The input span of the CS5516/CS5520 AIN input signal is determined by a combination of
the instrumentation amplifier gain (X25), the programmable amplifier gain setting (1,2,4 or 8),
and the magnitude of the voltage between the VREF+ and VREF- pins. The voltage into the
VREF pins can range from 2.0 to 3.8 volts and is determined by the resistor divider ratio
selection of the resistor network which divides down the bridge excitation voltage. The
CDB5516 and CDB5520 evaluation boards come with resistors which divide the excitation
supply (nominally 10 volts total) down to 2.5 volts between the VREF+ and VREF- input pins.
This 2.5 volt reference input is divided by the PGA gain setting and by the X25
instrumentation amplifier gain to determine the nominal full scale input span to the converter.
For example, if the PGA gain is set for a gain of 8, then the input span to the instrumentation
amplifier will be 2.5 volts (VREF+ - VREF-) divided by 8 X 25, or 2.5/(200) = 12.5 mV
nominal in unipolar mode, or ± 12.5 mV nominal in bipolar mode.