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ADuCM350 FAQ DFT / Impedance Measurement

Q: What is the frequency range ?

A: 80Hz is minimum. DFT requires at least 1 full period for accurate measurements. DFT completes the full 2048 sampling in 12.8mS.

There is antialiasing filters on receive stage with corner frequencies at 50KHz but these can vary down to ~30KHz. Its is

a gradual 1st order attenuation so some degradation in accuracy might start at ~30KHz. Maximum frequency is 79KHz.

Q: What is impedance range ?

A: Sub Ohm to Mohms. Advice reviewing AN-1271 on how to optimise ADuCM350 for Impedance Conversions.

A limitation on of +/-5mA max source/sink current exisits on the TIA. Ensure that the RTIA is scaled to ensure a +/-750mV on ADC range.

For MOhm measurements - leakage currents will have possible effects here and the Eval-ADuCM350EBZ board is not optimised for this range. Suggest using active shielding (at VBIAS) to limit leakage on current carrying signals (T node and N Node).

Q: Details on DFT Spectra ?

A: 2048 fixed sample point single frequency DFT using a Hann function. So no spectra data available. Signal power at a single frequency bin corresponding to the excitation frequency.

Q: How do I setup the part for Impedance Measurements ?

A: Use 2-Wire Impedance Measurement example in Software Development Kit along with applications board.Users guides available for both Hardware and Software.

Q: When to use 4-Wire, when to use 2-Wire and when to use the isolated 4-Wire solution with an IN-AMP ?

A: See Configurations section of AN-1302.

Q: Can I use an external amplifier if I need more current ?

A: System limitation is +/-5mA. For larger currents you may look at using an external drive and recive amplifier with the TIA buffer used a a voltage follower - short the IVS switch.