ADS1018 / input capacitance

Nagai-san,

Thank you for your query.

The ADS1018 uses a switched capacitor input stage as shown in "Figure 9. Simplified Analog Input Circuit"  of the "Analog Inputs" section in the datasheet.

 The input sampling capacitance is a function of the full-scale range (FSR) and the value of the capacitance can be calculated from the Differential Input Impedance provided in the "Electrical Characteristics" table.

Giving you an example, at FSR =  ±0.256 V, the differential input impedance is 710 kΩ. Since the modulator frequency is 250 kHz, and the differential input impedance is given by 1/(Csampling x modulator frequency), the input sampling capacitor (Csampling) can be calculated to be 5.63 pF. If we add some additional parasitics from bond pad and the input path, 10 pF is a reasonable estimate at FSR =  ±0.256 V. Similarly, we can also calculate the input sampling capacitance for other FSRs.

Let me know if you have any further questions.

Thanks,

Krunal