op-amp: is the "max" input offset voltage really the max? (CMRR/PSRR) - Page 1
Forum section: electronics / projects

author: incf
author_rank: Frequent Contributor
timestamp: March 27, 2025, 04:58:08 pm
content: When reading specs for op-amps a spec for max input offset voltage is often claimed. A few questions came up recently:Can changing the supply voltage increase the offset voltage a meaningful amount above the "max" spec? (assuming we stay within published limits, eg. 2-5.5V)If PSRR was 80dB at DC, does it increase the offset by 10^(-80/20)=100uV/V above the max?Can changing the input voltage of something like a unity gain buffer change the offset voltage a meaningful amount above the "max" spec? (assuming we respect the input/output voltage limits)If CMRR was 100dB at DC, does it increase the offset by 10^(-100/20)=10uV/V above the max ?I know AC changes in input common mode voltage, or power supply voltage can temporarily result in the offset voltage changing. But I'm interested in the effect that is has at DC.I also "know" that the max is usually given for certain extremely specific conditions like temperature and input voltages. And to subjecting the device to conditions where PSRR/CMRR matter means operating the device under slightly different conditions than what is stated for the max spec.*Power supply rejection ratio, common mode rejection ratio

author: jwet
author_rank: Frequent Contributor
timestamp: March 27, 2025, 07:48:19 pm
content: Having written and managed the writing of a few thousand op-amp and other data sheets at Maxim during my career- here is the skinny.A "Max" spec on a data sheet is the max (over temp, etc) for that parameter in the test configuration and is 100% production tested unless otherwise specified. You should never see a spec greater than the "Max" if you're operating the part within specs. The test conditions will sometimes specify a CM voltage and Vcc range, etc. Some vendors will kind of hide ugly specs in detailed language or test conditions in the data sheet. Quality suppliers like Analog Devices (Maxim, LTC) and TI don't play these games.Because a couple of seconds in a million dollar tester costs real money ( a cent or more per vector), a lot of testing is inferred by other tests and some parts have an asterisk by Max specs that says guaranteed by design or somesuch. This means that this spec is guaranteed to never be exceeded, but is not 100 percent tested. Sometimes these vectors are sample tested and ridiculous statistics are applied and sometime its straight GBD. This is a lot of what goes on in design review and final data sheet review. If you recieve parts that fail GBD specs at your receiving or in your application, you can return the lot but I've never seen it happen- good chip companies are pretty scrupulous. GBD specs are guard banded above a 6 sigma established range. Some premium grade parts are 100% tested. Most full military parts are 100% tested often at hot, cold and room. Often even commercial customers will request specials that are just to test some spec that the customer is concerned about- this increases their production yields. You can request such service at 10K pcs or if you're HP etc. A large fraction of parts in very high volume are "specials" in some way- testing, marking, etc, etc.One notoriously conservative spec is leakage current. A lot of leakage specs are grossly over specified. CMOS leakage in a clean process is often femto amps, but measuring these currents takes a lot of test time- its just a few electrons per second and is subject to all kind of statistics and is hard to fixture at high speed. You'll often see a max leakage spec of 1 nA in such cases- pretty conservative when the average is femto amps. An example, Maxim made some exotic switches (MAX329) that were tested to a pA, this added a dollar to the part cost but we could guarantee by 100% testing, 1 pA max.Another thing about leakage is that the leakage isn't generally gate leakage, its the difference in positive and negative leakage of the ESD structures used to protect the pin. If you have diode going to Vcc and another to ground, leakage will be dominated by the difference in leakage between the two devices!Changing Vcc, CM voltage etc can affect Vos, etc but it will always be less than the Max in the same test conditions. This is actually how some of these parameters are measured. You might see "inferred from CM test". A data sheet from a good supplier won't cherry pick test conditions to make a parameter look better.Most good analog data sheets also have "typ ops". These are graphs and data that establish distributions of parameters. It is actually correlation data taken from the first few wafers (perhaps 10000 units for something like an op-amp.) that design uses to get the test specs to align to their model and vice versa. In mature process, the models are pretty well wrung out but there are always things to learn. These represent the distribution and corners (extremes) seen in these first lots. These are valuable resources for the designer and give you a lot of info about how specs change.Too much writing. If you have something specific, I can respond.Edit- I went back and looked at MAX329- Oops- max spec is 10 pA, typical is 1 pA. Typs are real from good vendors but aren't guaranteed. I can tell you these parts have leakage well below a pA. System level analog engineers often use typicals based on their judgment of their own equipment's specs. If you know your operating environment is at cold, etc, such judgements are good engineering.

author: Smokey
author_rank: Super Contributor
timestamp: March 27, 2025, 09:36:43 pm
content: Quote from: jwet on March 27, 2025, 07:48:19 pmAmazing. Post of the day. So much good stuff. Thanks!

author: UnijunctionTransistor
author_rank: Frequent Contributor
timestamp: March 27, 2025, 10:19:00 pm
content: Exactly as JWET mentioned, testing is surprisingly expensive and definitely adds cost to the product.As Bob Pease remarked, one could purchase a LM358 from National Semiconductor (back then) or you can save a few pennies and purchase it from Hopefullyitworks Semiconductor. But when your product must be fully functioning at the operational corners, no ifs and buts or excuses allowed, who are you going to trust?Reputable companies have spent a lot of effort and good money fully characterizing the devices they sell, with significant margins.Speaking of the LM358: a previous employer of mine used tons of these devices, which are surprisingly good if you know its limitations, but with one caveat: Only Texas Instruments, which acquired National, was an approved source.

author: BrianHG
author_rank: Super Contributor
timestamp: March 28, 2025, 01:29:25 am
content: Quote from: UnijunctionTransistor on March 27, 2025, 10:19:00 pmI once built a device using a LM393 only to find out that since I designed it with a TI part, I could only make the device function properly with TI's LM393 after building 1k units with Motorola's equiv. It wasn't that the TI part was lower spec, it was that it outperformed the data sheet speed spec by quite a bit and my filter input design sort of became reliant on that spec.

author: Kurets
author_rank: Regular Contributor
timestamp: March 28, 2025, 07:13:26 am
content: I think that one thing kind of missed here is that certain parameters may be worse when you operate within spec, but away from the characterization point, which can surprise you.Today it is very common to use RR-input opamps, these almost universally have complementary input stages, which to some extent cancel eachothers input bias and affect offset. If you push the inputs all the way to one rail, one half of the input stage will switch off, and you will have very different specs to the "Vcm = 1/2Vsup" figure. One example of tjis is the classic OP484, which has specs for input bias vs common mode, but not input offset which degrades significantly when inputs are close to rails.In the end, this should not be surprising, but it is a case which can bite you in (as example) current sensing.

author: jwet
author_rank: Frequent Contributor
timestamp: March 28, 2025, 08:36:05 pm
content: If you look at something like the Microchip MCP6002- a cheap R-R op-amp, the specs are given like you say with a CM of Vss (low rail) though the general test is at Vcc/2. They give you 4.5mV max, pretty hideous but its a cheap part. This is ok but a little misleading- you don't operate amps biased at the low rail. A little squirelly. They don't hedge on the CM and sweep the CM .3v outside the rails, good.A few pages deeper, in the typicals, you'll see graphs of CM sweeps in some detail at different Vcc's. In the 1.8v graph, its just a bit of switchover in the middle (Vgs 1v). In the 5v graph, there is a pronounced step at 4v where the crossover happens. This is the sort of offset wiggles you will get as CM is swept. Its always below the max spec but it has some bumps. This is pretty responsible. People who want R-R amps should know this is the cost of that convenience and freedom. There is a funny graph in the typs there that has shows Vos vs CM at 1.8 and 5v. The 1.8V really takes off at the high rail. It doesn't show where it ends but its going straight up. No comments but that doesn't look very rail to rail to me. There was no way to explain that in the EC table without scaring you off. Note its all down at 10% of Max Vos but I would want to get some samples and bench test them if I was making some kind of high side current sense amp, etc. At the end of the typs, they give you input currents for inputs exceeding the negative rail- this is good- I wonder why there is none for the plus rail, another bench test I'd like to do.I'll let you compare that data sheet to that of the Max9914 series- a similar R-R fleapower LV amp. The global condition is CM=0 but its for all tests, not just the offset test. They also offer a guaranteed matching spec for the different parts in a package- this is nice touch. In the typ ops, they give you offset vs. CM, this time over temp, not that its best at hot?On this topic but not so much about data sheets is the MAX9613. These are rail to rail input amps that don't use the parallel P/N mos front end with all its ills and give great low Vos rail to rail inputs with no wiggles. They use a tiny, quiet charge pump and a P channel front end pair with their sources driven about a volt beyond the high rail. They also have a autozero type feature and give 150 uV Max VOS over temp. with no bumps. They do nice job of explaining this in the data sheet. You can see the Vos vs CM in the data sheet- pretty uninteresting.