Inverting amplifier to measure shunt voltage?

Question

I've built a motorhome-type vehicle, and wish to perform coulomb counting for the battery bank. I've placed a 100A/75mV shunt between the battery bank negative, and the negative bus (which is also connected to vehicle chassis to reduce wiring cost for alternator charging etc.)

^{simulate this circuit – Schematic created using CircuitLab}

How can I most accurately measure the mV across the shunt? The value will typically be 1-10 mV, up to 100 mV peak, and will be either positive or negative, depending on the direction of the current.

I understand that I may need an inverting amplifier, since the microcontroller will be grounded at the load level, e.g. unable to measure a negative voltage. Or is it sufficient just to use offsetting?

I'm new to amplifiers, and worried that I'll spend countless hours trying to get the wrong components to work, particularly having difficulties accurately reading the 1-5mV level.

Typical usage, as measured across the shunt with a multimeter: - Charging at 20-30A = (negative) 15-22mV - Constant loads, ventilation etc., 1-2A = 0.75-1.5mV - Peak charging from alternator 50-80A = (negative) 38-60mV

The multimeter reads the values quite well/stable.

Answer 1

I'd suggest this as prerequisite reading for you. There are several article parts (see end of page two) that cover most of the fundamentals.

TI make excellent High and Low side Current monitors INA138 (you need two of these to do bi-directional, see Figure 15 in datasheet), the automotive INA169 (again you need two) and INA210.

Configuring amps for low side detection can be more challenging, so I'd recommend using High side measurements (it makes no difference to your system).

If you are committed to using Low side detection them you can use the LM3900 Norton opamps to obviate having to use negative supplies. Again here, you'd have to have separate LM3900's for charge discharge sensing (though only a single sense shunt resistor).

I'd recommend use of the INA169, it's really easy to use and there is a small PCB version available from Sparkfun for $9.

Answer 2

You can get op-amps that are dc accurate down to a handful of micro volts so there's no problem there but, given that the current can be negative as well as positive I'd create a small negative supply to power the op-amp symmetrically about the ground point. Then I'd go for something like an OPA333 as the op-amp. If you need more bandwidth then something like an ADA4528 is also good.

If you are looking for budget parts - you get what you pay for.

Basically run the op-amp as a non-inverting amplifier and add offset to centralize the signal produced into the range of your microprocessor's ADC.

You might even consider a low offset-voltage Instrumentation Amplifier like the AD8222 fed from a symmetrical supply - it has a pin you can set for the offset called "REF".

You can get very cheap DC-to DC converters (very small too) that can be wired to take a positive supply voltage and produce a stable negative output voltage.

Answer 3

There's a product category called "current sense amplifiers" designed for just this scenario, where voltage across a shunt resistor must be read outside the supply rails of the amplifier.

Most of these parts are designed only to work with input voltages at or above the positive rail, but a few allow measurement below the negative rail. For example, INA193 can measure differential voltages offset from -16 to +80 V:

You can also look at the datasheets of these parts to see the circuit topology that allows measuring outside the rails, and reproduce it with discretes, although accuracy might suffer from not having components as well matched as you might get on-chip.