ADS7841-Q1: Recommended circuit design

>>>1. What is absolute maximum ratings of Vref?

VREF being an analog pin can tolerate -0.3V to (VCC + 0.3V )(per the Absolute Maximum Ratings).

>>>2. If Vref is larger than Vcc, how Vin (differential, full-scale) range is?

Vref larger than Vcc is invalid and out of spec condition.  Please limit Vref to less than or equal to Vcc.

>>>3. Regarding to CI (typ:25pF), do you have MIN/MAX data?

I checked and unfortunately we don't have information on this.

>>>4. Is it capable of adding input capacitance?  How much input capacitance can be possible?

>>>5. Do you have equivalent circuit of input ?  Is there any resistance?

Please refer to TI Precision Labs - ADC link below. Building a simplified SAR ADC model and input driver is discussed in Section 5

I have enclosed here a simplified TINA model for ADS7841 made on the steps explained in Precision Labs material. Also, enclosed here documentation for this simplified model.

As you can see in the documentation, based on the acquisition time (3clk cycles min @ 3.2MHz = 937.5ns) supported by the device, we use Rsw = 374ohm in the model. Also, note external input driver uses a 560pF cap.  FYI, we have bench tested the input driver (external R=100ohm, C=560pF) with ADS7841.

6330.ADS7841_Simplified_Model.pptx

1462.ADS7841_Settling.TSC

I am still looking into the last question.

Thanks,

Vishy

Hello,

>>>6.When the input channel is switched, is the last charge in sampling capacitor discharged before next sample?
No. In general, there is some residual charge left on the sampling capacitor and this can cause charge injection out to the AIN pin. The external filter capacitor helps reduce the error due to charge injection during sampling and acts as charge-storage reservoir. Please refer document below (pages 18 and 19) for a discussion on this
www.ti.com/.../slyp166.pdf

Thanks,
Vishy