UA315 Design Story. Part 2 Ratiometric use of AD5933

Block schematics

Block schematics

How to use AD5933 in ratiometric way

The Analog Devices Inc. (ADI) chip AD5933 has only one input and one output. The block schematics shows 2 signals.  The inputs are multiplexed (switched with additional analog switch to be measured one at a time).

 

Integrated Impedance Meter AD5933 is not simply a voltmeter, however in this project it is used as a voltmeter. In datasheet and other reference schematics published by Analog Devices Inc.(ADI), AD5933's input is connected to form a transimpedance amplifier (TIA). Transimpedance amplifier is a common way to convert I to V. The desired effect of using TIA is to have a virtual ground point at the input, where voltage is always zero relative to the ground. Having the virtual ground helps to the assumption that voltage on top of DUT is equal to voltage applied between top of DUT and ground.

 

In my schematics, I use instrumentation amplifiers PGA281 which are insensitive to common mode voltages and do not need TIA trick. However AD5933 can only be wired as TIA. The conversion of V-to-I and then from I-to-V (in TIA) is achieved with 2 identical resistors Rfb (feedback resistor from ADI datasheet) and second R=Rfb at the AD5933 input.

 

The presence of analog switch in precision signal path brings a challenge, non-ideality of switches, non-linearity and thermals. The symmetric inclusion of switches helps to eliminate the effect of Rsw. When 2 Rsw are in the signal path, the TIA cancels the distortions. Complete symmetry is achieved when both Rws channels are on the same die.

Cancellation of analog switch distortion in Transimpedance Amplifier. (Oct 10 2015)

Cancellation of analog switch distortion in Transimpedance Amplifier. (Oct 10 2015)

 

 

 

AD5933 was designed to measure I over time and integrate it using Single Bin Fourier Transformation (fancy name for Integrator of the output of I-Q Quadrature Synchronous Detector). The integrated value has physical units of Siemens (Conductance and Susceptance), but ADI is expressing the measured values through some magic unitless scaling coefficients to derive Ohms arithmetically.

 

With our rewiring, the device becomes Voltage integrator. We can express measured units in our software/firmware in Volts. Voltages are proportional to resistance for I-channel and proportional to impedance for V-channel. As described in Part 1, the ratios of this voltages is the only information needed to do the ratiometric calculation.

 

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