We are using AD5933 Evaluation board to measure impedance .
When using Output excitation @ 0.4 Vpp and 2 Vpp we are finding the result matching to impedance measured using different source .
But when we use Output excitation @ 0.2Vpp and 2 Vpp we find the result to be totally different.
Detail about setting:
-We are using external clock @ 250kHz as output frequency is 100 Hz
-when changing output every time we are using calibration( Resistor only R1 )
Please let me know if there are there any setting need to be taken in consideration .
With best regards and wishes
Is this the version of the Evaluation board you are using: https://www.analog.com/media/en/technical-documentation/user-guides/UG-364.pdf ?
As to the settings: what is your gain and number of settling cycles (cannot be 0)? When measuring at gain=1x and excitation 2V p-p both calibration resistor and the resistors you are measuring should be of greater value than the RFB. One more consideration is the value of C7 - did you increase it as you operate at low frequencies?
Yes we are using the evaluation board that you have specified .
Setting is as follows :
Start frequency : 100 Hz , Delta frequency 0.1 Hz , Number of Increments : 10 , Number of settling time cycles : 15 , MCLK _REf Clock frequency 250000, External Clock selected , PGA control :x1 , Calibration impedance :Resistor only R1 , DDS settling time cycle X1 , Calibration Gain factor is Multi - point .
Could you please let me know if we have to change C7 and as fc is less than 100Hz .
Could you please let me know when i can find more details with regards to external circuit .
If I understood you correctly regarding the external circuit, all the info is in the link in my previous message. Everything should work with the settings you indicated. Can you once again try to do the same measurements only at Vexc = 2V p-p and Rcal = RFB and measurement resistors of value larger than RFB and see if those are measured correctly?As to the C7, together with R7 and R8 it turns into a high-pass filter with cut-off frequency of 1/(2 Pi R C7), where R is R7||R8 = 25k, which is about 135 Hz, so you are operating below the cut-off. Should not matter much as you sweep a rather narrow frequency range, but I would increase C7 to, say, above 0.1uF.