Common mode voltage higher than power supply voltage

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Nuno
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Posted by Nuno: Tue. Nov 4, 2008 - 03:39 AM
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I want to measure a differential voltage in which the common mode voltage is higher than my circuit's supply voltage.

The measuring circuit and the circuit being measured share a common ground.

I'm designing a multi-channel volt- and amp-meter, this is the reason. I want to have a low power supply voltage.
Voltage measuring poses no problems. The tricky part is current measurement; although the (differential) voltages are quite small, the common mode voltage may not be.

I played a bit with AMPOPs but they seem to need a supply voltage higher than the common mode one... I need some ideas.

Thanks

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emuler
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Posted by emuler: Tue. Nov 4, 2008 - 04:45 AM
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The most common trick is to use voltage dividers to bring the voltages down within the allowed range. You can also use precision zener diodes (two terminal voltage references) to drop the voltage without the attenuation inherent in voltage dividers.

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nleahcim
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Posted by nleahcim: Tue. Nov 4, 2008 - 10:35 AM
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There are some over the top input parts - but typically they're, well, not particularly good. You could also boost your supply voltage, or give yourself a floating supply that floats at your inputs.

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Nuno
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Posted by Nuno: Tue. Nov 4, 2008 - 07:03 PM
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I already have the divider but for voltage measurement only. I don't want to use it on the current measurement because it will obviously remove lot's of resolution; the differential voltage I'm interested in can be as little as 0.5% of the common mode voltage.

Zener diodes... that could be a solution. They would have to be very tightly coupled because of temperature variations. If both zeners suffer the same amount of change, that won't influence the differential measurement. But... the current flowing will be very little to nothing, how will the Zener behave? No current means no voltage drop... I'll have to check that.

What do you mean with a "floating supply" at the inputs? If I need only 1 channel (no different than a DMM), that would mean connecting the measurer circuit's GND as one of the probes. But I need multi-channel, including voltage measurements (all channels can be used simultaneously, and I'll be having a few for voltage and a few for current), so I'll have to use a common ground (or at least that's how I see it). Yes, I could use separate power supplies for each channel, using batteries, but that's an ugly solution...

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Kleinstein
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Posted by Kleinstein: Tue. Nov 4, 2008 - 07:55 PM
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Some of the OP-amps with FET input (e.g. LF356 alow inputs of a few 100 mV above supply. Some CMOS allow about the same below the negative supply. This could be just enough for the shunt voltage.

There are also special amplifiers to work with a shunt at the supply. They are calls high side currend sensors/amplifiers.

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ka7ehk
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Posted by ka7ehk: Tue. Nov 4, 2008 - 08:00 PM
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Linear Tech has at least one "over the top" op-amp. It is pretty nice and should do what you are looking for. Sorry, no number in front of me.

Jim

Jim Wagner Oregon Research Electronics, Consulting Div. Tangent, OR, USA http://www.orelectronics.net

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Thomas Strand
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Posted by Thomas Strand: Tue. Nov 4, 2008 - 08:19 PM
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Analog Devices has some current sense amplifiers that can handle common-mode voltages up to 270V.

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Nuno
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Posted by Nuno: Wed. Nov 5, 2008 - 06:37 PM
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Ok, thanks for the hints. I found these 2:

Linear LT1990
Analog Devices AD22057

Yep, these are "specialty" chips so it gets quite expensive for me to get them (must get them abroad). However, they show their internal diagrams and it looks like the only thing they do differently is to have resistor dividers at the inputs, so I'm going to study them in order to replicate the behavior with more off-the-shelf chips.

Thanks!

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emuler
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Posted by emuler: Wed. Nov 5, 2008 - 06:58 PM
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Or you can try getting free samples. ;)

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Nuno
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Posted by Nuno: Sun. Nov 9, 2008 - 04:29 AM
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That's a way :)

Anyways, I ended up giving up on using such method. The input impedance is too low for the application; high common mode voltages with low common mode input impedance (500K on the chips above) leads to input currents 2 or 3 times bigger than the values I want to measure.
While "struggling" with this issue I found a small and free source for a 32V power supply: my old deskjet printer I just replaced and was going to trow into the recycle center.

Many thanks to all of you!

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