Hi

i have made an input ectension circuit on the ad7746 evaluation board.

i have calculated the extension circuit values for an input capacitance of 146pF

based on that my resistor values are R1=100.04k R2=125.7k

CDACEFF=1.24

Dac set=146/1.24

dac set(146pF)=0x76

F gain_factor=8.79

C_dynamic range=+/- 36.03

i have few confusion

Does the dynamic range means i can connect any capacitance within the range of (146+/-36.03)pF

Now i have connected 155pF to the input with

setting capdac A as 0x7D

enabling capacitive channel

exc setup as 1B

the data registers in continuous conversion mode read as=7A4572

capacitance is shown as(-0.183319)

how can i calculate the real capacitance based on these values?

thanks

Hello Adnan,

The capacitance shown in the evaluation software is based off the normal operating range of the device and cannot take into account your gain factor. You have to make the calculations yourself to account for your gain.

As per the calculations in your original post:

https://ez.analog.com/message/156352#156352

Capacitance = [(DecimalResult-DecimalMidScale) * TotalDynamicRange-in-pF/2^24] + CAPDACVALUE-in-pF

Capacitance = [(8013170-8388608) * 72.06pF/2^24] + 155pF = 153.387pF

This result is almost the same as the value calculated in your earlier post.

Regards,

Sean