Hey all,

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 R_{RFB} of 1 kΩ, R1 of 7.5 kΩ, R2 of 1 MΩ and R_{G} of the INA118 of 5.1 kΩ.

System Settings:

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?

Thanks,

Piers

Hi,

Could you check how it looks the in-amp output at high frequency?

In general 100nF could be a large capacitance to be driven by the inamp.

If this is teh case, I'd recommend to contact the amplifier team in EZ so, you can get some suggestions to replace the inamp.

Regards,

Miguel