Q
I use AD5933 to measure an impedance Z1,at the same time I connect a capacitor
between Pin Vout and Pin AGND for special purpose,and the value of capacitor is
changeable. Will AD5933 still work correctly to measure Z1?
Will the accuracy become worse?
And I want to know:
1.The maximum value of the capacitor that the Pin Vout can drive at
100kHZ,2Vp-p, VDD=3.3v.
2.The AC output impedance of Pin Vout.
3.The relationship between excitation voltage and AC/DC output impedance of Pin
Vout.
A
The AD5933 works by capturing the admittance signal current developed between
vin and vout (ac virtual earth WRT Vin.) the gain factor is essentially a
mapping of a known code to a know peak value at the adc input ( this is the
admittance signal). If the calibration gain factor is determined and the
dependence on the variation in capacitive reactance then the gain factor may or
may not be way out??? I have measured a grounded impedance in the past using
high side current sensing and reworked the match to get a new gain factor that
accounted for the sense resistor.Is this application measuring a grounded
impedance ? Is the impedance purely cap or res+cap?
1).The maxim value of the capacitor that the Pin Vout can drive at
100kHZ,2Vp-p, VDD=3.3v:
We did not characterise pure capacitors on vout-vin just resistors but the
device can drive up to 10M ohms in impedance. Again the impedance was not
grounded in this case!!!!
2).The AC output impedance of Pin Vout:
The dc output impedance was characterised. The DC output impedance was
calculated by sinking and sourcing current to the output of the DDS. A 30kHz
sinewave was generated on the output of the DDS using the external oscillator
configuration. A Keithley sourcemeter was used to sink and source current from
the output. The amount of current being drawn by the meter was incremented in
steps from -2mA to +2mA while monitoring the change in output voltage. The
output impedance was calculated by dividing the change in output voltage by the
change in current in this linear region. That aside we then characterised the
impedance for each of the parts across the different wafers and we plotted the
impedance.
