| Electronics > Beginners |
| Measuring analog signal voltage |
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| Rolypoly:
Short version: How would I record an analog signal voltage to my microcontroller if the two circuits have a different ground voltage? Long version: I have a device (a viscometer) that outputs on 3 pins, a ground and 2 analog signal voltages relative to the ground. I want to use this to automate the recording of the output from the device onto a computer. Running the signal voltage into the A/D converter on my arduino got the wrong readings, I realised this was because the two ground voltages are different, even when the arduino is running off battery. I tried an optocoupler but it turns out that it only outputs digitally and I lose my data. I've tried looking for some sort of circuit that can compensate for the different ground voltages but I'm not hitting the right keywords or something and have come up blank. Any ideas? TIA |
| jahonen:
Are you trying to measure the voltage between those two analog outputs? Maybe an instrumentation amplifier (something like AD620, AD8251 or perhaps AD623 etc.) is the device you'll need between your MCU ADC and the viscometer? That works if your ground voltage difference is inside instrumentation amplifier supply voltage, i.e. for example if the IA has supply voltage of ±15V, then the inputs can be at any voltage inside that range. Regards, Janne |
| Rolypoly:
Thanks for the response Janne, I want to be able to record the voltage between the devices ground and signal outputs (so I would have two bits of information, a comparison of each output to ground). I am unable to get the arduino to record both and just subtract the difference because the device ground is about -1V compared to arduino ground the A/D reads 0 (it only works from 0-5V relative to arduino ground). Essentially I want to offset the difference. This could be what I'm looking for :) I haven't used these before so they're a bit of a mysterious black box to me. This from the datasheet look promising: The reference terminal potential defines the zero output voltage and is especially useful when the load does not share a precise ground with the rest of the system. It provides a direct means of injecting a precise offset to the output. Just to be sure though, if I set this to a gain of 1 (by not connecting a resistor between +Rg and -Rg) and made the REF my arduino ground, -Vs = arduino ground, +Vs = arduino +5V, -IN = device ground, +IN = device signal then the voltage between OUTPUT and REF would be the same as the voltage between -IN and +IN? |
| GeekGirl:
Is there any reason you can not tie both grounds together ? In Engineering the big principle is KISS (Keep It Simple Stupid), over worked solutions can bite you. |
| Rolypoly:
I tried that, wondering if it would blow something up as I did it. I connected both ground outputs together but that didn't seem to effect the readings. |
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