ADS1220: Differential input impedance

Hi Seishin,

The ADS1114 and the ADS1220 have much different input architectures.  Also, the ADS1220 has a true PGA that can also be disabled and bypassed.  Impedance implies some kind of dynamic affect that can change over various operating conditions.  In other words, the impedance can change and is not linear like a true resistance. 

In the case of the ADS1114 the input is a switched cap.  The charging/discharging of these input caps create a small average current.  The cap values change depending on the FSR.

For the ADS1220 it was decided that the input current could be better described graphically showing the affects of the change of impedance over temperature and input voltage.  Note Figures 19 and 20 in the Typical Characteristics plots for when the PGA is enabled.  Here the information is much clearer as to how the differential input current changes over operating conditions.  Figures 25 and 26 show PGA disabled and bypassed.  The impedance will be quite high for the differential case as the current is shown to be quite low unless the device is operating at high temperatures.

There are also Absolute Input Current graphs.  In all cases the devices within the ADS1220 begin to be leaky at higher temperatures.  AIN3 is the worst case as this input has the low-side switch.

Best regards,

Bob B