4.3 Single-Supply Inverting Amplifier
Application Notes (Continued)
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 resis-
tor 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.
4.2.1 Three-Op-Amp Instrumentation Amplifier
The quad LMV324 can be used to build a three-op-amp
instrumentation amplifier as shown in Figure 8.
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.
10006085
FIGURE 8. Three-op-amp Instrumentation Amplifier
The first stage of this instrumentation amplifier is a
differential-input, differential-output amplifier, with two volt-
age followers. These two voltage followers assure that the
input impedance is over 100 MΩ. The gain of this instrumen-
tation 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.
10006013
10006020
FIGURE 10. Single-Supply Inverting Amplifier
4.4 ACTIVE FILTER
4.2.2 Two-op-amp Instrumentation Amplifier
4.4.1 Simple Low-Pass Active Filter
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 to R1 and R3 should equal R2.
The simple low-pass filter is shown in Figure 11. Its low-
→
frequency gain (ω
0) is defined by -R3/R1. This allows
low-frequency gains other than unity to be obtained. The
filter has a -20dB/decade roll-off after its corner frequency fc.
R2 should be chosen equal to the parallel combination of R1
and R3 to minimize errors due to bias current. The frequency
response of the filter is shown in Figure 12.
10006011
10006035
FIGURE 9. Two-Op-amp Instrumentation Amplifier
15
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