SBAS894A April 2018 – October 2018 ADS112C04
As with many amplifiers, the PGA has an absolute input voltage range requirement that cannot be exceeded. The maximum and minimum absolute input voltages are limited by the voltage swing capability of the PGA output. The specified minimum and maximum absolute input voltages (VAINP and VAINN) depend on the PGA gain, the maximum differential input voltage (VINMAX), and the tolerance of the analog power-supply voltages (AVDD and AVSS). Because gain on the ADS112C04 is implemented by both the PGA and a switched-capacitor gain circuit, there are two formulas that define the absolute input voltages. Use Equation 6 when the device gain is configured to less than or equal to 4. Use Equation 7 when the device gain is greater than 4. Use the maximum differential input voltage expected in the application for VINMAX.
Figure 44 graphically shows the relationship between the PGA input voltages to the PGA output voltages for gains larger than 4. The PGA output voltages (VOUTP, VOUTN) depend on the PGA gain and the differential input voltage magnitudes. For linear operation, the PGA output voltages must not exceed AVDD – 0.2 V or AVSS + 0.2 V. Figure 44 depicts an example of a positive differential input voltage that results in a positive differential output voltage.