8.3.3 Output Common-Mode Pin

The output voltages of the PGA855 are balanced with respect to the voltage on the output common-mode pin,

VOCM. The starting point for most designs is to assign an output common-mode voltage for the PGA855. For

ac-coupled signal paths, this voltage is often the default mid-supply voltage, so as to retain the most available

output swing around the voltage centered at VOCM. For dc-coupled signal paths, set this voltage between a

maximum of VLVDD – 1.5 V and minimum of VLVSS + 1.5 V. For precision ADC applications, this voltage is

typically the input common-mode voltage of the ADC.

The voltage at the VOCM pin is internally buffered to bias the fully differential output amplifier, eliminating the

need for an external VOCM buffer. In the event that the VOCM pin is left floating, the output common-mode

voltage is biased at output mid-supply using an internal 500-kΩ / 500-kΩ resistor divider network connected

between the output-stage power-supply pins.

8.3.4 Using the Fully Differential Output Amplifier to Shape Noise

Section 8.2 shows that the PGA855 output-stage fully-differential amplifier uses 5-kΩ feedback resistors

between the OUT+ and OUT– outputs and the inverting and noninverting inputs, respectively. External direct

access to the inverting and noninverting inputs of the fully differential amplifier is provided through the FDA_IN–

and FDA_IN+ pins, respectively. This option allows circuit designers to add external feedback capacitors in

parallel with the internal feedback resistors to implement noise-filtering or noise-shaping techniques. These

pins can also be used to implement customized attenuating gains for the output stage. Consider the following

important factors when designing parallel circuits with the internal feedback resistors:

• The accuracy of the internal resistor network is 0.01 % or better. This accuracy results in a common-mode

rejection (CMRR) of 80 dB or better. Mismatched leakage currents on these pins can cause CMRR

degradation.

• The internal resistors have ±15% absolute resistance variation and must be considered when implementing

custom attenuating gains or noise filters.

CAUTION

Do not treat these pins as outputs, nor use the pins to source or sink current. Excessive currents

through the feedback resistors can cause permanent damage to internal circuitry.

8.4 Device Functional Modes

The PGA855 has a single functional mode and operates when the input-stage power supply is greater than ±4 V

(8 V) and the output-stage power supply is greater than ±2.25 V (4.5 V); see also Section 7.3.

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PGA855

SBOSAE0B – APRIL 2023 – REVISED SEPTEMBER 2023

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