op-amp causing voltage divider output to change

[electronics man]

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?

[fcb]

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.

[codeboy2k]

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 !)

[Zero999]

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

[electronics man]

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

[electronics man]

all diodes are unpopulated

[electronics man]

the resistor values are R12 = 820K and R13 is 47k

[Zero999]

That schematic is far too small to read.

[electronics man]

yeah i know i don't know how to make it any bigger for you

[electronics man]

this may be better

[Zero999]

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.

[PSR B1257]

The op-amp power supplies aren't shown.

They are. Right down in the center 

[Zero999]

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?

[electronics man]

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.

[electronics man]

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

[free_electron]

common mode range of that stinker is vcc -4v to vcc- + 4 v ...

i never liked the TL7xx series.

[electronics man]

its the tl072

[electronics man]

just to be clear the opamps work fine. they are buffering, offsetting and amplifying just fine

[PSR B1257]

Have you tried a different opamp? Maybe your particular on is a little

[electronics man]

Have you tried a different opamp? Maybe your particular on is a little

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

[electronics man]

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

[PSR B1257]

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

[electronics man]

the TL072 is 18V tolerant and i will have a look at what is available

[PSR B1257]

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?

[electronics man]

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

[Kevin.D]

Poor gnd ? are you probing directly at the divider with both probes ?.

[electronics man]

Poor gnd ? are you probing directly at the divider with both probes ?.

yup right across the resistor

[Zero999]

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, the fact the voltage divider works without the op-amp in the circuit means the resistors are fine. The fact that the voltage rises when the op-amp is added suggests the fault lies with the op-amp.

At first I thought bias currents but the resistors values are far too low for that to be an issue with the TL072's tiny bias current.

This is why I think it's more likely to be a problem with the op-amp's power supply.,

Try swapping the op-amp for the LM358.

[electronics man]

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, the fact the voltage divider works without the op-amp in the circuit means the resistors are fine. The fact that the voltage rises when the op-amp is added suggests the fault lies with the op-amp.

At first I thought bias currents but the resistors values are far too low for that to be an issue with the TL072's tiny bias current.

This is why I think it's more likely to be a problem with the op-amp's power supply.,

Try swapping the op-amp for the LM358.

does the LM358 work with the split rails?

[Zero999]

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, the fact the voltage divider works without the op-amp in the circuit means the resistors are fine. The fact that the voltage rises when the op-amp is added suggests the fault lies with the op-amp.

At first I thought bias currents but the resistors values are far too low for that to be an issue with the TL072's tiny bias current.

This is why I think it's more likely to be a problem with the op-amp's power supply.,

Try swapping the op-amp for the LM358.

does the LM358 work with the split rails?

Why wouldn't it? Have you read the data sheet?

[DimitriP]

It's either a bad component or a construction/assembly error.
If the op amps are socketed have you replaced them with fresh ones already ?

Maybe you can post some pictures of the board area around the op-amps top and bottom....
 
I was intrigued enough to model it in LTSpice, here is the .asc file.
Took some guesses/made up some of the values.

The LT1057 is a plug in replacement for a TL072.
http://cds.linear.com/docs/en/datasheet/10578fd.pdf

Have fun
D


--------snip-o-matic-----------
Version 4
SHEET 1 1528 680
WIRE 144 -80 -144 -80
WIRE 528 -80 336 -80
WIRE 336 -16 336 -80
WIRE 400 -16 336 -16
WIRE -464 0 -464 -32
WIRE -336 0 -336 -32
WIRE 144 0 144 -80
WIRE 528 0 528 -80
WIRE 528 0 464 0
WIRE 736 0 528 0
WIRE 400 16 304 16
WIRE 432 80 432 32
WIRE -144 96 -144 -80
WIRE -464 112 -464 80
WIRE -336 112 -336 80
WIRE 144 160 144 80
WIRE 192 160 144 160
WIRE 304 160 304 16
WIRE 304 160 272 160
WIRE 144 224 144 160
WIRE 432 240 368 240
WIRE 544 240 512 240
WIRE 0 272 -16 272
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WIRE 544 336 448 336
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WIRE 144 416 144 304
WIRE 416 416 416 368
WIRE 448 416 416 416
WIRE 368 432 368 352
FLAG 432 -32 Vcc+
IOPIN 432 -32 BiDir
FLAG 416 304 Vcc+
IOPIN 416 304 BiDir
FLAG 432 80 Vcc-
FLAG 448 416 Vcc-
IOPIN 448 416 BiDir
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FLAG -336 -32 Vcc+
FLAG -464 112 0
FLAG -464 -32 Vcc-
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FLAG -144 176 0
FLAG 736 80 0
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WINDOW 3 32 56 VTop 2
SYMATTR InstName R3
SYMATTR Value 22K
SYMBOL res 160 320 R180
WINDOW 0 36 76 Left 2
WINDOW 3 36 40 Left 2
SYMATTR InstName R13
SYMATTR Value 47K
SYMBOL res 160 96 R180
WINDOW 0 36 76 Left 2
WINDOW 3 36 40 Left 2
SYMATTR InstName R2
SYMATTR Value 820K
SYMBOL voltage -144 80 R0
WINDOW 123 0 0 Left 2
WINDOW 39 0 0 Left 2
SYMATTR InstName V2
SYMATTR Value 20
SYMBOL voltage -464 -16 R0
WINDOW 123 0 0 Left 2
WINDOW 39 0 0 Left 2
SYMATTR InstName V3
SYMATTR Value -5
SYMBOL res 736 320 R0
SYMATTR InstName R9
SYMATTR Value 100K
SYMBOL res 352 416 R0
SYMATTR InstName R1
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SYMBOL res 720 -16 R0
SYMATTR InstName R4
SYMATTR Value 100K
SYMBOL voltage -336 -16 R0
WINDOW 123 0 0 Left 2
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SYMATTR InstName V1
SYMATTR Value 5
SYMBOL res 288 144 R90
WINDOW 0 0 56 VBottom 2
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SYMATTR InstName R5
SYMATTR Value 1
SYMBOL Opamps\\LT1057 432 -64 R0
SYMATTR InstName U3
SYMBOL Opamps\\LT1057 416 272 R0
SYMATTR InstName U4
TEXT 472 528 Left 2 !.tran 50m
TEXT 208 552 Top 1 ;This example schematic is supplied for informational/educational purposes only.
--------snip-o-matic-----------

[PChi]

Some other possibilities are:-
Oscillation, are the + and - 5V rails decoupled to GND? Can you check with an Oscillosope?
Have you tried checking with another meter? The impedances are relatively high and common mode mains hum pickup can cause read out errors.

[Zero999]

i am not concerned whit the opamps atm its what is on the output of voltage divider before the opamps

power rails are fine but that is not what matters here

If the  power supply connections to the op-amp are incorrect then it will affect what is connected to the input.

I've just done some tests on a breadboard and have confirmed that this is exactly what you should expect to happen if the input of the TL072 is taken too near the negative rail.

Note: although I used the TL082, it's exactly the same as the TL072, except noise specification isn't so good.

When -V = 0V then the gates of the op-amp's P channel J-FETs on the input stage will become forward biased, allowing current to flow into whatever is connected to the input.

[PSR B1257]

does the LM358 work with the split rails?

Evey opamp works with split rails, unless you excced its maximum V- to V+ voltage rating.

[electronics man]

i think i have found the problem the op amp is cactus

[Zero999]

I was intrigued enough to model it in LTSpice, here is the .asc file.
Took some guesses/made up some of the values.

The LT1057 is a plug in replacement for a TL072.

The LT1057 has a better specification than the TL072.

The LTSPICE model for the LT1057 doesn't leak current from its inputs when their voltage is below the minimum common mode range.

Here's a model for the TL072

Code: [Select]

* TL072 OPERATIONAL AMPLIFIER "MACROMODEL" SUBCIRCUIT
* CREATED USING PARTS RELEASE 4.01 ON 06/16/89 AT 13:08
* (REV N/A)      SUPPLY VOLTAGE: +/-15V
* CONNECTIONS:   NON-INVERTING INPUT
*                | INVERTING INPUT
*                | | POSITIVE POWER SUPPLY
*                | | | NEGATIVE POWER SUPPLY
*                | | | | OUTPUT
*                | | | | |
.SUBCKT TL072    1 2 3 4 5
*
  C1   11 12 3.498E-12
  C2    6  7 15.00E-12
  DC    5 53 DX
  DE   54  5 DX
  DLP  90 91 DX
  DLN  92 90 DX
  DP    4  3 DX
  EGND 99  0 POLY(2) (3,0) (4,0) 0 .5 .5
  FB    7 99 POLY(5) VB VC VE VLP VLN 0 4.715E6 -5E6 5E6 5E6 -5E6
  GA    6  0 11 12 282.8E-6
  GCM   0  6 10 99 8.942E-9
  ISS   3 10 DC 195.0E-6
  HLIM 90  0 VLIM 1K
  J1   11  2 10 JX
  J2   12  1 10 JX
  R2    6  9 100.0E3
  RD1   4 11 3.536E3
  RD2   4 12 3.536E3
  RO1   8  5 150
  RO2   7 99 150
  RP    3  4 2.143E3
  RSS  10 99 1.026E6
  VB    9  0 DC 0
  VC    3 53 DC 2.200
  VE   54  4 DC 2.200
  VLIM  7  8 DC 0
  VLP  91  0 DC 25
  VLN   0 92 DC 25
.MODEL DX D(IS=800.0E-18)
.MODEL JX PJF(IS=15.00E-12 BETA=270.1E-6 VTO=-1)
.ENDS

i think i have found the problem the op amp is cactus

What has lead you to that conclusion? Did you try a different TL072 or a completely different op-amp?

[electronics man]

I was intrigued enough to model it in LTSpice, here is the .asc file.
Took some guesses/made up some of the values.

The LT1057 is a plug in replacement for a TL072.

The LT1057 has a better specification than the TL072.

The LTSPICE model for the LT1057 doesn't leak current from its inputs when their voltage is below the minimum common mode range.

Here's a model for the TL072

Code: [Select]

* TL072 OPERATIONAL AMPLIFIER "MACROMODEL" SUBCIRCUIT
* CREATED USING PARTS RELEASE 4.01 ON 06/16/89 AT 13:08
* (REV N/A)      SUPPLY VOLTAGE: +/-15V
* CONNECTIONS:   NON-INVERTING INPUT
*                | INVERTING INPUT
*                | | POSITIVE POWER SUPPLY
*                | | | NEGATIVE POWER SUPPLY
*                | | | | OUTPUT
*                | | | | |
.SUBCKT TL072    1 2 3 4 5
*
  C1   11 12 3.498E-12
  C2    6  7 15.00E-12
  DC    5 53 DX
  DE   54  5 DX
  DLP  90 91 DX
  DLN  92 90 DX
  DP    4  3 DX
  EGND 99  0 POLY(2) (3,0) (4,0) 0 .5 .5
  FB    7 99 POLY(5) VB VC VE VLP VLN 0 4.715E6 -5E6 5E6 5E6 -5E6
  GA    6  0 11 12 282.8E-6
  GCM   0  6 10 99 8.942E-9
  ISS   3 10 DC 195.0E-6
  HLIM 90  0 VLIM 1K
  J1   11  2 10 JX
  J2   12  1 10 JX
  R2    6  9 100.0E3
  RD1   4 11 3.536E3
  RD2   4 12 3.536E3
  RO1   8  5 150
  RO2   7 99 150
  RP    3  4 2.143E3
  RSS  10 99 1.026E6
  VB    9  0 DC 0
  VC    3 53 DC 2.200
  VE   54  4 DC 2.200
  VLIM  7  8 DC 0
  VLP  91  0 DC 25
  VLN   0 92 DC 25
.MODEL DX D(IS=800.0E-18)
.MODEL JX PJF(IS=15.00E-12 BETA=270.1E-6 VTO=-1)
.ENDS

i think i have found the problem the op amp is cactus

What has lead you to that conclusion? Did you try a different TL072 or a completely different op-amp?

i only populated agh op amp and the voltage divider on a spare board and it worked fine. i think i will change the op amp anyway. since i have had some other unrelated problems with it 

[Zero999]

i only populated agh op amp and the voltage divider on a spare board and it worked fine. i think i will change the op amp anyway. since i have had some other unrelated problems with it

Perhaps it's the board?

What other problems have you had with the op-amp?