AN-733 [ADI]

Universal Precision Op Amp Evaluation Board in MSOP Package; 通用精密运算放大器评估板，采用MSOP封装


型号：	AN-733
厂家：	ADI
描述：	Universal Precision Op Amp Evaluation Board in MSOP Package 通用精密运算放大器评估板，采用MSOP封装运算放大器
文件：	总4页 (文件大小：631K)
中文：	中文翻译
下载：	下载PDF数据表文档文件

AN-733

APPLICA

TION NOTE

One Technology Way •

O. Box 9106 • Norwood, MA 02062-9106 • Tel: 781/329-4700 • Fax: 781/326-8703 •

www.analog.com

Universal Precision Op Amp Evaluation Board in MSOP Package

by Giampaolo Marino, Souﬁane Bendaoud, and Steve Ranta

INTRODUCTION

LOW-PASS FILTER

The EVAL-PRAOPAMP-1RM is an evaluation board which

accommodates single op amps in MSOP packages. It is

meant to provide the user with multiple choices and

extensive ﬂexibility for different application circuits

and conﬁgurations. This board is not intended to be

used with high frequency components or high speed

ampliﬁers. However, it provides the user with many

combinations for various circuit types, including active

ﬁlters, differential ampliﬁers, and external frequency

compensation circuits. A few examples of application

circuits are given in this application note.

Figure 1 is a typical representation of a ﬁrst-order low-

pass ﬁlter. This circuit has a 6 dB per octave roll-off

after a close-loop –3 dB point deﬁned by f_C

. Gain below

this frequency is deﬁned as the magnitude of R7 to R2.

The circuit might be considered as an ac integrator for

frequencies well above f_C; however, the time domain

response is that of a single RC, rather than an integral.

f_C= 1/(2  R7  C7); –3 dB frequency

f_L= 1/(2  R2  C7); unity gain frequency

Acl = –(R7/R2); close loop gain

R6 should be chosen equal to the parallel combination

between R7 and R2 in order to minimize errors due to

bias currents.

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Figure 2. Difference Ampliﬁer

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DIFFERENCE AMPLIFIER AND PERFORMANCE

OPTIMIZATION

Figure 2 shows an op amp conﬁgured as a difference

ampliﬁer. The difference ampliﬁer is the complement

of the summing ampliﬁer, and allows the subtraction

of two voltages or the cancellation of a signal common

to both inputs. The circuit shown in Figure 2 is useful

as a computational ampliﬁer in making a differential

to single-ended conversion or in rejecting a common-

mode signal. The output voltage V_OUTis comprised of

two separate components:

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1. A component V_OUT1 due to V_IN1 acting alone (V_IN2

short circuited to ground.)

Figure 1. Simple Low-Pass Filter

2. A component V_OUT2 due to V_IN2 acting alone (V_IN1

short circuited to ground.)

REV. A

AN-733

The algebraic sum of these two components should be

equal to V_OUT. By applying the principles expressed in

the output voltage V_OUTcomponents, and by letting R4

= R2 and R7 = R6, then:

CURRENT-TO-VOLTAGE CONVERTER

Current may be measured in two ways with an opera-

tional ampliﬁer: It can be converted to a voltage with a

resistor and then ampliﬁed, or injected directly into a

summing node.

V_OUT1 = V_IN1 R7/R2

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_OUT

2 = –V

_IN

2 R7/R2

_OUT

= V

_OUT

1 + V

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1 – V

_IN2) R7/R1

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Difference amplifiers are commonly used in high

accuracy circuits to improve the common-mode rejec-

tion ratio, typically known as CMRR.

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For this type of application, CMRR depends upon how

tightly matched resistors are used; poorly matched resis-

tors result in a low value of CMRR.

Figure 3. Current-to-Voltage Converter

Figure 3 is a typical representation of a current-to-voltage

transducer. The input current is fed directly into the sum-

ming node and the ampliﬁer output voltage changes to

exactly the same current from the summing node through

R7. The scale factor of this circuit is R7 volts per amps.

The only conversion error in this circuit is I_BIAS, which is

summed algebraically with I_IN

To see how this works, consider a hypothetical source

of error for resistor R7 (1 – error). Using the superposi-

tion principle and letting R4 = R2 and R7 = R6, the output

voltage would be as follows:

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R7  R2 + 2R7 error

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V_DD=V_IN2 −V_IN1

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From this equation, A_CMand A_DMcan be deﬁned as

follows:

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A_CM= R7/(R7 – R2)  error

A_DM= R7/R2  {1 – [(R2+2R7/R2+R7)  error/2]}

These equations demonstrate that when there is not an

error in the resistor values, the A_CM= 0 and the ampliﬁer

responds only to the differential voltage being applied to

its inputs; under these conditions, the CMRR of the circuit

becomes highly dependent on the CMRR of the ampliﬁer

selected for this job.

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As mentioned above, errors introduced by resistor

mismatch can be a big drawback of discrete differential

ampliﬁers, but there are different ways to optimize this

circuit conﬁguration:

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1. The differential gain is directly related to the ratio

R7/R2; therefore, one way to optimize the perfor-

mance of this circuit is to place the ampliﬁer in a high

gain conﬁguration. When larger values for resistors

R7 and R6 and smaller values for resistors R2 and R4

are selected, the higher the gain, the higher the CMRR.

For example, when R7 = R6 = 10 k, and R2 = R4 = 1 k

and error = 0.1%, CMRR improves to better than 80 dB.

For high gain conﬁguration, select ampliﬁers with

very low I_BIASand very high gain (such as the AD8551,

AD8571, AD8603, and AD8605) to reduce errors.

Figure 5. Output Response

GENERATION OF SQUAREWAVEFORMS USING A

BISTABLE MULTIVIBRATOR

A square waveform can be simply generated by arrang-

ing the ampliﬁer for a bistable multivibrator to switch

states periodically as Figure 5 shows.

Once the output of the ampliﬁer reaches one of two pos-

sible levels, such as L+, capacitor C9 charges toward this

level through resistor R7. The voltage across C9, which

is applied to the negative input terminal of the ampli-

ﬁer denoted as V–, then rises exponentially toward L+

with a time constant  = C9R7. Meanwhile, the voltage

2. Select resistors that have much tighter tolerance and

accuracy. The more closely they are matched, the better

the CMRR. For example, if a CMRR of 90 dB is needed,

then match resistors to approximately 0.02%.

–2–

REV. A

AN-733

at the positive input terminal of the ampliﬁer, denoted as

V+ = BL+. This continues until the capacitor voltage

reaches the positive threshold V_TH, at which point the

bistable multivibrator switches to the other stable state

in which V_O= L– and V+ = BL–. This is shown in Figure 5.

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The capacitor then begins to discharge, and its voltage,

V–, decreases exponentially toward L–. This continues

until V– reaches the negative threshold V_TL, at which time

the bistable multivibrator switches to the positive output

state, and the cycle repeats itself.

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It is important to note that the frequency of the square

wave being generated, f_O, depends only on the external

components being used. Any variation in L+ will cause

V+ to vary in proportion, ensuring the same transition

time and the same oscillation frequency. The maximum

operating frequency is determined by the amplifier

speed, which can be increased signiﬁcantly by using

faster devices.

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Figure 8. Capacitive Load Drive with Resistor

EXTERNAL COMPENSATIONTECHNIQUES

Series Resistor Compensation

The use of external compensation networks may be

required to optimize certain applications. Figure 6 is a

typical representation of a series resistor compensation

for stabilizing an op amp driving capacitive load. The sta-

bilizing effect of the series resistor isolates the op amp

output and the feedback network from the capacitive

load. The required amount of series resistance depends

on the part used, but values of 5  to 50  are usually

sufﬁcient to prevent local resonance. The disadvantages

of this technique are a reduction in gain accuracy and

extra distortion when driving nonlinear loads.

The lowest operating frequency depends on the practical

upper limits set by R7 and C9.

Using the name convention outlined on the PRA OPAMP

evaluation board, the circuit should be connected

as follows:

B = R4/(R4 + R9); feedback factor (noninverting input)

T = 2R7  C9  ln((1 + B)/(1 – B)); period of oscillation

f_O= 1/T; oscillation frequency

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Figure 6. Series Resistor Compensation

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Figure 7. Capacitive Load Drive Without Resistor

Figure 10. Capacitive Load Drive Without Snubber

REV. A

–3–

AN-733

Snubber Network

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Another way to stabilize an op amp driving a capacitive

load is with the use of a snubber, as shown in Figure

9. This method presents the signiﬁcant advantage of

not reducing the output swing because there is not

any isolation resistor in the signal path. Also, the use

of the snubber does not degrade the gain accuracy or

cause extra distortion when driving a nonlinear load.

The exact R

and C

combinations can be determined

experimentally.

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Figure 11. Capacitive Load Drive with the Snubber

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Figure 13. EVAL-PRAOPAMP-1RM Board Layout Patterns

–4–

REV. A

AN-733 [ADI]

相关型号：

AN-7508

AN-756

AN-759

AN-763

AN-768

AN-7701

AN-7730

AN-7732

AN-780

AN-7820

AN-784

AN-793