THS4531A_15 Datasheet, PDF (24/59 Pages) Texas Instruments – THS4531A Ultra Low-Power, Rail-to-Rail Output, Fully Differential Amplifier

THS4531A
SLOS823B – DECEMBER 2012 – REVISED JUNE 2015
8 Detailed Description
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8.1 Overview
As shown in the Functional Block Diagram, the THS4531A device is comprised of three functional blocks: a fully-
differential amplifier with high open-loop gain of 114 dB, a servo amplifier to set the common-mode voltage of the
output equal to the V_OCM input, and a power-down circuit to greatly reduce the power consumption when the
device is idle.
The common-mode voltage servo has impressive performance specifications of ±1% maximum gain error, ±5-mV
maximum voltage offset, and 24-MHz bandwidth.
8.2 Functional Block Diagram
V_S+
V_IN±
V_IN+
±
High-A_ol +
Differential I/O
Amplifier ±
+
6 k
6 k
V_OUT+
V_OUT±
PD
CMOS
Buffer
V_S+
625 k
±
V_CM
Error
Amplifier
+
V_OCM
625 k
V_S±
8.3 Feature Description
8.3.1 Input Common-Mode Voltage Range
The input common-model voltage of a fully differential op amp is the voltage at the positive and negative (+
and –) input pins of the op amp.
Do not violate the input common-mode voltage range (V_ICR) of the op amp. Assuming the op amp is in linear
operation, the voltage across the input pins is only a few millivolts at most. Therefore finding the voltage at one
input pin determines the input common-mode voltage of the op amp.
Use Equation 1 to calculate the voltage with the negative input as a summing node.
æ
ç ^V_OUT+
è
´
R_G
R_G + R_F
ö
÷
ø
+
æ
ç ^V_IN-
è
´
R_F
R_G + R_F
ö
÷
ø
(1)
To determine the V_ICR of the op amp, the voltage at the negative input is evaluated at the extremes of V_OUT+.
As the gain of the op amp increases, the input common-mode voltage becomes closer and closer to the input
common-mode voltage of the source.
24
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