Measuring voltages from different sources sharing Common ground that might differ slightly

Question

I have a design with resistor dividers and low pass RC filters dropping down 0-32V to ADC range of 0-1.8V. With multiple channels, I attempt to measure voltages from different voltage sources on a boat - battery voltage, voltage over the engine temperature sensor, voltage over the oil pressure sensor etc. All sources share common ground, but the problem on fibreglass boats is that all devices and grounded with a wire, not a chassis therefore it's easy to get ground level difference. Prime example is when someone flushes the toilet, the voltage measure on the engine temperature sensor drops slightly down, thus the calibrated temperature value goes up (the higher voltage on the sensor represents lower temperature) and leads to triggering false alarms. Here is my current solution:

^{simulate this circuit – Schematic created using CircuitLab}

How do I resolve the issue? I have been thinking of the following new implementation:

^{simulate this circuit}

where V1 is the ground connection of the measured voltage (0V) and V2 is the positive Connection of the measured voltage (0 to +32VDC) The Op Amps I have selected are NCV20074DTBR2G https://www.onsemi.com/pub/Collateral/NCS20071-D.PDF I'm using 6 ADC channels of STM32 Wroom32U https://www.espressif.com/sites/default/files/documentation/esp32-wroom-32d_esp32-wroom-32u_datasheet_en.pdf

The requirements I'm trying to meet are:

• Measure 0-32V DC voltage of 6 to 8 independent channels and display the value 10per second on a separate gauges (my SW already does the display portion).
• Resolution 0.05V
• Avoid ground loop measuring issues. Tolerate about +/- 0.5V ground differences.
• When wires on the input are reversed by mistake, do not cause short circuit (this is a potential problem with my current solution as it has ground connection on the one of the input connectors)
• Ideally have an offset on the output of the op amp to avoid the non linear portion of the ADC (downfall of using cheap uC). On the Op Amp the input should be 0-32VDC and the output should be 0.15V to 1.8V in order to compress the output and to use the almost linear portion of the ADC, otherwise my lower voltage measurement (below 5V) will be questionable and unfortunately this range is where the most sensor output voltages are.

I have some doubts of my new differential measurement solution as I'm not sure if using isolated PS for powering the ADC and the OpAmp and having floating ground with regards to the ground I'm measuring is or could be an issue.

• What happens if the wires on the input are reversed and I feed negative to the positive input of the Op Amp and vice a versa.
• How do I protect the input for EMI and other transient Voltages?
• do I add caps to ground on each input after the input resistors?

Thank you in advance for your time reading my post! Cheers!

Answer 1

If you have ground offsets, then use an instrumentation amplifier and do a differential measurement.

An instrumentation (or instrumentational) amplifier (sometimes shorthanded as In-Amp or InAmp) is a type of differential amplifier that has been outfitted with input buffer amplifiers, which eliminate the need for input impedance matching and thus make the amplifier particularly suitable for use in measurement and test equipment. Additional characteristics include very low DC offset, low drift, low noise, very high open-loop gain, very high common-mode rejection ratio, and very high input impedances. Instrumentation amplifiers are used where great accuracy and stability of the circuit both short and long-term are required. Ref: Wikipedia

Instrument op-amp takes differential voltage as input and you don't need to worry about your circuit (just make sure you stay within the rails), as it can be quite complex if you want to achieve accuracy. They have very good CMMR and are very easy to use (a single resistor set the amplification).

It's fairly simple as they come altogether in a chip like AD8220 and will be much more accurate.

Make sure the in-amp is able to drive the rest of your circuit (like an A/D) you may need to add an opamp as buffer.

Answer 2

Use differential ADCs

I had a similar problem with instrumenting a generator, which meant taking readings from various oil pressure, temperature and current gauges, whilst tolerating a large ground rise when the starter motor was running.

In my case the easiest solution was just to use differential ADCs, bringing back each signal as a twisted pair, and measuring the true delta to each "local" ground. The use of twisted pair and differential measurement also provides good protection against RFI pickup on low level signals.

This is similar to what is suggested in an earlier answer which talks about a quiet ground, but is effectively a quiet ground per sensor.

There are very nice, cheap, 18bit quad differential DACs such as the MCP3424 which are easy to interface to a range of systems over I2C. As long as the ADC is powered with a ground going directly to the most negative point in the systems, you know all the other negative references / "grounds" will be above zero, so compatible with the differential input.

In my case resistive dividers with clamp diodes were all that I needed to achieve scaling and good readings in a hostile environment.

The reason I selected the MCP3424 is that they are cheap, configurable to allow you to chose the best balance between conversion speed and resolution, and have programable gain amps for low level signals.

Finally, being accessed over I2C means you have the option of using buffered I2C, placing the ADCs near the sources, and connecting to them remotely from a central location.

Answer 3

Your cleanest implementation here would be to run a separate "quiet" ground for your sensors which doesn't carry significant current. Then you would use differential amplifiers with the negative coming from the quiet ground.

Since the quiet ground isn't carrying significant current, you won't get the I-R losses that you'll see with a common ground.

Answer 4

How about using a 3-resistor ladder at each point where you want to take a measurement, then you can measure the voltages on each side of the middle resistor and determine both the local supply voltage and the supply and return losses, which may be useful to know.