I have been trying to recreate the 4-electrode analog front end for the 5933-EVAL board presented in the 2008 paper titled “An analog front-end enables electrical impedance spectroscopy system on-chip for biomedical applications” by Seoane et al. (http://iopscience.iop.org/article/10.1088/0967-3334/29/6/S23/meta )
My circuit is shown in the image attached (Circuit.PNG) and has the following components an RRFB of 1 kΩ, R1 of 7.5 kΩ, R2 of 1 MΩ and RG of the INA118 of 5.1 kΩ.
VDD = 3.3V
Rcal of 1 kOhm
RRFB of 1 kOhm
Gain of x1
Vpp of 2 V
Voltage at ADC with Z of 1K Ohm = 3.09 Vpp
Voltage at ADC with Z of 50 Ohms = 0.144 Vpp
At low frequencies my results are very accurate. For example, at 5kHz a 51 Ω resistor has a measured impedance of 50.9 Ω . However, as my frequency increases the error increases; at 100kHz the AD5933 measures the impedance as 57 Ohms. In the graphs below, the impedance, reactance and resistance of a series 2R-1C circuit, with Rs = 68 Ω , Rp = 130 Ω and C = 100 nF. The results show increasing divergence from the theoretical values as the frequency increases. I measured two points per frequency (with a frequency step of 2 Hz) and recalibrated the AD5933 with the 1kOhm resistor before every measurement at each given frequency, so I don't think it's a calibration issue.
Can anyone comment on why there is this increasing error with frequency? And also how this error may be minimised?