Weird Value : Real Value of Nyquist is Negative

Hi,

    Is there a particular reason why you have chosen RTIA =40kOhms?

With below config:

Output for 100kOhms load:

Magnitude:                                                                         Phase:                                                                           Nyquist plot:

Waveform at CE0:

Hi Akila, I tried to use your config, with 100k resistor load.

And got a measurement result for several frequencies, as shown below

Looks like the result is same with yours at freq 80kHz, but at freq 15kHz and 20kHz show that it can reach up to 200k Ohm for several samples. Could you tell me why does it happen?

The reason why I chosen RTIA = 40k in the previous test is I got the best SNR at that setup in my application. I also collected data for RTIA=40k (all the setup is same with yours, but the RTIA only I chosen 40k), and the following fig. is the comparison with yours as well.

What do you think about the data? The white noise looks like better. But you could see that the real value is getting lower after I increase the freq = 80kHz.

Hi,

    RTIA value ensures that ADC input is within allowed limits and hence ADC doesn't get saturated.  

    Impedance magnitude is expected to get lower with higher frequency due to the stray capacitance in measurement path.

Hello Akila, I also collected data use your SensorPal setup, but I change the DFT value into 1024. And obtained the following result.

In the fig, seems like the real value is getting worse for freq 2kHz after I changed the DFT number into 1024.

Is it possible the negative real value issue coming from the DFT number? Could you explain this phenomenon?

Hi,

AD5940's DFT block follows the below equation:

where,

x(i) = discrete input data from ADC,

N = DFT number,

R = Real part of result

I = Imaginary part of result

Fs = ADC Data sampling rate (ADCRATE_800KHZ or ADCRATE_1P6MHZ)

f = excitation frequency

Hi Akila,

just random question about the switch matrix, how long does each switch matrix last?

Switches in matrix stay in current set state until you change their set from on to off or vice versa.