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.