# Issues about power supply of op-amps OP07CP

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#### Minh Thanh

##### New Member
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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#### ronsimpson

##### Well-Known Member
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?

supply is equal to 17.5v. That makes unstable values in ADC converter.
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?

#### Minh Thanh

##### New Member
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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#### ronsimpson

##### Well-Known Member
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.

#### ronsimpson

##### Well-Known Member
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.

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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#### Minh Thanh

##### New Member
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.
View attachment 107553
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.
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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#### ronsimpson

##### Well-Known Member
I didn't see what problem is
Look for bad solder connection for:
Diode + 1k resistor
AND
Diode + 1k resistor (other diode)

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#### schmitt trigger

##### Well-Known Member
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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