Vin MAX for Op-amp

Discussion in 'General Electronics Chat' started by JBmtk, Feb 3, 2011.

  1. JBmtk

    Thread Starter Member

    Jun 29, 2009
    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.
  2. kubeek


    Sep 20, 2005
    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?
  3. Ron H

    AAC Fanatic!

    Apr 14, 2005
    You will need precision resistors if you use conventional op amps. Google "high voltage difference amplifier" for some amplifiers that have the matched resistors built in.
  4. Adjuster

    Late Member

    Dec 26, 2010
    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."&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.