Datasheet

LPV321, LPV324-N, LPV358-N
SNOS413D –AUGUST 2000–REVISED MARCH 2013 www.ti.com
Two-op-amp Instrumentation Amplifier
A two-op-amp instrumentation amplifier can also be used to make a high-input-impedance DC differential
amplifier (Figure 46). As in the three-op-amp circuit, this instrumentation amplifier requires precise resistor
matching for good CMRR. R
4
should equal to R
1
and R
3
should equal R
2
.
Figure 46. Two-op-amp Instrumentation Amplifier
(2)
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 R
3
and R
4
is implemented to bias the amplifier so the
input signal is within the input common-common voltage range of the amplifier. The capacitor C
1
is placed
between the inverting input and resistor R
1
to block the DC signal going into the AC signal source, V
IN
. The
values of R
1
and C
1
affect the cutoff frequency,
fc = 1/2π R
1
C
1
(3)
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.
Figure 47. Single-Supply Inverting Amplifier
(4)
Active Filter
Simple Low-Pass Active Filter
The simple low-pass filter is shown in Figure 48. Its low-frequency gain(ω → o) is defined by −R
3
/R
1
. This allows
low-frequency gains other than unity to be obtained. The filter has a −20 dB/decade roll-off after its corner
frequency fc. R
2
should be chosen equal to the parallel combination of R
1
and R
3
to minimize errors due to bais
current. The frequency response of the filter is shown in Figure 49
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