THS4522IPWR datasheet(40/71 Pages) TI2

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S+

OCM

S–

275 k

EXT

To internal

circuit

OCM

EXT

V )

50k

OCM

S+

S-

OUT+

R +R

IN- ´

R +R

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 (VICR) 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:

(8)

To determine the VICR of the amplifier, the voltage at the negative input is evaluated at the extremes of VOUT+. 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 VOCM 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 VOCM input. The internal VOCM 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:

where:

•

VOCM is the voltage applied to the VOCM pin

(9)

Figure 78. VOCM Input Circuit

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