op-amp causing voltage divider output to change  - Page 1
Forum section: electronics / beginners

author: electronics man
author_rank: Frequent Contributor
timestamp: August 23, 2015, 05:10:59 pm
content: 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?

author: fcb
author_rank: Super Contributor
timestamp: August 23, 2015, 05:20:03 pm
content: 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.

author: codeboy2k
author_rank: Super Contributor
timestamp: August 23, 2015, 05:40:03 pm
content: 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 !)

author: Zero999
author_rank: Super Contributor
timestamp: August 23, 2015, 05:42:10 pm
content: 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

author: electronics man
author_rank: Frequent Contributor
timestamp: August 23, 2015, 05:49:02 pm
content: 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

author: electronics man
author_rank: Frequent Contributor
timestamp: August 23, 2015, 05:49:42 pm
content: all diodes are unpopulated

author: electronics man
author_rank: Frequent Contributor
timestamp: August 23, 2015, 05:50:54 pm
content: the resistor values are R12 = 820K and R13 is 47k

author: Zero999
author_rank: Super Contributor
timestamp: August 23, 2015, 05:54:40 pm
content: That schematic is far too small to read.

author: electronics man
author_rank: Frequent Contributor
timestamp: August 23, 2015, 05:58:25 pm
content: yeah i know i don't know how to make it any bigger for you

author: electronics man
author_rank: Frequent Contributor
timestamp: August 23, 2015, 05:59:41 pm
content: this may be better

author: Zero999
author_rank: Super Contributor
timestamp: August 23, 2015, 06:08:39 pm
content: 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.

author: PSR B1257
author_rank: Frequent Contributor
timestamp: August 23, 2015, 06:18:21 pm
content: QuoteThey are. Right down in the center

author: Zero999
author_rank: Super Contributor
timestamp: August 23, 2015, 06:48:39 pm
content: 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?

author: electronics man
author_rank: Frequent Contributor
timestamp: August 23, 2015, 07:01:42 pm
content: Quote from: Hero999 on August 23, 2015, 06:48:39 pmi 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.

author: electronics man
author_rank: Frequent Contributor
timestamp: August 23, 2015, 07:02:54 pm
content: Quote from: Hero999 on August 23, 2015, 06:08:39 pmpower rails are fine but that is not what matters here

author: free_electron
author_rank: Super Contributor
timestamp: August 23, 2015, 07:04:08 pm
content: common mode range of that stinker is vcc -4v to vcc- + 4 v ...i never liked the TL7xx series.

author: electronics man
author_rank: Frequent Contributor
timestamp: August 23, 2015, 07:06:15 pm
content: its the tl072

author: electronics man
author_rank: Frequent Contributor
timestamp: August 23, 2015, 07:08:28 pm
content: just to be clear the opamps work fine. they are buffering, offsetting and amplifying just fine

author: PSR B1257
author_rank: Frequent Contributor
timestamp: August 23, 2015, 07:25:21 pm
content: Have you tried a different opamp? Maybe your particular on is a little

author: electronics man
author_rank: Frequent Contributor
timestamp: August 23, 2015, 07:27:18 pm
content: Quote from: PSR B1257 on August 23, 2015, 07:25:21 pmno because it worked fine on breadboard ext and that would require me getting the pcbs remade

author: electronics man
author_rank: Frequent Contributor
timestamp: August 23, 2015, 07:30:29 pm
content: 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

author: PSR B1257
author_rank: Frequent Contributor
timestamp: August 23, 2015, 07:32:53 pm
content: QuoteNo, I meant a different TL072.Aside of that, a whole lot different op amps share the same pinout

author: electronics man
author_rank: Frequent Contributor
timestamp: August 23, 2015, 07:34:25 pm
content: the TL072 is 18V tolerant and i will have a look at what is available

author: PSR B1257
author_rank: Frequent Contributor
timestamp: August 23, 2015, 07:46:07 pm
content: QuoteIt 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?

author: electronics man
author_rank: Frequent Contributor
timestamp: August 23, 2015, 07:48:09 pm
content: Quote from: PSR B1257 on August 23, 2015, 07:46:07 pmyes i have and the output of the opamp is fine i really don't understand

author: Kevin.D
author_rank: Frequent Contributor
timestamp: August 23, 2015, 07:57:59 pm
content: Poor gnd ? are you probing directly at the divider with both probes ?.

author: electronics man
author_rank: Frequent Contributor
timestamp: August 23, 2015, 09:43:33 pm
content: Quote from: Kevin.D on August 23, 2015, 07:57:59 pmyup right across the resistor

author: Zero999
author_rank: Super Contributor
timestamp: August 23, 2015, 09:49:45 pm
content: Quote from: PSR B1257 on August 23, 2015, 07:46:07 pmYes, 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.

author: electronics man
author_rank: Frequent Contributor
timestamp: August 23, 2015, 10:11:30 pm
content: Quote from: Hero999 on August 23, 2015, 09:49:45 pmdoes the LM358 work with the split rails?

author: Zero999
author_rank: Super Contributor
timestamp: August 23, 2015, 10:18:24 pm
content: Quote from: electronics man on August 23, 2015, 10:11:30 pmWhy wouldn't it? Have you read the data sheet?

author: DimitriP
author_rank: Super Contributor
timestamp: August 23, 2015, 10:35:56 pm
content: 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.pdfHave funD--------snip-o-matic-----------Version 4SHEET 1 1528 680WIRE 144 -80 -144 -80WIRE 528 -80 336 -80WIRE 336 -16 336 -80WIRE 400 -16 336 -16WIRE -464 0 -464 -32WIRE -336 0 -336 -32WIRE 144 0 144 -80WIRE 528 0 528 -80WIRE 528 0 464 0WIRE 736 0 528 0WIRE 400 16 304 16WIRE 432 80 432 32WIRE -144 96 -144 -80WIRE -464 112 -464 80WIRE -336 112 -336 80WIRE 144 160 144 80WIRE 192 160 144 160WIRE 304 160 304 16WIRE 304 160 272 160WIRE 144 224 144 160WIRE 432 240 368 240WIRE 544 240 512 240WIRE 0 272 -16 272WIRE 368 320 368 240WIRE 384 320 368 320WIRE 544 336 544 240WIRE 544 336 448 336WIRE 752 336 544 336WIRE 304 352 304 160WIRE 368 352 304 352WIRE 384 352 368 352WIRE 144 416 144 304WIRE 416 416 416 368WIRE 448 416 416 416WIRE 368 432 368 352FLAG 432 -32 Vcc+IOPIN 432 -32 BiDirFLAG 416 304 Vcc+IOPIN 416 304 BiDirFLAG 432 80 Vcc-FLAG 448 416 Vcc-IOPIN 448 416 BiDirFLAG -336 112 0FLAG -336 -32 Vcc+FLAG -464 112 0FLAG -464 -32 Vcc-FLAG 752 416 0FLAG 368 512 0FLAG 144 416 0FLAG -144 176 0FLAG 736 80 0SYMBOL res 528 224 R90WINDOW 0 0 56 VBottom 2WINDOW 3 32 56 VTop 2SYMATTR InstName R3SYMATTR Value 22KSYMBOL res 160 320 R180WINDOW 0 36 76 Left 2WINDOW 3 36 40 Left 2SYMATTR InstName R13SYMATTR Value 47KSYMBOL res 160 96 R180WINDOW 0 36 76 Left 2WINDOW 3 36 40 Left 2SYMATTR InstName R2SYMATTR Value 820KSYMBOL voltage -144 80 R0WINDOW 123 0 0 Left 2WINDOW 39 0 0 Left 2SYMATTR InstName V2SYMATTR Value 20SYMBOL voltage -464 -16 R0WINDOW 123 0 0 Left 2WINDOW 39 0 0 Left 2SYMATTR InstName V3SYMATTR Value -5SYMBOL res 736 320 R0SYMATTR InstName R9SYMATTR Value 100KSYMBOL res 352 416 R0SYMATTR InstName R1SYMATTR Value 1000kSYMBOL res 720 -16 R0SYMATTR InstName R4SYMATTR Value 100KSYMBOL voltage -336 -16 R0WINDOW 123 0 0 Left 2WINDOW 39 0 0 Left 2SYMATTR InstName V1SYMATTR Value 5SYMBOL res 288 144 R90WINDOW 0 0 56 VBottom 2WINDOW 3 32 56 VTop 2SYMATTR InstName R5SYMATTR Value 1SYMBOL Opamps\\LT1057 432 -64 R0SYMATTR InstName U3SYMBOL Opamps\\LT1057 416 272 R0SYMATTR InstName U4TEXT 472 528 Left 2 !.tran 50mTEXT 208 552 Top 1 ;This example schematic is supplied for informational/educational purposes only.--------snip-o-matic-----------

author: PChi
author_rank: Frequent Contributor
timestamp: August 24, 2015, 09:03:09 am
content: 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.

author: Zero999
author_rank: Super Contributor
timestamp: August 24, 2015, 11:29:56 am
content: Quote from: electronics man on August 23, 2015, 05:49:02 pmQuote from: electronics man on August 23, 2015, 07:02:54 pmIf 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.

author: PSR B1257
author_rank: Frequent Contributor
timestamp: August 24, 2015, 02:50:03 pm
content: QuoteEvey opamp works with split rails, unless you excced its maximum V- to V+ voltage rating.

author: electronics man
author_rank: Frequent Contributor
timestamp: August 24, 2015, 04:41:00 pm
content: i think i have found the problem the op amp is cactus

author: Zero999
author_rank: Super Contributor
timestamp: August 24, 2015, 04:52:33 pm
content: Quote from: DimitriP on August 23, 2015, 10:35:56 pmThe 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 TL072Code:* 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-12C2 6 7 15.00E-12DC 5 53 DXDE 54 5 DXDLP 90 91 DXDLN 92 90 DXDP 4 3 DXEGND 99 0 POLY(2) (3,0) (4,0) 0 .5 .5FB 7 99 POLY(5) VB VC VE VLP VLN 0 4.715E6 -5E6 5E6 5E6 -5E6GA 6 0 11 12 282.8E-6GCM 0 6 10 99 8.942E-9ISS 3 10 DC 195.0E-6HLIM 90 0 VLIM 1KJ1 11 2 10 JXJ2 12 1 10 JXR2 6 9 100.0E3RD1 4 11 3.536E3RD2 4 12 3.536E3RO1 8 5 150RO2 7 99 150RP 3 4 2.143E3RSS 10 99 1.026E6VB 9 0 DC 0VC 3 53 DC 2.200VE 54 4 DC 2.200VLIM 7 8 DC 0VLP 91 0 DC 25VLN 0 92 DC 25.MODEL DX D(IS=800.0E-18).MODEL JX PJF(IS=15.00E-12 BETA=270.1E-6 VTO=-1).ENDSQuote from: electronics man on August 24, 2015, 04:41:00 pmWhat has lead you to that conclusion? Did you try a different TL072 or a completely different op-amp?

author: electronics man
author_rank: Frequent Contributor
timestamp: August 24, 2015, 05:46:37 pm
content: Quote from: Hero999 on August 24, 2015, 04:52:33 pmi 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

author: Zero999
author_rank: Super Contributor
timestamp: August 24, 2015, 07:00:06 pm
content: Quote from: electronics man on August 24, 2015, 05:46:37 pmPerhaps it's the board?What other problems have you had with the op-amp?