Author Topic: op-amp causing voltage divider output to change  (Read 11612 times)

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Offline electronics manTopic starter

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op-amp causing voltage divider output to change
« on: August 23, 2015, 05:10:59 pm »
i have built the circuit in the diagram. the voltage divider is divider (R12 and R13)is divide by 20 so when i put 20V into it i expect 1V out and i do get this value however when the op-amp is installed (its a TL072) the voltage from the divider increases to 1.5V with a 20v input an 1.7V at 30V. why is this happening and how can i fix it?
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Online fcb

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Re: op-amp causing voltage divider output to change
« Reply #1 on: August 23, 2015, 05:20:03 pm »
Without seeing the whole diagram it's going to be a guess.

1. Sounds like something is clamping your signal.
2. What value are your resistors?
3. Is your opamp supply referenced to the divider?
4. Check your wiring.
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Offline codeboy2k

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Re: op-amp causing voltage divider output to change
« Reply #2 on: August 23, 2015, 05:40:03 pm »
Please post a complete circuit, not a fragment.

I see the voltage divider center (the "output" ) is also going off to the left somewhere... where does that go, and how does it influence the voltage divider?  If that goes off to any node that has current coming from the supply rail then it will cause the voltage to go up (since it's injecting current into the divider's output node).

Any current being removed from the node will cause the voltage to go down, not up, so I am expecting that something in your circuit is injecting current into that point.

Definitely we need resistance values and it's best if we can see a complete circuit (at least more than you show there !)
 

Online Zero999

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Re: op-amp causing voltage divider output to change
« Reply #3 on: August 23, 2015, 05:42:10 pm »
What voltage is the negative rail of the op-amp connected to? If it's 0V then this is exactly what you should expect to happen.

The TL072 is not a single supply op-amp. Look at the P-channel differential pair at the input stage of the TL072 - see page 13 of the datasheet. If the input voltage is too near the negative rail, the gate will become forward biased.
http://www.ti.com/lit/ds/symlink/tl074.pdf

 

Offline electronics manTopic starter

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Re: op-amp causing voltage divider output to change
« Reply #4 on: August 23, 2015, 05:49:02 pm »
the diodes to the left of the divider are not populated on the board i am working on. the op amps have a +5v and -5v supply and the dividers referenced to 0V however i am not concerned whit the opamps atm its what is on the output of voltage divider before the opamps
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Offline electronics manTopic starter

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Re: op-amp causing voltage divider output to change
« Reply #5 on: August 23, 2015, 05:49:42 pm »
all diodes are unpopulated
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Offline electronics manTopic starter

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Re: op-amp causing voltage divider output to change
« Reply #6 on: August 23, 2015, 05:50:54 pm »
the resistor values are R12 = 820K and R13 is 47k
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Online Zero999

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Re: op-amp causing voltage divider output to change
« Reply #7 on: August 23, 2015, 05:54:40 pm »
That schematic is far too small to read.
 

Offline electronics manTopic starter

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Re: op-amp causing voltage divider output to change
« Reply #8 on: August 23, 2015, 05:58:25 pm »
yeah i know i don't know how to make it any bigger for you
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Offline electronics manTopic starter

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Re: op-amp causing voltage divider output to change
« Reply #9 on: August 23, 2015, 05:59:41 pm »
this may be better
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Online Zero999

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Re: op-amp causing voltage divider output to change
« Reply #10 on: August 23, 2015, 06:08:39 pm »
What CAD software did you use to draw the schematic?

The op-amp power supplies aren't shown. I know you said -5V and +5V but have you checked this? Connecting something which should go to -V, to 0V is easilly done.
 

Offline PSR B1257

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Re: op-amp causing voltage divider output to change
« Reply #11 on: August 23, 2015, 06:18:21 pm »
Quote
The op-amp power supplies aren't shown.
They are. Right down in the center  ;)
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Online Zero999

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Re: op-amp causing voltage divider output to change
« Reply #12 on: August 23, 2015, 06:48:39 pm »
All right, the PSU rails are shown but it's not very clear.

The component values are missing and what's IC2?

What is the circuit supposed to do?
 

Offline electronics manTopic starter

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Re: op-amp causing voltage divider output to change
« Reply #13 on: August 23, 2015, 07:01:42 pm »
All right, the PSU rails are shown but it's not very clear.

The component values are missing and what's IC2?

What is the circuit supposed to do?
i gave you the values of the resistors that are required, Dip trace doesn't put the values in the schematic, they are shown when a component is selected. I'm not here to talk about my schematic skills i just want to sir the is problem out. IC 2 is an ad8210 current sense amp.
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Offline electronics manTopic starter

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Re: op-amp causing voltage divider output to change
« Reply #14 on: August 23, 2015, 07:02:54 pm »
What CAD software did you use to draw the schematic?

The op-amp power supplies aren't shown. I know you said -5V and +5V but have you checked this? Connecting something which should go to -V, to 0V is easilly done.
power rails are fine but that is not what matters here
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Offline free_electron

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Re: op-amp causing voltage divider output to change
« Reply #15 on: August 23, 2015, 07:04:08 pm »
common mode range of that stinker is vcc -4v to vcc- + 4 v ...

i never liked the TL7xx series.
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Offline electronics manTopic starter

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Re: op-amp causing voltage divider output to change
« Reply #16 on: August 23, 2015, 07:06:15 pm »
its the tl072
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Offline electronics manTopic starter

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Re: op-amp causing voltage divider output to change
« Reply #17 on: August 23, 2015, 07:08:28 pm »
just to be clear the opamps work fine. they are buffering, offsetting and amplifying just fine
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Offline PSR B1257

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Re: op-amp causing voltage divider output to change
« Reply #18 on: August 23, 2015, 07:25:21 pm »
Have you tried a different opamp? Maybe your particular on is a little :-BROKE

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Offline electronics manTopic starter

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Re: op-amp causing voltage divider output to change
« Reply #19 on: August 23, 2015, 07:27:18 pm »
Have you tried a different opamp? Maybe your particular on is a little :-BROKE

no because it worked fine on breadboard ext and that would require me getting the pcbs remade
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Offline electronics manTopic starter

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Re: op-amp causing voltage divider output to change
« Reply #20 on: August 23, 2015, 07:30:29 pm »
it may be the resistors, i don't know why it would be though, its a real mystery why an opamp would affect a voltage divider like this
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Offline PSR B1257

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Re: op-amp causing voltage divider output to change
« Reply #21 on: August 23, 2015, 07:32:53 pm »
Quote
and that would require me getting the pcbs remade
No, I meant a different TL072.
Aside of that, a whole lot different op amps share the same pinout ;)
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Offline electronics manTopic starter

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Re: op-amp causing voltage divider output to change
« Reply #22 on: August 23, 2015, 07:34:25 pm »
the TL072 is 18V tolerant and i will have a look at what is available
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Offline PSR B1257

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Re: op-amp causing voltage divider output to change
« Reply #23 on: August 23, 2015, 07:46:07 pm »
Quote
its a real mystery why an opamp would affect a voltage divider like this
It shouldn't. And it will not, if the op amp is intact. Therefore the op amp is most likely defect. Have you checked its orientation btw?
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Offline electronics manTopic starter

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Re: op-amp causing voltage divider output to change
« Reply #24 on: August 23, 2015, 07:48:09 pm »
Quote
its a real mystery why an opamp would affect a voltage divider like this
It shouldn't. And it will not, if the op amp is intact. Therefore the op amp is most likely defect. Have you checked its orientation btw?
yes i have and the output of the opamp is fine i really don't understand
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