The VGA is easily the most important part of our analog front-end as it both imposes numerous constraints on the system, some of which have already been laid out, while simultaneously greatly increasing the resolution of the system.
After consulting with Dr. Richard he suggested we look into the AD8331 Ultralow Noise VGA. Functionally, it takes in a single ended signal, sends it through a fixed gain pre-amplifier (19dB), uses a variable attenuator (48dB) to reduce that signal, and finally sends it through a post-amp (21dB) that has differential outputs. This allows us to take in a wide range of signals from the probe input circuit and scale them so that they fill the entire range of the ADC, giving us the best possible resolution for our configuration.The value of the internal attenuator is set using Vgain, an external analog voltage reference. Vgain can be 0.04-1 V before it begins forcefully saturating the outputs. Since we will need to control all parts of the system from the FPGA we implemented a 6-bit resistive summing DAC, shown below. The set size of the DAC was approximately 15 mV, shown below, making it ideal for setting Vgain to roughly 40 mV.