Precision AC to DC conversion for voltage measurement.

Discussion in 'General Electronics Chat' started by gmcmurry, Mar 29, 2010.

  1. gmcmurry

    Thread Starter Member

    Mar 29, 2010
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    I need to measure a low level (0 - 8 vac) voltage with a A to D converter. I have tried offsetting the AC with an op amp then rectifying that AC as I would in a power supply. Trouble is that the ripple is greater than I can tolerate.

    Does anyone know of a clean way to create a smooth dc voltage, lets day 0 to +10 vdc with very low ripple that represents an AC signal that is approximately 0 to 10 vac?

    Thank You
     
  2. beenthere

    Retired Moderator

    Apr 20, 2004
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  3. gmcmurry

    Thread Starter Member

    Mar 29, 2010
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    This looks like just what I need... I will give it a try.

    THANKS
     
  4. beenthere

    Retired Moderator

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    You're most welcome.
     
  5. gmcmurry

    Thread Starter Member

    Mar 29, 2010
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    The LM101a is obsolete. Do you have any idea if I can just use a couple of 741's...

    Maybe I can even use a 1458 (2 741s in a package with no compensation pins)(also I have many of them) since I will be working at low frequencies 20-60hz.

    What do you think?
     
  6. beenthere

    Retired Moderator

    Apr 20, 2004
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    Unless there is some reason to stay with obsolete designs, you might think of using an OPA2134 dual op amp. Much better in every respect than a 1458 or a pair of 741's. They will work, but both are getting to be 40 years out of date.

    Even the very inexpensive TL072 is way better than a 1458.
     
  7. gmcmurry

    Thread Starter Member

    Mar 29, 2010
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    Here is the application that was suggested by "beenthere" earlier. It looks exactly like what I need. I don't want to use the LM101 as in this schematic... You can't get them and this is part of a larger item that will be around for a while.

    [​IMG]

    Most replacement options do not anything on pin 8 so I cannot use C3, otherwise there are many options.

    If I use one of the dual options, I will not be able to use C1 either.

    I want to try this schematic but I don't really have any suitable parts on hand. I have LM1458 and LM358 on hand. If I am going to the trouble to go get something new, is there something that more accurately replaces the LM101 pin for pin?

    I have been looking but don't see anything.

    Greg
     
  8. beenthere

    Retired Moderator

    Apr 20, 2004
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    Oh, dear - those LM101's are like 709's with external compensating caps. Those went out about 40 years ago.

    If you want to use that 1458 dual, just hook up the second section in place of A2. Use pin 5 for the non-inverting input, 6 for the inverting input, and 7 for the output. The other section has the output from pin 1. Use Google to get a data sheet to follow the different pinout.

    Sorry - I've been using the circuit for so long I forgot about the unavailable op amp specified.
     
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