz
kHz
Bandwidth
–3dB, CLK = 8MHz
4.24
Serial Clock (SCLK)
System Clock Input (CLK)
8
8
MHz
MHz
ACCURACY
Integral Nonlinearity
Differential Input
±0.0002
±0.0015
% of FSR
THD
Noise
1kHz Input; 0.1dB below FS
105
1.5
dB
ppm of FSR, rms
Bits
2.5
Resolution
No Missing Codes
24
24
90
Bits
Common-Mode Rejection
Gain Error
60Hz, AC
98
0.1
±30
1:1
80
dB
1
% of FSR
ppm of FSR
Offset Error
±100
Gain Sensitivity to VREF
Power-Supply Rejection Ratio
70
dB
PERFORMANCE OVER TEMPERATURE
Offset Drift
Gain Drift
0.07
0.4
ppm/°C
ppm/°C
ADS1251
2
SBAS184A
www.ti.com
ELECTRICAL CHARACTERISTICS (Cont.)
All specifications at TMIN to TMAX, VDD = +5V, CLK = 8MHz, and VREF = 4.096, unless otherwise specified.
ADS1251U
TYP
PARAMETER
CONDITIONS
MIN
MAX
UNITS
VOLTAGE REFERENCE
VREF
0.5
4.096
32
VDD
V
Load Current
µA
DIGITAL INPUT/OUTPUT
Logic Family
CMOS
Logic Level: VIH
+4.0
–0.3
+4.5
+VDD + 0.3
+0.8
V
V
V
V
V
VIL
VOH
VOL
IOH = –500µA
IOL = 500µA
0.4
Input (SCLK, CLK) Hysteresis
Data Format
0.6
Offset Binary Two’s Complement
POWER-SUPPLY REQUIREMENTS
Operation
+4.75
+5
1.5
7.5
0.4
+5.25
VDC
mA
Quiescent Current
VDD = +5VDC
2
10
1
Operating Power
mW
µA
Power-Down Current
TEMPERATURE RANGE
Operating
–40
–60
+85
°C
°C
Storage
+100
PIN CONFIGURATION
PIN DESCRIPTIONS
PIN
NAME
PIN DESCRIPTION
Top View
SO
1
+VIN
Analog Input: Positive Input of the Differen-
tial Analog Input
2
–VIN
Analog Input: Negative Input of the Differ-
ential Analog Input.
3
4
+VDD
CLK
Input: Power-Supply Voltage, +5V
Digital Input: Device System Clock. The
system clock is in the form of a CMOS-
compatible clock. This is a Schmitt-Trigger
input.
+VIN
1
VREF
GND
SCLK
8
7
6
5
5
DOUT/DRDY
Digital Output: Serial Data Output/Data
Ready. This output indicates that a new
output word is available from the ADS1251
data output register. The serial data is
clocked out of the serial data output shift
register using SCLK.
–VIN
2
ADS1251U
+VDD
3
6
SCLK
Digital Input: Serial Clock. The serial clock
is in the form of a CMOS-compatible clock.
The serial clock operates independently
from the system clock, therefore, it is pos-
sible to run SCLK at a higher frequency
than CLK. The normal state of SCLK is
LOW. Holding SCLK HIGH will either ini-
tiate a modulator reset for synchronizing
multiple converters or enter power-down
mode. This is a Schmitt-Trigger input.
CLK
4
DOUT/DRDY
7
8
GND
VREF
Input: Ground
Analog Input: Reference Voltage Input
ADS1251
SBAS184A
3
www.ti.com
TYPICAL CHARACTERISTICS
At TA = +25°C, VDD = +5V, CLK = 8MHz, and VREF = 4.096, unless otherwise specified.
RMS NOISE vs DATA RATE
2.0
EFFECTIVE RESOLUTION vs DATA OUTPUT RATE
20.0
19.5
19.0
18.5
18.0
1.6
1.2
0.8
0.4
0
10
100
1k
10k
100k
100
1k
10k
100k
Data Rate (Hz)
Data Output Rate (Hz)
RMS NOISE vs TEMPERATURE
EFFECTIVE RESOLUTION vs TEMPERATURE
2.0
1.8
1.6
1.4
1.2
1.0
19.5
19.0
18.5
18.0
–40
–20
0
20
40
60
80
100
–40
–20
0
20
40
60
80
100
Temperature (°C)
Temperature (°C)
RMS NOISE vs VREF VOLTAGE
RMS NOISE vs VREF VOLTAGE
18
16
14
12
10
8
14
12
10
8
6
6
4
4
2
2
0
0
0.5
1
1.5
2
2.5
3
3.5
4
4.5
5
0
1
2
3
4
5
VREF Voltage (V)
V
REF Voltage (V)
ADS1251
4
SBAS184A
www.ti.com
TYPICAL CHARACTERISTICS (Cont.)
At TA = +25°C, VDD = +5V, CLK = 8MHz, and VREF = 4.096, unless otherwise specified.
INTEGRAL NONLINEARITY vs TEMPERATURE
RMS NOISE vs INPUT VOLTAGE
2.0
4.0
3.5
3.0
2.5
2.0
1.5
1.0
0.5
0
1.5
1.0
0.5
0
–40
–20
0
20
40
60
80
100
–5
–4
–3
–2
–1
0
1
2
3
4
5
Temperature (°C)
Input Voltage (V)
OFFSET vs TEMPERATURE
INTEGRAL NONLINEARITY vs DATA OUTPUT RATE
40
35
30
25
20
15
10
5
5
4
3
2
1
0
0
–40
–20
0
20
40
60
80
100
100
1k
10k
100k
Temperature (°C)
Data Output Rate (Hz)
POWER-SUPPLY REJECTION RATIO
vs CLK FREQUENCY
GAIN ERROR vs TEMPERATURE
650
625
600
575
550
525
500
–60
–65
–70
–75
–80
–85
–90
–95
–100
–40
–20
0
20
40
60
80
100
1
2
3
4
5
6
7
8
Temperature (°C)
Clock Frequency (MHz)
ADS1251
SBAS184A
5
www.ti.com
TYPICAL CHARACTERISTICS (Cont.)
At TA = +25°C, VDD = +5V, CLK = 8MHz, and VREF = 4.096, unless otherwise specified.
COMMON-MODE REJECTION RATIO
COMMON-MODE REJECTION RATIO
vs COMMON-MODE FREQUENCY
vs CLK FREQUENCY
–60
–65
–70
–75
–80
–85
–90
–95
–100
–105
–60
–65
–70
–75
–80
–85
–90
–95
–100
–105
1
2
3
4
5
6
7
8
10
100
1k
10k
100k
Clock Frequency (MHz)
Common-Mode Signal Frequency (Hz)
CURRENT vs TEMPERATURE
POWER DISSIPATION vs CLK FREQUENCY
1.65
1.60
1.55
1.50
1.45
1.40
9
8
7
6
5
4
3
2
1
0
–40
–20
0
20
40
60
80
100
0
1
2
3
4
5
6
7
8
Temperature (°C)
Clock Frequency (MHz)
TYPICAL FFT
(1kHz input at 0.1dB less than full-scale)
VREF CURRENT vs CLK FREQUENCY
0
–20
35
30
25
20
15
10
5
–40
–60
–80
–100
–120
–140
–160
0
0
1
2
3
4
5
6
7
8
9
10 11
0.1
1
10
Frequency (kHz)
Clock Frequency (MHz)
ADS1251
6
SBAS184A
www.ti.com
clock frequency of 8MHz, the data-output rate is 20.8kHz
with a –3dB frequency of 4.24kHz. The –3dB frequency
scales with the system clock frequency.
THEORY OF OPERATION
The ADS1251 is a precision, high-dynamic range, 24-bit,
delta-sigma, A/D converter capable of achieving very
high-resolution digital results at high data rates. The analog
input signal is sampled at a rate determined by the frequency
of the system clock (CLK). The sampled analog input is
modulated by the delta-sigma A/D modulator, which is fol-
lowed by a digital filter. A sinc5 digital low-pass filter processes
the output of the delta-sigma modulator and writes the result
into the data-output register. The DOUT/DRDY pin is pulled
LOW, indicating that new data is available to be read by the
external microcontroller/microprocessor. As shown in the block
diagram on the front page, the main functional blocks of the
ADS1251 are the 4th-order delta-sigma modulator, a digital
filter, control logic, and a serial interface. Each of these
functional blocks is described in the following sections.
To ensure the best linearity of the ADS1251, and to maxi-
mize the elimination of even-harmonic noise errors, a fully
differential signal is recommended.
For more information about the ADS1251 input structure,
please refer to application note SBAA086 found at www.ti.com.
BIPOLAR INPUT
Each of the differential inputs of the ADS1251 must stay
between –0.3V and VDD. With a reference voltage at less
than half of VDD, one input can be tied to the reference
voltage, and the other input can range from 0V to
2 • VREF. By using a three op amp circuit featuring a single
amplifier and four external resistors, the ADS1251 can be
configured to accept bipolar inputs referenced to ground. The
conventional ±2.5V, ±5V, and ±10V input ranges can be
interfaced to the ADS1251 using the resistor values shown in
Figure 1.
ANALOG INPUT
The ADS1251 contains a fully differential analog input. In
order to provide low system noise, common-mode rejection
of 98dB, and excellent power-supply rejection, the design
topology is based on a fully differential switched-capacitor
architecture. The bipolar input voltage range is from –4.096
to +4.096V, when the reference input voltage equals +4.096V.
The bipolar range is with respect to –VIN, and not with respect
to GND.
R1
The differential input impedance of the analog input changes
with the ADS1251 system clock frequency (CLK). The rela-
tionship is:
10kΩ
+IN
OPA4350
20kΩ
ADS1251
VREF
Bipolar
Input
–IN
Impedance (Ω) = (8MHz/CLK) • 210,000
See application note Understanding the ADS1251, ADS1253,
and ADS1254 Input Circuitry (SBAA086), available for down-
load from TI’s web site www.ti.com.
R
2
OPA4350
With regard to the analog-input signal, the overall analog
performance of the device is affected by three items. First,
the input impedance can affect accuracy. If the source
impedance of the input signal is significant, or if there is
passive filtering prior to the ADS1251, a significant portion of
the signal can be lost across this external impedance. The
magnitude of the effect is dependent on the desired system
performance.
OPA4350
REF
2.5V
BIPOLAR INPUT
R1
R2
±10V
±5V
2.5kΩ
5kΩ
5kΩ
10kΩ
20kΩ
Second, the current into or out of the analog inputs must be
limited. Under no conditions should the current into or out of
the analog inputs exceed 10mA.
±2.5V
10kΩ
Third, to prevent aliasing of the input signal, the bandwidth of
the analog-input signal must be band-limited; the bandwidth
is a function of the system clock frequency. With a system
FIGURE 1. Level-Shift Circuit for Bipolar Input Ranges.
ADS1251
SBAS184A
7
www.ti.com
DELTA-SIGMA MODULATOR
REFERENCE INPUT
The ADS1251 operates from a nominal system clock fre-
quency of 8MHz. The modulator frequency is fixed in relation
to the system clock frequency. The system clock frequency
is divided by 6 to derive the modulator frequency (fMOD).
Therefore, with a system clock frequency of 8MHz, the
modulator frequency is 1.333MHz. Furthermore, the
oversampling ratio of the modulator is fixed in relation to the
modulator frequency. The oversampling ratio of the modula-
tor is 64, and with the modulator frequency running at
1.333MHz, the data rate is 20.8kHz. Using a slower system
clock frequency will result in a lower data output rate, as
shown in Table I.
The reference input takes an average current of 32µA with a
8MHz system clock. This current will be proportional to the
system clock. A buffered reference is recommended for the
ADS1251. The recommended reference circuit is shown in
Figure 2.
Reference voltages higher than 4.096V will increase the full-
scale range, while the absolute internal circuit noise of the
converter remains the same. This will decrease the noise in
terms of ppm of full-scale, which increases the effective
resolution (see typical characteristic “RMS Noise vs VREF
Voltage”).
DIGITAL FILTER
CLK (MHz)
DATA OUTPUT RATE (Hz)
The digital filter of the ADS1251, referred to as a Sinc5 filter,
computes the digital result based on the most recent outputs
from the delta-sigma modulator. At the most basic level, the
digital filter can be thought of as averaging the modulator
results in a weighted form and presenting this average as the
digital output. The digital output rate, or data rate, scales
directly with the system clock frequency. This allows the data
output rate to be changed over a very wide range (five orders
of magnitude) by changing the system clock frequency.
However, it is important to note that the –3dB point of the
filter is 0.2035 times the data output rate, so the data output
rate should allow for sufficient margin to prevent attenuation
of the signal of interest.
8(1)
20,833
19,200
16,000
15,625
12,800
9600
8000
6400
4800
2400
1200
1000
500
100
60
50
30
25
20
16.67
15
12.50
10
7.372800(1)
6.144000(1)
6.000000(1)
4.915200(1)
3.686400(1)
3.072000(1)
2.457600(1)
1.843200(1)
0.921600
0.460800
0.384000
0.192000
0.038400
0.023040
0.019200
0.011520
0.009600
0.007680
0.006400
0.005760
0.004800
0.003840
As the conversion result is essentially an average, the
data-output rate determines the location of the resulting
notches in the digital filter (see Figure 3). Note that the first
notch is located at the data output rate frequency, and
subsequent notches are located at integer multiples of the
data output rate; this allows for rejection of not only the
fundamental frequency, but also harmonic frequencies. In
this manner, the data output rate can be used to set specific
notch frequencies in the digital filter response.
NOTE: (1) Standard Clock Oscillator.
TABLE I. CLK Rate versus Data Output Rate.
For example, if the rejection of power-line frequencies is
desired, then the data output rate can simply be set to the
power-line frequency. For 50Hz rejection, the system clock
+5V
+5V
0.10µF
7
0.1µF
2
3
To VREF
Pin 8 of
the ADS1251
1
6
OPA350
10kΩ
2
REF3040
+
10µF
0.1µF
+
10µF
0.10µF
0.1µF
4
3
FIGURE 2. Recommended External Voltage Reference Circuit for Best Low-Noise Operation with the ADS1251.
ADS1251
8
SBAS184A
www.ti.com
frequency must be 19.200kHz, and this sets the data output
rate to 50Hz (see Table I and Figure 4). For 60Hz rejection, the
system CLK frequency must be 23.040kHz, and this sets the
data-output rate to 60Hz (see Table I and Figure 5). If both
50Hz and 60Hz rejection is required, then the system CLK
must be 3.840kHz; this sets the data output rate to 10Hz and
rejects both 50Hz and 60Hz (see Table I and Figure 6).
The digital filter requires five conversions to fully settle. The
modulator has an oversampling ratio of 64; therefore, it
requires 5 • 64, or 320 modulator results (or clocks) to fully
settle. As the modulator clock is derived from the system
CLK (modulator clock = CLK ÷ 6), the number of system
clocks required for the digital filter to fully settle is
5 • 64 • 6, or 1920 CLKs. This means that any significant step
change at the analog input requires five full conversions to
settle. However, if the step change at the analog input occurs
asynchronously to the DOUT/DRDY pulse, six conversions
are required to ensure full settling.
There is an additional benefit in using a lower data output
rate. It provides better rejection of signals in the frequency
band of interest. For example, with a 50Hz data output rate,
a significant signal at 75Hz may alias back into the passband
at 25Hz. This is due to the fact that rejection at 75Hz may
only be 66dB in the stopband—frequencies higher than the
first notch frequency (see Figure 4). However, setting the
data output rate to 10Hz provides 135dB rejection at 75Hz
(see Figure 6). A similar benefit is gained at frequencies near
the data output rate (see Figures 7, 8, 9, and 10). For
example, with a 50Hz data output rate, rejection at 55Hz may
only be 105dB (see Figure 7). With a 10Hz data output rate,
however, rejection at 55Hz will be 122dB (see Figure 8). If a
slower data output rate does not meet the system require-
ments, then the analog front-end can be designed to provide
the needed attenuation to prevent aliasing. Additionally, the
data output rate may be increased and additional digital
filtering may be done in the processor or controller.
CONTROL LOGIC
The control logic is used for communications and control of
the ADS1251.
Power-Up Sequence
Prior to power-up, all digital and analog input pins must be
LOW. At the time of power-up, these signal inputs can be
biased to a voltage other than 0V; however, they should
never exceed +VDD
.
Once the ADS1251 powers up, the DOUT/DRDY line will
pulse LOW on the first conversion for which the data is valid
from the analog input signal.
Application note A Spreadsheet to Calculate the Frequency
Response of the ADS1250-54 (SBAA103) available for down-
load from TI’s web site www.ti.com provides a simple tool for
calculating the ADS1250’s frequency response for any CLK
frequency.
DOUT/DRDY
The DOUT/DRDY output signal alternates between two
modes of operation. The first mode of operation is the Data
Ready mode (DRDY) to indicate that new data has been
loaded into the data output register and is ready to be read.
The second mode of operation is the Data Output (DOUT)
mode and is used to serially shift data out of the Data Output
Register (DOR). See Figure 11 for the time domain partition-
ing of the DRDY and DOUT function.
The digital filter is described by the following transfer function:
5
π • f •64
sin
fMOD
H(f) =
See Figure 12 for the basic timing of DOUT/DRDY. During
the time defined by t2, t3, and t4, the DOUT/DRDY pin
functions in DRDY mode. The state of the DOUT/DRDY pin
π • f
64• sin
fMOD
or
5
1– z–64
H(z) =
64• 1– z–1
(
)
ADS1251
SBAS184A
9
www.ti.com
NORMALIZED DIGITAL FILTER RESPONSE
DIGITAL FILTER RESPONSE
0
–20
0
–20
–40
–40
–60
–60
–80
–80
–100
–120
–140
–160
–180
–200
–100
–120
–140
–160
–180
–200
0
1
2
3
4
5
6
7
8
9
10
0
50
100
150
200
250
300
Frequency (Hz)
Frequency (Hz)
FIGURE 3. Normalized Digital Filter Response.
FIGURE 4. Digital Filter Response (50Hz).
DIGITAL FILTER RESPONSE
0
DIGITAL FILTER RESPONSE
0
–20
–40
–20
–40
–60
–60
–80
–80
–100
–120
–140
–160
–180
–200
–100
–120
–140
–160
–180
–200
0
50
100
150
200
250
300
0
10
20
30
40
50
60
70
80
90 100
Frequency (Hz)
Frequency (Hz)
FIGURE 5. Digital Filter Response (60Hz).
FIGURE 6. Digital Filter Response (10Hz Multiples).
DIGITAL FILTER RESPONSE
0
DIGITAL FILTER RESPONSE
0
–20
–40
–20
–40
–60
–60
–80
–80
–100
–120
–140
–160
–180
–200
–100
–120
–140
–160
–180
–200
45
46
47
48
49
50
51
52
53
54
55
45
46
47
48
49
50
51
52
53
54
55
Frequency (Hz)
Frequency (Hz)
FIGURE 7. Expanded Digital Filter Response (50Hz with a
50Hz data output rate).
FIGURE 8. Expanded Digital Filter Response (50Hz with a
10Hz data output rate).
ADS1251
10
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DIGITAL FILTER RESPONSE
DIGITAL FILTER RESPONSE
0
–20
0
–20
–40
–40
–60
–60
–80
–80
–100
–120
–140
–160
–180
–200
–100
–120
–140
–160
–180
–200
55
56
57
58
59
60
61
62
63
64
65
55
56
57
58
59
60
61
62
63
64
65
Frequency (Hz)
Frequency (Hz)
FIGURE 9. Expanded Digital Filter Response (60Hz with a
60Hz data output rate).
FIGURE 10. Expanded Digital Filter Response (60Hz with a
10Hz data output rate).
is HIGH prior to the internal transfer of new data to the DOR.
The result of the A/D conversion is written to the DOR from
the Most Significant Bit (MSB) to the Least Significant Bit
(LSB) in the time defined by t1 (see Figures 11 and 12). The
DOUT/DRDY line then pulses LOW for the time defined by
t2, and then pulses HIGH for the time defined by t3 to indicate
that new data is available to be read. At this point, the
function of the DOUT/DRDY pin changes to DOUT mode.
Data is shifted out on the pin after t7. The device communi-
cating with the ADS1251 can provide SCLKs to the ADS1251
after the time defined by t6. The normal mode of reading data
from the ADS1251 is for the device reading the ADS1251 to
latch the data on the rising edge of SCLK (because data is
shifted out of the ADS1251 on the falling edge of SCLK). In
order to retrieve valid data, the entire DOR must be read
before the DOUT/DRDY pin reverts back to DRDY mode.
The internal data pointer for shifting data out on DOUT/DRDY
is reset on the falling edge of the time defined by t1 and t4.
This ensures that the first bit of data shifted out of the
ADS1251 after DRDY mode is always the MSB of new data.
SYNCHRONIZING MULTIPLE CONVERTERS
The normal state of SCLK is LOW; however, by holding SCLK
HIGH, multiple ADS1251s can be synchronized. This is accom-
plished by holding SCLK HIGH for at least four, but less than 20,
consecutive DOUT/DRDY cycles (see Figure 13). After the
ADS1251 circuitry detects that SCLK has been held HIGH for
four consecutive DOUT/DRDY cycles, the DOUT/DRDY pin
pulses LOW for one CLK cycle and then is held HIGH, and the
modulator is held in a reset state. The modulator will be
released from reset and synchronization occurs on the falling
edge of SCLK. With multiple converters, the falling edge tran-
sition of SCLK must occur simultaneously on all devices. It is
important to note that prior to synchronization, the DOUT/DRDY
pulse of multiple ADS1251s in the system could have a differ-
ence in timing up to one DRDY period. Therefore, to ensure
synchronization, the SCLK must be held HIGH for at least five
DRDY cycles. The first DOUT/DRDY pulse after the falling
edge of SCLK occurs at t14. The first DOUT/DRDY pulse
indicates valid data.
If SCLKs are not provided to the ADS1251 during the DOUT
mode, the MSB of the DOR is present on the DOUT/DRDY
line until the time defined by t4. If an incomplete read of the
ADS1251 takes place while in DOUT mode (that is, less than
24 SCLKs were provided), the state of the last bit read is
present on the DOUT/DRDY line until the time defined by t4.
If more than 24 SCLKs are provided during DOUT mode, the
DOUT/DRDY line stays LOW until the time defined by t4.
ADS1251
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POWER-DOWN MODE
SERIAL INTERFACE
The normal state of SCLK is LOW; however, by holding
SCLK HIGH, the ADS1251 will enter power-down mode. This
is accomplished by holding SCLK HIGH for at least 20
consecutive DOUT/DRDY periods (see Figure 14). After the
ADS1251 circuitry detects that SCLK has been held HIGH for
four consecutive DOUT/DRDY cycles, the DOUT/DRDY pin
pulses LOW for one CLK cycle and then is held HIGH, and
the modulator is held in a reset state. If SCLK is held HIGH
for an additional 16 DOUT/DRDY periods, the ADS1251
enters power-down mode. The part will be released from
power-down mode on the falling edge of SCLK. It is impor-
tant to note that the DOUT/DRDY pin is held HIGH after four
DOUT/DRDY cycles, but power-down mode is not entered
for an additional 16 DOUT/DRDY periods. The first
DOUT/DRDY pulse after the falling edge of SCLK occurs at
The ADS1251 includes a simple serial interface which can be
connected to microcontrollers and digital signal processors in
a variety of ways. Communications with the ADS1251 can
commence on the first detection of the DOUT/DRDY pulse
after power up.
It is important to note that the data from the ADS1251 is a
24-bit result transmitted MSB-first in Offset Binary Two’s
Complement format, as shown in Table III.
The data must be clocked out before the ADS1251 enters
DRDY mode to ensure reception of valid data, as described
in the DOUT/DRDY section of this data sheet.
DIFFERENTIAL VOLTAGE INPUT
DIGITAL OUTPUT (HEX)
+Full-Scale
Zero
–Full-Scale
7FFFFFH
000000H
800000H
t
16 and indicates valid data. Subsequent DOUT/DRDY pulses
will occur normally.
TABLE III. ADS1251 Data Format (Offset Binary Two’s
Complement).
SYMBOL
DESCRIPTION
MIN
TYP
MAX
UNITS
tDRDY
DRDY Mode
DOUT Mode
Conversion Cycle
DRDY Mode
DOUT Mode
DOR Write Time
384 • CLK
36 • CLK
348 • CLK
6 • CLK
6 • CLK
6 • CLK
ns
ns
ns
ns
ns
ns
ns
ns
ns
ns
ns
ns
ns
ns
ns
ns
ns
ns
ns
ns
t1
t2
t3
t4
t5
t6
t7
t8
t9
t10
t11
t12
t13
t14
t15
t16
t17
DOUT/DRDY LOW Time
DOUT/DRDY HIGH Time (Prior to Data Out)
DOUT/DRDY HIGH Time (Prior to Data Ready)
Rising Edge of CLK to Falling Edge of DOUT/DRDY
End of DRDY Mode to Rising Edge of First SCLK
End of DRDY Mode to Data Valid (Propagation Delay)
Falling Edge of SCLK to Data Valid (Hold Time)
Falling Edge of SCLK to Next Data Out Valid (Propagation Delay)
SCLK Setup Time for Synchronization or Power Down
DOUT/DRDY Pulse for Synchronization or Power Down
Rising Edge of SCLK Until Start of Synchronization
Synchronization Time
24 • CLK
30
30
30
30
5
30
1 • CLK
1537 • CLK
0.5 • CLK
7679 • CLK
6143.5 • CLK
Falling Edge of CLK (After SCLK Goes LOW) Until Start of DRDY Mode
Rising Edge of SCLK Until Start of Power Down
Falling Edge of CLK (After SCLK Goes LOW) Until Start of DRDY Mode
Falling Edge of Last DOUT/DRDY to Start of Power Down
2042.5 • CLK
7681 • CLK
2318.5 • CLK
6144.5 • CLK
TABLE II. Digital Timing.
DRDY Mode
t4
DRDY Mode
DOUT Mode
DOUT Mode
t2
t3
DATA
DATA
DATA
DOUT/DRDY
t1
FIGURE 11. DOUT/DRDY Partitioning.
ADS1251
12
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ADS1251
13
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ISOLATION
SYSTEM CONSIDERATIONS
The serial interface of the ADS1251 provides for simple
isolation methods. The CLK signal can be local to the
ADS1251, which then only requires two signals (SCLK, and
DOUT/DRDY) to be used for isolated data acquisition.
The recommendations for power supplies and grounding will
change depending on the requirements and specific design
of the overall system. Achieving 24 bits of noise performance
is a great deal more difficult than achieving 12 bits of noise
performance. In general, a system can be broken up into four
different stages:
LAYOUT
POWER SUPPLY
•
•
•
•
Analog Processing
Analog Portion of the ADS1251
Digital Portion of the ADS1251
Digital Processing
The power supply must be well-regulated and low-noise. For
designs requiring very high resolution from the ADS1251,
power supply rejection will be a concern. Avoid running
digital lines under the device as they may couple noise onto
the die. High-frequency noise can capacitively couple into
the analog portion of the device and will alias back into the
passband of the digital filter, affecting the conversion result.
This clock noise will cause an offset error.
For the simplest system consisting of minimal analog signal
processing (basic filtering and gain), a microcontroller, and
one clock source, one can achieve high resolution by power-
ing all components from a common power supply. In addi-
tion, all components could share a common ground plane.
Thus, there would be no distinctions between analog power
and ground, and digital power and ground. The layout
should still include a power plane, a ground plane, and
careful decoupling. In a more extreme case, the design could
include:
GROUNDING
The analog and digital sections of the system design should
be carefully and cleanly partitioned. Each section should
have its own ground plane with no overlap between them.
GND should be connected to the analog ground plane, as
well as all other analog grounds. Do not join the analog and
digital ground planes on the board, but instead connect the
two with a moderate signal trace. For multiple converters,
connect the two ground planes at one location as central to
all of the converters as possible. In some cases, experimen-
tation may be required to find the best point to connect the
two planes together. The printed circuit board can be de-
signed to provide different analog/digital ground connections
via short jumpers. The initial prototype can be used to
establish which connection works best.
•
•
•
Multiple ADS1251s
Extensive Analog Signal Processing
One or More Microcontrollers, Digital Signal Processors,
or Microprocessors
•
•
Many Different Clock Sources
Interconnections to Various Other Systems
High resolution will be very difficult to achieve for this design.
The approach would be to break the system into as many
different parts as possible. For example, each ADS1251 may
have its own analog processing front end.
DECOUPLING
DEFINITION OF TERMS
Good decoupling practices should be used for the ADS1251
and for all components in the design. All decoupling capaci-
tors, and specifically the 0.1µF ceramic capacitors, should be
placed as close as possible to the pin being decoupled. A
1µF to 10µF capacitor, in parallel with a 0.1µF ceramic
capacitor, should be used to decouple VDD to GND.
An attempt has been made to use consistent terminology in
this data sheet. In that regard, the definition of each term is
provided here:
Analog-Input Differential Voltage—for an analog signal
that is fully differential, the voltage range can be compared to
that of an instrumentation amplifier. For example, if both
analog inputs of the ADS1251 are at 2.048V, the differential
voltage is 0V. If one analog input is at 0V and the other
ADS1251
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The 2 • VREF figure in each calculation represents the full-
scale range of the ADS1251. This means that both units are
absolute expressions of resolution—the performance in dif-
ferent configurations can be directly compared, regardless of
the units.
analog input is at 4.096V, then the differential voltage mag-
nitude is 4.096V. This is the case regardless of which input
is at 0V and which is at 4.096V. The digital-output result,
however, is quite different. The analog-input differential volt-
age is given by the following equation:
f
MOD—frequency of the modulator and the frequency the
+VIN – (–VIN)
input is sampled.
A positive digital output is produced whenever the analog-
input differential voltage is positive, whereas a negative
digital output is produced whenever the differential is nega-
tive. For example, a positive full-scale output is produced
when the converter is configured with a 4.096V reference,
and the analog-input differential is 4.096V. The negative full-
scale output is produced when the differential voltage is –
4.096V. In each case, the actual input voltages must remain
within the –0.3V to +VDD range.
CLK Frequency
6
fMOD
=
fDATA—Data output rate.
fMOD CLK Frequency
fDATA
=
=
64
384
Noise Reduction—for random noise, the ER can be im-
proved with averaging. The result is the reduction in noise by
the factor √N, where N is the number of averages, as shown
in Table IV. This can be used to achieve true 24-bit perfor-
mance at a lower data rate. To achieve 24 bits of resolution,
more than 24 bits must be accumulated. A 36-bit accumulator
is required to achieve an ER of 24 bits. The following uses
Actual Analog-Input Voltage—the voltage at any one ana-
log input relative to GND.
Full-Scale Range (FSR)—as with most A/D converters, the
full-scale range of the ADS1251 is defined as the input which
produces the positive full-scale digital output minus the input
which produces the negative full-scale digital output. For
example, when the converter is configured with a 4.096V
reference, the differential full-scale range is:
VREF = 4.096V, with the ADS1251 outputting data at 20kHz, a
4096 point average will take 204.8ms. The benefits of averag-
ing will be degraded if the input signal drifts during that 200ms.
[4.096V (positive full-scale) – (–4.096V) (negative full-scale)] = 8.192V
N
NOISE
REDUCTION
FACTOR
ER
IN
Vrms
ER
IN
BITS rms
Least Significant Bit (LSB) Weight—this is the theoretical
amount of voltage that the differential voltage at the analog
input would have to change in order to observe a change in
the output data of one least significant bit. It is computed as
follows:
(Number
of Averages)
1
2
1
1.414
2
16µV
11.3µV
8µV
19.26
19.75
20.26
20.76
21.26
21.76
22.26
22.76
23.26
23.76
24.26
24.76
25.26
4
8
2.82
4
5.66µV
4µV
Full−ScaleRange 2• VREF
16
LSBWeight =
=
2N –1
2N –1
32
5.66
8
2.83µV
2µV
64
128
256
512
1024
2048
4096
11.3
16
1.41µV
1µV
where N is the number of bits in the digital output.
Conversion Cycle—as used here, a conversion cycle refers
to the time period between DOUT/DRDY pulses.
22.6
32
0.71µV
0.5µV
0.35µV
0.25µV
45.25
64
Effective Resolution (ER)—of the ADS1251, in a particular
configuration, can be expressed in two different units:
bits rms (referenced to output) and µVrms (referenced to
input). Computed directly from the converter’s output data,
each is a statistical calculation based on a given number of
results. Noise occurs randomly; the rms value represents a
statistical measure, which is one standard deviation. The ER
in bits can be computed as follows:
TABLE IV. Averaging for Noise Reduction.
2 • VREF
20 • log
Vrms noise
ER in bits rms =
6.02
ADS1251
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