AD5934 measuring impedances at low frequencies

Document created by analog-archivist Employee on Feb 23, 2016
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I am having difficulty measuring capacitor impedances at low frequencies (200Hz)

My specifications are:
VDD = 3.3V
Capacitor 6.8nF
Rfeedback = 179K
Excitation frequency: 100Hz-200Hz


Assuming VDD = 3.3V,

Vp = 1.48+1 = 2.48V

ADC input voltage = Rfeedback/Runknown * Vp

Calibrating the  system,
ADC input voltage =  179k/178.8k * Vp = 2.48V

With the cap (6.8nF @ 200Hz = 117k)

ADC input voltage = 179k / 117k * Vp = 3.79V

VDD is less than 3.79V, which is where the problem stems from.