7.3.3 Output Common-Mode Pin

The output voltages of the PGA854 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 PGA854. 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, and low voltage output supply,

set this voltage between a maximum of VLVDD – 1.5V and minimum of VLVSS + 1.5V. for higher voltage supplies,

set VOCM between VLVDD – 2V and minimum of VLVSS + 2V. Note that 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

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

between the output-stage power-supply pins.

7.3.4 Using the Fully Differential Output Amplifier to Shape Noise

Section 7.2 shows that the PGA854 output-stage fully-differential amplifier uses 6.67kΩ feedback resistors

between the OUT+ output and the inverting input, and the OUT– output and the noninverting input. External

direct access to inverting input is provided through the FDA_IN+ pin, and to the noninverting input through

the FDA_IN- pin. 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 are also usable

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 ratio (CMRR) of 80dB or better. Mismatched leakage currents on these pins potentially causes

CMRR degradation.

• The internal resistors have ±15% absolute resistance variation; consider this variation 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 potentially cause permanent damage to internal circuitry.

7.4 Device Functional Modes

The PGA854 has a single functional mode and operates when the input-stage power supply is greater than

±4.5V (9V) and the output-stage power supply is greater than ±2.25V (4.5V); see also Section 6.3.

PGA854

SBOSAN2A – AUGUST 2025 – REVISED DECEMBER 2025

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