LM6132/34 Application Hints

(Continued)

cellent response is obtained with a C

4 Vp-p and C

capacitor should be established after the board layout is fin-

ished because the value is dependent on board stray capac-

ity, the value of the feedback resistor, the closed loop gain

and, to some extent, the supply voltage.

Another effect that is common to all opamps is the phase

shift caused by the feedback resistor and the input capaci-

tance. This phase shift also reduces phase margin. This ef-

fect is taken care of at the same time as the effect of the ca-

pacitive load when the capacitor is placed across the

feedback resistor.

The circuit shown in

Figure 5 was used for these scope

photos.

Figure 6 shows a method for compensating for load capaci-

tance (C

the output and a feedback capacitor C

output and the inverting input pin. Feedback capacitor C

F

compensates for the pole introduced by R

ing ringing in the output waveform while the feedback resis-

tor R

Depending on the size of the load capacitance, the value of

R

Typical Applications

3 OPAMP INSTRUMENTATION AMP WITH

RAIL-TO-RAIL INPUT AND OUTPUT

Using the LM6134, a 3 opamp instrumentation amplifier with

rail-to-rail inputs and rail to rail output can be made. These

features make these instrumentation amplifiers ideal for

single supply systems.

Some manufacturers use a precision voltage divider array of

5 resistors to divide the common-mode voltage to get an in-

put range of rail-to-rail or greater. The problem with this

method is that it also divides the signal, so to even get unity

gain, the amplifier must be run at high closed loop gains.

This raises the noise and drift by the internal gain factor and

lowers the input impedance. Any mismatch in these preci-

sion resistors reduces the CMR as well. Using the LM6134,

all of these problems are eliminated.

In this example, amplifiers A and B act as buffers to the dif-

ferential stage (

Figure 7). These buffers assure that the input

impedance is over 100 M

Ω and they eliminate the require-

ment for precision matched resistors in the input stage. They

also assure that the difference amp is driven from a voltage

source. This is necessary to maintain the CMR set by the

matching of R1–R2 with R3–R4.

DS012349-45

FIGURE 3.

DS012349-42

FIGURE 4.

DS012349-43

FIGURE 5.

DS012349-37

FIGURE 6.

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