Here my first update:

Got the analog part up and running. Unfortunaltely I had to do some "bugfixes" & optimizations... Nevertheless, tests have proven that it supports up to 100 kHz test frequencies.

The low level stuff of the software is also done:
- Communication with PGA
- UART debug output
- LC-Display control, including "printf"
- signal generation and synchronous sampling
- phase angle and amplitude measurement
- automatic setting of PGA according to the input signal amplitude

Measurement results

The readings are raw and uncensored data, i.e. without any compensation of imperfections of the ADC, DAC, PGA and imbalance of current measurement, which will be added in a next step.

The imbalance and non perfections of the current measurement lead to great errors in the phase and amplitude measurement when the current signal is small. The root source is, that both inputs of the subtractor will never be 100% balanced. But the good news is, that this error can be compensated in the digital domain.

When viewing the results, also consider, that a 10K resistor does not have a value of 10K. I double checked for example the 10K resistor which measures as 9950 Ohm with a multimeter.

Interpretation of the display content

Line 1: phase of voltage signal
Line 2: amplitude of voltage signal
Line 3: phase of current signal
Line 4: amplitude of current signal
Line 5: phase between voltage and current signal
Line 6: resistance in Ohm or capacitance in pF

Here some other measurement results:

Next steps

- Realize different measurement frequencies (currently fixed to 10kHz)
- Get more precision
- autodetection of equivalent circuit (in this version only R L or C only are supported)
- update schematic according to new changes
- make a fancy menu system, nicer font, ...