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Issues about power supply of op-amps OP07CP

Discussion in 'Electronic Projects Design/Ideas/Reviews' started by Minh Thanh, Aug 14, 2017.

  1. Minh Thanh

    Minh Thanh New Member

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    I am measuring EC of solutions. I supply dual voltage of +/- 9v to op-amps . However, when I check power pin of op-amps , I just read power supply is equal to 17.5v. That makes unstable values in ADC converter. May you help me about this issues ,please ? Tks u very much.
     

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  2. ronsimpson

    ronsimpson Well-Known Member Most Helpful Member

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    I think this picture is true. +9V and -8.5V.
    What is the load? How much current for -8.5V? What is load for U7?

    upload_2017-8-14_19-51-34.png
    Why is ADC unstable? There are many things you have not told us.
    What is the voltage range for U7-pin6?
    What does "unstable ADC" look like?
     
  3. Minh Thanh

    Minh Thanh New Member

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    This is my entire circuit. Range of voltage of Vo is 0v to 5v ,you can adjust by potentiometer to get required Vo. It's not -15v in first Vin in zero and span circuit , it is -9v. About unstable ADC, for example : 700, 618, 623 , 720,725, 680 , 590......
     

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  4. dave

    Dave New Member

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  5. ronsimpson

    ronsimpson Well-Known Member Most Helpful Member

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    There are several places where noise is getting into your circuit.
    Maybe your -9v is not clean. Any noise will go though the POT to the opamp. Try using a 9V battery to power the POT. Or add a "LPF circuit" in between the POT and -9V. (-9V 10k, 10uf to ground, 500k POT to opamp).

    Please look at Vout. (set scope to AC and turn to smallest voltage) I want to see the noise. What frequency? I want to know if it is 10khz, noise from -9V, or high frequency.

    No time now, maybe in 5 hours I can come back. There is another source of noise.
     
  6. ci139

    ci139 Active Member

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  7. ronsimpson

    ronsimpson Well-Known Member Most Helpful Member

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    I think you are using a ICL7660 to get the -9V. It is not a good clean supply.
    Here I added a RC low pass filter to the POT to remove the 10khz noise that is on the power supply.
    upload_2017-8-15_18-8-8.png
    It is possible that 10khz noise on the -9V can get into the op-amp. Probably not likely. Can be reduced by adding a RC filter.

    The LF351 (and its ground) might not be connected well to the micro computer. Add a capacitor from ADC-input to the analog ground at the micro. Add a resistor to make a RC low pass filter. This filter will average the noise out.

    AND: Add a 0.1uF cap from +9V to ground and -8.5V to ground on each op-amp.
     

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    Last edited: Aug 15, 2017
  8. Minh Thanh

    Minh Thanh New Member

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    Thank you for your sharing very much . Now , I have another problem and I need your help is that Absolute Circuit 's Frequence of Vout is similar to Frequence of Vin . It is 5khz. Meanwhile, correct result is 10Khz. I got Vout of 10Khz in absolute cỉcuit before . However, It was not correct 2 weeks later. I checked my circuit a lot , yet I didn't see what problem is .Could you help me about this problem ?
     

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  9. ronsimpson

    ronsimpson Well-Known Member Most Helpful Member

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    Look for bad solder connection for:
    Diode + 1k resistor
    AND
    Diode + 1k resistor (other diode)
     
  10. ci139

    ci139 Active Member

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  11. schmitt trigger

    schmitt trigger Active Member

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    A trick I use when I suspect that the power supply is creating the interference:
    -> Temporarily use batteries. And since your circuit requires +/- 9 volt, a pair of common batteries will suffice.
     

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