7.3.3 Using the Output Difference Amplifier to Shape Noise

Section 7.2 shows that the PGA849 output-stage difference amplifier uses a 5kΩ feedback resistor between the

output and the inverting input. External direct access to the inverting and noninverting inputs of the difference

amplifier is provided through the DA_IN– and DA_IN+ pins, respectively. This option allows circuit designers

to add external capacitors in parallel with the internal resistors to implement noise-filtering or noise-shaping

techniques. These pins are also used to implement customized attenuating gains for the output stage. Consider

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

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

mode rejection (CMRR) of 80dB 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.

7.4 Device Functional Modes

The PGA849 has a single functional mode and operates when the input-stage power supply is greater than ±4V

(8V) and the output-stage power supply is greater than ±2.25V (4.5V); see also Section 6.3.

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PGA849

SBOSAG3 – MARCH 2024

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