I'm actually asking 2 questions - one about sine wave generation and another about high rate / precision voltage measurement.

I'm working on a project where we have to generate a sine wave on top of a DC bias - say, 3.2V with a +/- .1V amplitude sine wave - for the purposes of EIS. Because we will be measuring low impedance devices, a small voltage difference can result in a large current, so just generating the signal is not enough, it has to be able to deliver fairly high current (~4 amps max) . Right now, we are doing low-side N-type FET control with a PWM sine wave generated by an arduino or a C2000 TI board, with a low-pass filter on the high side of the circuit. The results are alright, but the low-side FET control causes a fair amount of noise. Would this be improved by physically hiding the FET behind the filter and doing high-side control?

So far, we are generating a sine wave that reduces in resolution as the frequency increases, but it should be able to generate between 1 Hz and 50 kHz. The sampling rate has quickly become the bottleneck, as measurements taken with the 10-bit adc on the arduino are next to useless, so we are using an external ADC, which is limited to about 400hz sampling rate. Are there any recommendations for a faster, less noisy ADC than the ADS1115?

Is this a decent approach to the sine wave generation, or would using another form of generation and then maybe using an op-amp give better results? Our THD right now is around 10% with a .1 ohm load at low frequency, and we would like to get that as close to 0 as possible. Any advice would be greatly appreciated.