LMV321 datasheet(14/27 Pages) NSC

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Application Notes (Continued)

In Figure 3 , the isolation resistor R

ISO and the load capacitor

L form a pole to increase stability by adding more phase

margin to the overall system. The desired performance de-

pends on the value of R

ISO. The bigger the RISO resistor

value, the more stable V

OUT will be. Figure 4 is an output

waveform of Figure 3 using 620

Ω for R

ISO and 510pF for CL..

The circuit in Figure 5 is an improvement to the one in Figure

3 because it provides DC accuracy as well as AC stability. If

there were a load resistor in Figure 3, the output would be

voltage divided by R

ISO and the load resistor. Instead, in

Figure 5,R

provides the DC accuracy by using feed-

forward techniques to connect V

IN to RL. Caution is needed

in choosing the value of R

F due to the input bias current of

the LMV321/358/324. C

F and RISO serve to counteract the

loss of phase margin by feeding the high frequency compo-

nent of the output signal back to the amplifier’s inverting

input, thereby preserving phase margin in the overall feed-

back loop. Increased capacitive drive is possible by increas-

ing the value of C

F . This in turn will slow down the pulse

response.

3.0 INPUT BIAS CURRENT CANCELLATION

The LMV321/358/324 family has a bipolar input stage. The

typical input bias current of LMV321/358/324 is 15nA with 5V

supply. Thus a 100k

Ω input resistor will cause 1.5mV of error

voltage. By balancing the resistor values at both inverting

and non-inverting inputs, the error caused by the amplifier’s

input bias current will be reduced. The circuit in Figure 6

shows how to cancel the error caused by input bias current.

4.0 TYPICAL SINGLE-SUPPLY APPLICATION CIRCUITS

4.1 Difference Amplifier

The difference amplifier allows the subtraction of two volt-

ages or, as a special case, the cancellation of a signal

common to two inputs. It is useful as a computational ampli-

fier, in making a differential to single-ended conversion or in

rejecting a common mode signal.

4.2 Instrumentation Circuits

The input impedance of the previous difference amplifier is

set by the resistors R

1,R2,R3, and R4. To eliminate the

problems of low input impedance, one way is to use a

voltage follower ahead of each input as shown in the follow-

ing two instrumentation amplifiers.

Time (2

µs/div)

10006099

FIGURE 4. Pulse Response of the LMV324 Circuit in

Figure 3

10006005

FIGURE 5. Indirectly Driving A Capacitive Load with

DC Accuracy

10006006

FIGURE 6. Cancelling the Error Caused by Input Bias

Current

10006007

10006019

FIGURE 7. Difference Amplifier

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