voltage level conversion

Discussion in 'The Projects Forum' started by GRC, Apr 20, 2009.

  1. GRC

    Thread Starter Member

    Apr 20, 2009
    21
    0
    I'm working on a project that uses a PLL as a discrimator to show on a meter when the signal is tuned to a certain frequency. The demod out of the PLL varies from around 2 to up to 3 volts, when it is at 2.5 volts the input signal is correct. What I an trying to do is figure out an opamp cct that will drive a 0 -1 ma meter movement(I don't want to use a -1 -0 +1 centre zero meter) As the voltage starts going up from 2 volts the meter will decrease until it is 0 at 2.5 volts then the meter will start going up again when the voltage increases towards 3 volts.
    Any help greatly appreciatted.
    Thanks
    Geoff
     
  2. Darren Holdstock

    Active Member

    Feb 10, 2009
    262
    11
    Should be pretty simple - feed the PLL output voltage into a full-wave precision rectifier (2 op-amps, 2 diodes, some resistors), but instead of the op-amp input references going to 0V, connect them to a 2.5 V reference. Take the ammeter (+ series resistor so it reads voltage) return to the +2.5 V too.

    There are a number of precision rectifier (AKA absolute value) circuits out there and they all have their pros and cons, but if you don't need very high precision or speed and you're not working with just a few mV then any one of them should do, as long as there's a means to change the input reference to +2.5 V. Figure 4 from ESP will do nicely.
     
  3. GRC

    Thread Starter Member

    Apr 20, 2009
    21
    0
    Tried it out works beautiful. Thanks for the help.

    Geoff
     
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