DC-DC converter for a instrumentation amp

Discussion in 'The Projects Forum' started by Mark_varney, Jan 30, 2015.

  1. Mark_varney

    Thread Starter New Member

    Dec 1, 2013
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    I have to design a circuit with a ad8221 Instrumentation amplifier which is supplied from +/- 15V.

    however the only supplies i have on the board are 5v and -5v. i have looked at DC-DC converters to boast the voltage up to +/- 15V, however i am aware that DC-DC converters can be noisey and might be to noisey for a IA.
    would any one be able to recommended a low noise DC-DC converter or better still a better idea of creating the +/- 15v from +/-5v.

    thanks

    Mark
     
  2. ronv

    AAC Fanatic!

    Nov 12, 2008
    3,292
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    Maybe a little LC low pass filter on the supply to knock down the switching frequency.
     
  3. crutschow

    Expert

    Mar 14, 2008
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    The AD8221 will work from ±5V supplies.
     
  4. Mark_varney

    Thread Starter New Member

    Dec 1, 2013
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    0
    Thanks for the replies, the signal going into the IA is from 0v to around 12v so i need the higher voltage. the set up is 0-12v varying voltage into the IA then into a an adc then fpga and also also from the IA to a fast comparator which will create a ttl pulse
     
  5. crutschow

    Expert

    Mar 14, 2008
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    So I see two choices:

    Use a DC-DC inverter with a careful layout using a ground plane and a lot of filtering to minimize the noise.

    Reduce the 12V input voltage with a resistive divider to the maximum input the IA can linearly handle.
    (Why do you need an IA amplifier for a 0-12V signal?)
     
  6. AnalogKid

    Distinguished Member

    Aug 1, 2013
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    How often is the A/D converter being triggered, and how long does a conversion take? If the duty cycle is low, like an a/d conversion time of 10 us and a sample rate of 10 samples per second, you can use the signal that starts a conversion to shut down the dc/dc converters and restart them when the conversion is complete. That way the DC/DC converters are not running when the signal is being sampled and converted. Makes for a super low noise power system when it counts.

    Old analog trick from old analog kid.

    ak
     
  7. crutschow

    Expert

    Mar 14, 2008
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    That is a good trick.
    A similar technique is to synchronize the A/D sample so it occurs just before the next dc/dc converter transition. But, of course, that requires some way to detect the dc/dc converter switching frequency.
     
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