Vin MAX for Op-amp

Thread Starter

JBmtk

Joined Jun 29, 2009
23
I have two signals (~70V in reference to GND) that I want to take the difference of and get a DC signal out. Is it possible to feed them into a differential amplifier circuit and get out a DC value (assuming rails of op-amp are greater than the difference) ? The typical DC value that I am already seeing is around 120mV, although it is noisy and I have had some trouble in finding a good filtering circuit, or rather an LC chock circuit that can effectively eliminate the noise well enough.

I have found that an RC circuit does a good job because I have the parts available here, but it loads down to different values depending on the scope or DMM used.
 

kubeek

Joined Sep 20, 2005
5,794
First you need resistive dividers to get both signals down to a level that the opamp can use. Then use a standard instrumentation amplifier and filtering. What is the frequency of the noise?
 

Adjuster

Joined Dec 26, 2010
2,148
If you require a relatively wide bandwidth it may be better to use a differential amplifier whose ground reference voltage is close to the potential to be measured. The amplified output is then transferred to ground potential. A Google of "Isolation Amplifier" should show you some devices made for this purpose.

http://www.google.fr/search?hl=fr&c...Amplifier"&btnG=Rechercher&aq=f&aqi=&aql=&oq=

These devices may be more expensive and less simple to use than standard instrumentation amplifiers: the best choice will depend on the detail of your application.
 
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