Q9. What’s the equivalent input impedance for a pair of voltage input pins of AD4111/AD4112/AD4114/AD4115?
Below is the detailed figure of the voltage input stage. Please take note that the 10MΩ is not relevant to the VIN 1/10X transfer function, and is already accounted for in the 1 MΩ minimum input impedance spec. The resistor die used the 10MΩ resistors for the Open-Wire-Detect function on AD4111. It is present on the AD4112 VINs, but the other end is internally connected to AVSS. Therefore, the differential input impedance of AD4111/AD4112 has an approximate nominal value of 2.34 MΩ and may vary from part to part. The differential resistance range is higher on AD4114/AD4115 because without the10MΩ, the nominal input impedance is 1.33MΩ, therefore the differential impedance is 2.66MΩ.
Figure 4. Simplified Voltage input circuit of AD4111/AD4112 (Note that COMPA and COMPB pins are DNC pins in AD4112 but nodes in the diagram are internally connected to AVSS)
Figure 5. Simplified Voltage input circuit of AD4114/AD4115