THS4521 Datasheet, PDF (40/65 Pages) Texas Instruments – VERY LOW POWER, NEGATIVE RAIL INPUT, RAIL-TO-RAIL OUTPUT, FULLY DIFFERENTIAL AMPLIFIER

THS4521, THS4522, THS4524
SBOS458H – DECEMBER 2008 – REVISED JUNE 2015
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8.5 Programming
8.5.1 Input Common-Mode Voltage Range
The input common-mode voltage of a fully-differential amplifier is the voltage at the + and – input pins of the
device.
It is important to not violate the input common-mode voltage range (V_ICR) of the amplifier. Assuming the amplifier
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 amplifier.
Treating the negative input as a summing node, the voltage is given by Equation 8:
R
V_OUT+ ´ _R
G
+R
G
F
+
R
V_IN- ´ _R
F
+R
G
F
(8)
To determine the V_ICR of the amplifier, the voltage at the negative input is evaluated at the extremes of V_OUT+. As
the gain of the amplifier increases, the input common-mode voltage becomes closer and closer to the input
common-mode voltage of the source.
8.5.1.1 Setting the Output Common-Mode Voltage
The output common-model voltage is set by the voltage at the V_OCM pin. The internal common-mode control
circuit maintains the output common-mode voltage within 5-mV offset (typ) from the set voltage. If left
unconnected, the common-mode set point is set to midsupply by internal circuitry, which may be overdriven from
an external source.
Figure 78 represents the V_OCM input. The internal V_OCM circuit has typically 23 MHz of –3 dB bandwidth, which is
required for best performance, but it is intended to be a dc bias input pin. A 0.22-μF bypass capacitor is
recommended on this pin to reduce noise. The external current required to overdrive the internal resistor divider
is given approximately by the formula in Equation 9:
I_EXT =
2V_OCM - (V_S+ - V_S-)
50 kW
where:
• V_OCM is the voltage applied to the V_OCM pin
(9)
V_S+
To internal
V_OCM circuit
275 kΩ
275 kΩ
I_EXT
V_OCM
V_S–
Figure 78. V_OCM Input Circuit
40
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