I've read AN-1271 (Optimizing the ADuCM350 for Impedance Conversion) and from what I understand the following should be done:

- Rcal - Set equal to the lowest expected impedance being measured
- Rtia - Calculate to provide max voltage at minimum impedance
- Ctia - Calculate based on Rtia

My questions are:

- What happens if the device sees a value lower than the expected minimum impedance
- In the 2-Wire example I don't see a call to set the Rtia anywhere, why is this not used?
- Is there any limit how high an impedance can be measured? For instance, with an Rcal of 1k ohm would I have trouble measuring impedances on the order of 10s of k ohms?

Hi,

1. What happens if the device sees a value lower than the expected minimum impedance

Ans: If the device sees a lower impedance than the minimum expected impedance, the TIA will be overloaded. Note that the TIA maximum output is only 750mV.

2. In the 2-Wire example I don't see a call to set the Rtia anywhere, why is this not used?

Ans: Impedance measurement uses a ratio-metric approach in measuring impedance. That is each measurement of Unknown Z is compared to a known impedance (RCAL). Therefore RTIA is not needed to calculate Z-unknown.

3. Is there any limit how high an impedance can be measured? For instance, with an Rcal of 1k ohm would I have trouble measuring impedances on the order of 10s of k ohms?

Ans: Yes, there is a limit but you should be fine on measuring 10s of k ohms with RCAL=1K

-Mark