Current Sense Amplifier for Differential ADC (low-side, remote sensor)

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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.

TI Filter Pro - 100Hz 2nd order filter with a gain of 25 and real-world components.
TI Filter Pro – 100Hz 2nd order filter with a gain of 25 and real-world components.
 

The two input voltages are shifted up by 1.25V, gained by 25, and filtered to 100Hz. The current shunt is .25mOhm, so 200A corresponds to 50mA; 25 times that is 1.25V, the full scale voltage the ADC will accept.
The two input voltages are shifted up by 1.25V, gained by 25, and filtered to 100Hz. The current shunt is .25mOhm, so 200A corresponds to 50mA; 25 times that is 1.25V, the full scale voltage the ADC will accept.
 

This is the differential voltage as seen by the ADC.  V+ minus V-.
This is the differential voltage as seen by the ADC. V+ minus V-.
 

We need to make sure both V+ and V- stay within the allowable voltage range for the ADC.
We need to make sure both V+ and V- stay within the allowable voltage range for the ADC.