DFT window simulator

How the AD5940 turns a sampled current into one impedance point, and how eis-testbed picks the DFT size, window and frequency for each point.

How to use this page: work through the six steps in order. Read the explanation here, then follow Try this beside the plot. Press Do it, or use the control outlined in orange yourself, and compare the plot with what the step says you should see. Every control stays live, so you can wander off and come back.

The measurement

Everything the firmware decides before the DFT runs. Defaults match EisSensorDriver.

Requested frequency
What you'd type in eis measure <hz> or a sweep's point list. The firmware may move it; see snapping below.

DFT set-up

Auto-DFTFirmware picks filters and N per frequency using AD5940_GetFreqParameters. On by default.
DFT size N (dft-num)
DFT source (dft-src)
SINC3 decimation (sinc3-osr)
SINC2 decimation (sinc2-osr)
High-speed ADC (hs-adc)1.6 MHz ADC clock instead of 800 kHz. On by default.
Hann window (han-window)Fades samples in and out at the window edges. Off by default.
Frequency snappingMoves the frequency to a whole number of cycles per window. On by default; seq … qc0 turns it off.
Minimum cycles (k-min)
Snapping never goes below k-min whole cycles in the window. Default 4.
Waveform generator word
The generator can only make frequencies on a 16 MHz ÷ 2bits grid. The firmware reads the chip ID to choose.

What else reaches the ADC

Levels are relative to the excitation signal's amplitude at the ADC.

DC offset
2nd harmonic (cell non-linearity)
Mains pickup
Noise (rms per sample)

What the firmware does at this frequency

Reproduce on the testbed console


        
      

    

  


  

    
Choosing settings, in short

    
  


  
The DFT maths is exact for the tones shown (closed-form windowed sums, the same single-frequency correlation the AD5940 DFT block performs). Filter responses, the 50/60 Hz notch, RTIA gain and ADC quantisation aren't modelled, and interference levels are illustrative. The test cell is an ideal Randles circuit, not real electrode data.