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How to use a single supply amplifier?

Discussion in 'Electronic Design' started by [email protected], Aug 17, 2007.

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  1. Guest

    Hello NG
    I'd like to amplify the signal of a photodiode to read it with a ad-
    converter.

    Signal photodiode: 0-50mV
    Voltage range adc: 0-5V

    I tried to use two types of amplyfiers, AD623 and INA126u. But in the
    tests some very strange clipping happened. (See below.) I think
    there's something basic about single supply amplifiers which I
    understand wrong.

    Could you please tell me where the mistake is with my wiring or make a
    better proposal. If possible I want to use one of these amplifiers
    since they are lying right on my desk...

    Thank you

    Test1 with AD623
    Pin1 -RG is connected to Pin8 +RG by a 1kOhm resistor (Gain = 100)
    Pin2 -IN is connected to GND
    Pin4 -VS is connected to GND
    Pin5 REF is connected to GND
    Pin7 +VS in connected to +5V or +10V (The result doesn't depend on
    this. - Capacitors to GND are applied...)

    Test results:
    Pin3 +IN Pin6 OUT
    0mV 0V
    10mV 1V
    20mV 1.2V
    30mV 1.3V
    40mV 1.5V
    50mV 1.7V

    Test2 with INA127u
    Pin1 RG is NC (Gain = 5 for testing purpous)
    Pin2 V-IN is connected to GND
    Pin4 V- is connected to GND
    Pin5 REF is connected to GND
    Pin7 V+ in connected to +5V
    Pin8 RG is NC

    Test results:
    Pin3 V+IN Pin6 VO
    0V 3.6V
    1V 3.6V
    2V 3.6V
    3V 3.6V
    3.6V 3.6V
    3.7V 4.1V
    3.8V 4.6V
    3.9V 4.6V
     
  2. Tom Bruhns

    Tom Bruhns Guest

    For the INA126, you're trying to operate it outside its input common
    mode range, and until that's fixed, all bets are off.

    For the AD623, look at Figure 21 of the data sheet. At even 1V out,
    you're outside the range suggested by that figure.

    I don't see why you don't just use a common rail-to-rail input/output
    op amp to do the job. You can configure for a non-inverting gain
    easily. I'm a bit surprised you are using the photodiode in a voltage
    mode; it's common to run them into a low impedance current-to-voltage
    converter for good linearity. It should be easy to find op amp ap
    noted about that application.

    Cheers,
    Tom
     
  3. Guest

    Hi Tom

    The input common mode range was exactly my problem, for both
    amplifiers.

    Thank you so much for your answer!

    have a good day,
    Gabriel
     
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