Quick update: last weekend I was trying to design up a current sense amplifier design for our boat’s new electrical system, which needs to sense from -40A to 200A.
16-Bit Delta-Sigma ADCs
After the post last weekend I started looking at 16-bit delta-sigma ADCs, and decided that was a great way to get the absurdly-accurate mA-level fidelity I was after. (that’s assuming noise doesn’t kill it). Those devices generally have differential inputs, so I was thinking about this a little bit and finished up a new amplifier design today (Sunday).
Since ultimately we need to measure the difference between the two input voltages, a differential ADC is actually perfect, and saves us needing to compute the difference with an op-amp. Our main job is to make sure the common-mode voltages (i.e. each voltage with respect to ground) do not exceed the range allowed for the ADC. In this case, anything inside the ADC’s rail will be fine. I selected the LTC2472 from Linear because it looks like it’ll get us the 100Hz of bandwidth we want.
TI Filter Pro – Highly Recommended
I also noticed that we we’re under-utilizing the op-amps we did need — why not use a 2nd order filter and get some better filtering (specifically -40dB/decade rather than -20dB/decade)? Big shout out to TI’s awesome Filter Pro software! This program is absolutely awesome for active filter design — both because the program is super clean and simple, plus it helps you to select optimal real-world resistor and capacitor values. Active filters are great, but as you may know the op-amp bandwidth required is always many times the filter cutoff frequency. This means you’ll need to use some pretty fancy op-amps if you want have pass-bands that are high frequency (in many cases passive filters are the only choice because of this). TI Filter Pro reports that we need an OpAmp with a minimum gain-bandwidth-product of 185 kHz. Note that we’re applying a gain of x25 so that our maximum 50mV current shunt voltages scales to 1.25V, the maximum differential input voltage.