Instrumentation Circuits
The input impedance of the previous difference amplifier is
set by the resistors R1, R2, R3, and R4. To eliminate the prob-
lems of low input impedance, one way is to use a voltage
follower ahead of each input as shown in the following two
instrumentation amplifiers.
Three-Op-Amp Instrumentation Amplifier
The quad LMV324 can be used to build a three-op-amp in-
strumentation amplifier as shown in Figure 8.
10006011
10006035
FIGURE 9. Two-Op-Amp Instrumentation Amplifier
Single-Supply Inverting Amplifier
There may be cases where the input signal going into the
amplifier is negative. Because the amplifier is operating in
single supply voltage, a voltage divider using R3 and R4 is
implemented to bias the amplifier so the input signal is within
the input common-mode voltage range of the amplifier. The
capacitor C1 is placed between the inverting input and resistor
R1 to block the DC signal going into the AC signal source,
VIN. The values of R1 and C1 affect the cutoff frequency, fc =
1/2πR1C1.
10006085
FIGURE 8. Three-Op-Amp Instrumentation Amplifier
As a result, the output signal is centered around mid-supply
(if the voltage divider provides V+/2 at the non-inverting input).
The output can swing to both rails, maximizing the signal-to-
noise ratio in a low voltage system.
The first stage of this instrumentation amplifier is a differential-
input, differential-output amplifier, with two voltage followers.
These two voltage followers assure that the input impedance
is over 100 MΩ. The gain of this instrumentation amplifier is
set by the ratio of R2/R1. R3 should equal R1, and R4 equal
R2. Matching of R3 to R1 and R4 to R2 affects the CMRR. For
good CMRR over temperature, low drift resistors should be
used. Making R4 slightly smaller than R2 and adding a trim
pot equal to twice the difference between R2 and R4 will allow
the CMRR to be adjusted for optimum performance.
Two-Op-Amp Instrumentation Amplifier
A two-op-amp instrumentation amplifier can also be used to
make a high-input-impedance DC differential amplifier (Fig-
ure 9). As in the three-op-amp circuit, this instrumentation
amplifier requires precise resistor matching for good CMRR.
R4 should equal R1 and, R3 should equal R2.
10006013
10006020
FIGURE 10. Single-Supply Inverting Amplifier
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