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Amplifiers
A.3.12 High Common-Mode Range Differential Amplifier
When all resistors are equal, Equation A–15 reduces to Equation A–16.
V OUT
V IN2
R 2 R F
R 1
R G1
V REF1
R 1 R 2 R F
R 1
V IN1
R F
R G1
R G2
V REF2
R F R G1 R G2
V OUT
VIN2 V IN1
2VREF1 3V REF2
R G1
R G2
A–15
A–16
R 1 and R G2 are equal-value resistors terminated into a virtual ground, hence, the input
sources are equally terminated. This configuration has high common-mode capability because
R 1 and R G2 limit the current that can flow into or out of the op amp. Thus, the input
voltage can rise to any value that does not exceed the op amp’s drive capability. The voltage
references, V REF1 and V REF2 , are added for bias purposes. Without bias, the output
voltage of the op amps would saturate at ground, and the bias voltages keep the
output voltage of the op amp positive.
+ 5 V
R 1
R 2
V IN2
V REF1
+V CC
_
+
R G1
R G2
V IN1
R F
+V CC
_
+
V OUT
V OUT — 0.5 V/Div
V REF2
0 V
– 2.5 V
V IN2 — 0.5 V/Div
+ 2.5 V
V IN1 = 1 V
V IN2 = 5 Vp-p
V REF1 = 1.25 V V REF2 = 1.67 V
V CC = 5 V Op Amp = TLV247x
R 1 = R 2 = R F = R G1 = R G2 = 10 k
Figure A–16. High Common-Mode Range Differential Amplifier
A-16