# high voltage buffer

Discussion in 'General Electronics Chat' started by kevin.cheung19, May 10, 2012.

1. ### kevin.cheung19 Thread Starter New Member

Nov 22, 2011
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0
I'm trying to use a diff amp to measure the real time differential voltage of two location (high voltage). The voltage can get up to 270V. So I was using AD629 as a diff Amp as it can do differential voltage well up to 270V. But I found out that the input impedance is lower than what I wanted (200K common mode and 800K for diff mode). Because at high voltage it will draw up to 1mA of current which will affects our measuring result by a good amount, so I'm looking at somewhere 5-10Gohm input impedance for better result.
I don't think I can just put a resistor in the inputs because I think it will change the input voltage to the diff amp. So I'm thinking about a buffer that can handle that voltage. But I couldn't find one, any suggestion? Or if you can suggest me another way to increase the input impedance of this diff amp, that'll be great too.

Thanks a lot. Any help will be appreciated.

Kevin

2. ### #12 Expert

Nov 30, 2010
18,076
9,680
I think you are looking for an instrumentation amplifier. They have high impedance buffers on each input and THEN the signal is presented to the differential amp.

3. ### crutschow Expert

Mar 14, 2008
21,757
6,271
Exactly what voltage are you trying to measure? Is it the difference between two 270V sources or just the value of the voltage to ground?

Depending upon the voltage difference you are trying to measure perhaps an isolation amp such as one of these may work. The have essentially an infinite common-mode voltage impedance to ground.

4. ### kevin.cheung19 Thread Starter New Member

Nov 22, 2011
24
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The two locations are two low impedance sources that generally having 5v or less differential voltages -A series of cells that produce power.

5. ### crutschow Expert

Mar 14, 2008
21,757
6,271
If you want to measure the voltage difference between those two sources then check the isolation amps I referenced.

6. ### kevin.cheung19 Thread Starter New Member

Nov 22, 2011
24
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But is there such thing as high voltage buffer/follower?

7. ### kevin.cheung19 Thread Starter New Member

Nov 22, 2011
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At unity gain, they have about 2.5% gain error max, which is not feasible in my case. If my inputs are around 200v, the output will have about 5v max of error, which is a lot.

8. ### Lundwall_Paul Active Member

Oct 18, 2011
226
20
Is this 270 volt supply by chance an aircraft supply?

9. ### kevin.cheung19 Thread Starter New Member

Nov 22, 2011
24
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No. Why? Does this make a difference?