OPAMP and low level voltages - Page 1
Forum section: electronics / beginners

author: alltheway
author_rank: Contributor
timestamp: September 13, 2017, 11:44:40 am
content: Hi,I just played around with the TLV2369 opamp on a project of mine and I found something strange which I don't really understand.I'm using the opamp in a voltage follower configuration as a buffer for a small signal (1mV).The issue is that the output voltage of the opamp (Vout) is not matching the input voltage as it should on an ideal opamp.The measured values are:OPAMP offset  = 4uVInput voltage  = 2.518 mVOutput voltage  = 2.371 mVThe error is ~150 uV which is not a big deal but amplified by 100 we get 15 mV which is way to much.What is causing this error? where shall I look in datasheet to chose a better opamp.PS: I know TLV2369 is not a chopper stabilized opamp and the input offset voltage is pretty high for my application but I hand selected one and the Vos on my setup is ~4 uV.Thank you,

author: HB9EVI
author_rank: Frequent Contributor
timestamp: September 13, 2017, 12:15:28 pm
content: Hi thereThis phenomenon is called input offset voltage; it's a classical example which shows the difference between the ideal opamp model and the real world devices we have available.Depending on the requirements you'll have to watch out for (ultra) low offset opamps; there are models which achieve Vos <10uV, but of course other parameters e.g. supply voltage are maybe not fitting for your needs anymore.

author: Dave
author_rank: Super Contributor
timestamp: September 13, 2017, 01:00:41 pm
content: Are you using a bipolar supply for the opamp?If the voltages that you're measuring are near the supply rails, you can forget about the tight offset specs.

author: albert22
author_rank: Regular Contributor
timestamp: September 13, 2017, 01:07:20 pm
content: I may by wrong but, As far as I know the input offset varies with the common mode voltage. Which in your case is around 2 volts. There is also a CMRR that may be contributing.

author: Yansi
author_rank: Super Contributor
timestamp: September 13, 2017, 01:10:22 pm
content: albert22, you are completely off and mix things together.

author: AlfBaz
author_rank: Super Contributor
timestamp: September 13, 2017, 01:23:22 pm
content: Quote from: alltheway on September 13, 2017, 11:44:40 amData sheet's most min offset value is 400uV, you sure your offset measurement is right?

author: alltheway
author_rank: Contributor
timestamp: September 13, 2017, 01:46:50 pm
content: Hi,I used a single 5v supply.I changed the power supply to 2.500V split supply.I increased the input voltage just for tests and I get the following results:OPAMP offset  = 124 uVInput voltage  = 2.494 mVOutput voltage  = 2.388 mVOPAMP offset  = 124 uVInput voltage  = 24.939 mVOutput voltage  = 24.891 mVOPAMP offset  = 124 uVInput voltage  = 249.19 mVOutput voltage  = 249.26 mVIt looks like the offset is not the same, I expect to get something close to opamp offset (124uV) on each test.I did not mentioned before that I have a voltage divider on the input of the opamp.I have four resistors in series to divide a 2.5V voltage reference, in this way I get four steps (2.5mV, 25mV, 250mV, 2.5V).Can this resistors play a role here?I'm going to post the schematic later this day.EDIT:Alfbaz:On 5v single supply the offset is ~4 uVChanging the pover supply to 2.5 split supply the offset changed.I think I may have some issues with the input bias current and the resistor divider but I'm not sure and I don't know how to measure it.I can deal with the offset but I expect it to be the same on each range.Thank you.

author: tszaboo
author_rank: Super Contributor
timestamp: September 13, 2017, 02:07:11 pm
content: VO Voltage output swing from rail RL = 10 k? 25 mVTable at 6.6.Use dual supplies, or a small negative voltage for the negative rail. LM7705 or so is ideal for this.

author: StillTrying
author_rank: Super Contributor
timestamp: September 13, 2017, 02:51:54 pm
content: The easiest way to measure the offset in the voltage follower is to measure the voltagedirectly betweenthe opamp's +ve input and output.With +-2.5V supplies you could measure the offset with the +ve input at 0V, 2.5mV, 25mV and 250mV. But at 2.5V input the output will probably be too close to the +2.5V supply for the offset reading to mean much, unfortunately even rail to railoutputsdon't go all the way to the rail.

author: alltheway
author_rank: Contributor
timestamp: September 13, 2017, 06:00:54 pm
content: Hello all,Attached is my current schematic.It is clear for me that TLV2369 is not suitable for my aplication but why the offset is not the same on all ranges?For example:Let's assume the OPAMP input offset is 1mV.According to my schematic i should get the following results, right?2.5mV + 1mV = 3.5mV25mV + 1mV = 26mV250mV + 1mV = 251mVI also tried with other TLV's.Decoupling capacitors are close to the opamp.Tomorrow i will change the opamp with an OPA177.Thank you,

author: Zero999
author_rank: Super Contributor
timestamp: September 13, 2017, 09:38:57 pm
content: Quote from: alltheway on September 13, 2017, 06:00:54 pmThe OPA177 might not be suitable either. It isn't specified to work below a total supply voltage of 6V (+3V -3V) and its common mode voltage is limited to within 2V worst case, 1V typical, of the supply rails.

author: Circlotron
author_rank: Super Contributor
timestamp: September 13, 2017, 11:29:16 pm
content: The output offset may change on different ranges because of the input bias current flowing through different value voltage divider resistors.

author: AlfBaz
author_rank: Super Contributor
timestamp: September 13, 2017, 11:53:16 pm
content: Quote from: alltheway on September 13, 2017, 06:00:54 pmAt a guess I would suspect input bias currents through the different input resistor configurations adding to Vos

author: AlfBaz
author_rank: Super Contributor
timestamp: September 13, 2017, 11:55:08 pm
content: Quote from: Circlotron on September 13, 2017, 11:29:16 pmLOL.... always check for new posts before hitting post

author: danadak
author_rank: Super Contributor
timestamp: September 13, 2017, 11:58:33 pm
content: 2.5mV + 1mV = 3.5mV25mV + 1mV = 26mV250mV + 1mV = 251mVor could be2.5mV - 1mV = 1.5mV25mV - 1mV = 24mV250mV - 1mV = 249mVOr anything between the two cases.Regards, Dana.

author: danadak
author_rank: Super Contributor
timestamp: September 14, 2017, 12:01:46 am
content: Typical offset due to Ibias = 10 pA x 1 Mohm = 10 uVAn excellent book on low level measurements -https://www.google.com/url?sa=t&rct=j&q=&esrc=s&source=web&cd=3&cad=rja&uact=8&ved=0ahUKEwis0Nr0r6PWAhWpv1QKHdIpCcAQFggzMAI&url=https%3A%2F%2Fwww.tek.com%2Fdocument%2Fhandbook%2Flow-level-measurements-handbook&usg=AFQjCNHQ4zBZiNYmexVJsq-eyESBdE0cFwRegards, Dana.

author: David Hess
author_rank: Super Contributor
timestamp: September 14, 2017, 03:24:24 am
content: Quote from: alltheway on September 13, 2017, 06:00:54 pmThe common mode rejection ratio (CMRR) could be as low as 80dB producing 100 microvolts of input voltage offset change for every 1 volt of change at the non-inverting input.Quote from: Circlotron on September 13, 2017, 11:29:16 pmThe TLV2369 is a MOSFET input operational amplifier so the input bias currents are insignificant. See below.Quote from: danadak on September 14, 2017, 12:01:46 amIn this case, 10pA x 90k = 0.1uV so errors from input bias current may be ignored.

author: Zero999
author_rank: Super Contributor
timestamp: September 14, 2017, 10:46:39 am
content: Yes, it's a MOSFET input op-amp, so the bias current is not a factor.I suspect it's more likely because it's a rail-to-rail op-amp, which actually has two input stages: one of voltages near the positive rail and another for voltages near the negative rail. As the common mode voltage varies, the op-amp will switch from one input stage to the other, possibly using both at the same time, at some point. The two input stages will have slightly different offset voltages, which would explain why it varies, as the common mode voltage is changed.

author: alltheway
author_rank: Contributor
timestamp: September 14, 2017, 12:00:54 pm
content: Hi,I want to thank you all for your support.I changed the power supply to +/- 5V split supply.First I experimented with OPA177 but as it was spotted out earlier the results weren't so good.I got ~70uV offset and still varied when I changed the ranges.Then I changed the opamp with TC7650 and .... surprise surprise ... it is spot on.OPAMP offset  = 0 uVInput voltage  = 2.5168 mVOutput voltage  = 2.5172 mVOPAMP offset  = 0 uVInput voltage  = 25.1721 mVOutput voltage  = 25.1722 mVOPAMP offset  = 0 uVInput voltage  = 251.532 mVOutput voltage  = 251.531 mVNow the question is what opamp shall I chose for my application?TC7650 is not ok because I can find it only in DIP package (at a reasonable price on my location) and is not a dual opamp.Other options will be:OPA2333 - input bias current may be to high.MCP6V26If someone have the time and is willing to explain me a little about CMRR (what means, advantages/disadvantages, on which application is this parameter important, or more important)I did not work before with precision opamp's just with really cheap ones and the offset where corrected in software.Thank you,

author: StillTrying
author_rank: Super Contributor
timestamp: September 14, 2017, 12:06:59 pm
content: The tlv2369's rail to rail input uses a charge pump - so there's only 1 input stage.I think(again) about the only way to measure any changes of uVs in the OP's small 0V to 250mV common mode range would be by measuringdirectlybetween the opamp's input and output.Edit"OPAMP offset  = 0 uVInput voltage  = 251.532 mVOutput voltage  = 251.531 mV"How exactly are you measuring the divider's 250mV. If your DVM's impedance isonly10M the 250mV reading will read low by ~2.3mV, while measuring the divider, but not while measuring the opamp's output.

author: tszaboo
author_rank: Super Contributor
timestamp: September 14, 2017, 12:33:56 pm
content: CMRR: You measure the common mode voltage. This is the input voltage of the amplifier. You multiply it with the CMRR. CMRR is often expressed (wrongly) in dB, so -80dB is 10 exp (-80/20) = 0.0001.It is wrong, because CMRR should be 0.0001 and -80dB is called CMR.But whatever. Add this as an input offset. Different frequencies have different CMRR.

author: alltheway
author_rank: Contributor
timestamp: September 14, 2017, 12:52:37 pm
content: Hello,Quote from: StillTrying on September 14, 2017, 12:06:59 pmI use a keithley 2700, it suppose to have an input impedance of more than 10Gohm.I measured the offset as you proposed and I get the same values.Quote from: NANDBlog on September 14, 2017, 12:33:56 pmthank you, I will also dig a bit on this topic.

author: David Hess
author_rank: Super Contributor
timestamp: September 14, 2017, 02:29:43 pm
content: Quote from: alltheway on September 14, 2017, 12:00:54 pmThe OPA177 should be a lot better with its CMRR of 120dB or higher which yields less than 1uV/V of change. It might be time to look for other causes like noise being coupling into the circuit via your multimeter leads. Decouple the input to the operational amplifier with a capacitor to ground and the output with a low value series resistor, say 1k, and capacitor to ground.QuoteThe OPA2333 will probably work fine and the MCP6V26 certainly will be. There are too many suitable parts to list.QuoteCMRR matters in any application where the non-inverting input changes level like non-inverting amplifiers, buffers or voltage followers, and instrumentation or difference amplifiers. A high CMRR is one of the major characteristics which makes a precision operational amplifier "precision" and a low CMRR is often the most significant error term.

author: albert22
author_rank: Regular Contributor
timestamp: September 14, 2017, 06:21:15 pm
content: Quote from: Yansi on September 13, 2017, 01:10:22 pmThank you.

author: danadak
author_rank: Super Contributor
timestamp: September 14, 2017, 08:08:21 pm
content: I need to go back to school for ohms law it seems.Attached offset calculation.Regards, Dana.

author: StillTrying
author_rank: Super Contributor
timestamp: September 14, 2017, 10:38:11 pm
content: Dana, you don't need to know the value of the 2.5V, only that its impedance is zero the same as 0V. So all you've got is 10pA flowing into 90k. (900K || 100K) * 10pA = 0.9uV is a bit shorter/quicker!Quote from: alltheway on September 14, 2017, 12:52:37 pmIn Reply#6 you've got a TLV2369 on +-2.5V supply. Over your small CM test range of 25mV to 250mV you should only see a change in the input offset of < 3uV (which is the yellow square the graph) but you measured a change of +106uV to -70uA, I don't know why you getting such a big change in the TLV2369's offset.

author: David Hess
author_rank: Super Contributor
timestamp: September 15, 2017, 02:57:34 am
content: Quote from: StillTrying on September 14, 2017, 10:38:11 pmThe worst case CMRR of the TLV2369 could contribute 25uV of error. My guess would have been that being a very low power part, the TLV2369 was having difficulty driving the meter so the extra error is a measurement problem but the OPA177 did not work either.

author: alltheway
author_rank: Contributor
timestamp: September 15, 2017, 10:35:47 am
content: Hi,I don't know what's the problem here.Maybe because I build the schematic on breadboard, but why is working fine with TC7650.I ordered a few pcb's and I will continue the tests.At the moment the topic is on hold.I have a feeling that the issues are related to the breadboard.Thank you,

author: danadak
author_rank: Super Contributor
timestamp: September 15, 2017, 10:54:29 am
content: StillTrying, yes for sure I took the long way home on calculations. No "shortcuts,simplifications" were used.For all here is a pretty good summary of imperfect OpAmp considerations -http://www.ti.com/lit/sg/slyt701/slyt701.pdfRegards, Dana.

author: alltheway
author_rank: Contributor
timestamp: November 27, 2017, 07:34:42 am
content: Hello,I'm back with newsIt's been some time but finally I had the time to work on this topic.This time I have a proper pcb board for this project and the results look really good.It works how it supposed to work with a very small error on the output (basically the opamp offset error).I still don't believe that the problem can be the breadboard.Thank you,