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Old 23rd October 2006, 08:04 PM   #1
xiaonan is offline xiaonan  China
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Default About Op-Amp input range

If I use an Op-Amp which power supply is +/-18V, can I apply a voltage Vin1=50V and Vin2=51V to its two input? (You can see the attachment.) My meaning is that the differencial input voltage is in the range, though the input voltage will go out of range.

Thanks!
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Old 23rd October 2006, 08:24 PM   #2
AndrewT is offline AndrewT  Scotland
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Hi,
the data sheet will tell you the common mode limits.

They may call them maximum input voltage +ve & -ve and are likely to be about the rail voltage, maybe a little below (+18-2V=+16V) but sometimes a little above (+18+1V =+19V).
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Old 23rd October 2006, 08:29 PM   #3
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Typical opamp would in best case change polarity of inputs, and at worst case burn. Some opamps can work with voltages exceeding supplies, but I doubt if any can do it in 50V range.

In short: No, don't do it.
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Old 23rd October 2006, 08:55 PM   #4
xiaonan is offline xiaonan  China
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Yes, I think so.

But my problem is I want to apply two different voltage to two pins of my load (you can see the upper schematic of the attachment), and at same time test the current. For example, one pin is applied to 100V and the other is applied to 50V. The nether schematic of the attachment is my former solution. I think it can not work.

Is there some other method to do that? I think Angilent 4256 can do that, but I don't know the details about that. Is there anyone who know that or other solutions?

Thanks a lot!
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Old 23rd October 2006, 10:54 PM   #5
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I think you are confusing op-amps and instrumentation amps.

In general, by itself, an opamp cannot support input (or output!) voltages outside its supply rails.

An instrumentation amplifier ( one or more opamps with resistors) can easily do this and cope with a common mode voltage over 100v.

Or have I missed the point?
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Old 23rd October 2006, 11:56 PM   #6
xiaonan is offline xiaonan  China
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Thank you to answer me.

Yes, I want to know how the instrument amp do that. And then set up the circuit to test current. Do you know how did they do that?
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Old 24th October 2006, 12:38 AM   #7
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An instrumentation amplifier may be made up from several opamps and some very high prrecision resistors (matched to 0.001%, for example.)

These can be found conveniently in single package, with the resistors laser trimmed.

See here

http://www.allaboutcircuits.com/vol_3/chpt_8/10.html

for example

Essentially an IA can sense the voltage across a resistor (your current sense resistor) in the presence of a high (+/-100V?) common mode voltage.

Read the link then ask more questions
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Old 24th October 2006, 01:15 AM   #8
xiaonan is offline xiaonan  China
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Thank you.

So should every amps in the instrumentation amplifier have +100 common mode voltage range?
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Old 24th October 2006, 07:49 AM   #9
AndrewT is offline AndrewT  Scotland
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read the datasheet
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