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.](http://blog.saleae.com/wp-content/uploads/2013/03/TI-Filter-Pro-700x441.png)](http://blog.saleae.com/wp-content/uploads/2013/03/TI-Filter-Pro.png)

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.](http://blog.saleae.com/wp-content/uploads/2013/03/Remote-Low-Side-Current-Amplifier-for-Differential-ADC-700x433.png)](http://blog.saleae.com/wp-content/uploads/2013/03/Remote-Low-Side-Current-Amplifier-for-Differential-ADC.png)

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-.](http://blog.saleae.com/wp-content/uploads/2013/03/Differential-Voltage-at-ADC-700x411.png)](http://blog.saleae.com/wp-content/uploads/2013/03/Differential-Voltage-at-ADC.png)

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.](http://blog.saleae.com/wp-content/uploads/2013/03/Common-mode-voltages-at-ADC-inputs-700x354.png)](http://blog.saleae.com/wp-content/uploads/2013/03/Common-mode-voltages-at-ADC-inputs.png)

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