Opamp suggestion for 5V RRIO with flat offset - Page 1
Forum section: electronics / projects

author: dobsonr741
author_rank: Frequent Contributor
timestamp: October 17, 2022, 05:01:51 am
content: Building an AFE and the offset “curl up” up of the Cmos RRIO opamps, at the top of the common mode range really bothers me. Like MCP6xxx or TLV2372, or any other jellybean op amps I looked at.The requirement is to work from single 5.5V supply, have flat offset in the 0.25 to 5.25V common mode range (it’s a buffer). Offset flatness is expected to be 10uV. Offset by itself is not an issue, will be auto zeroed out. Neither the bandwidth. Bias <10pA.What part would you recommend?

author: exmadscientist
author_rank: Frequent Contributor
timestamp: October 17, 2022, 05:15:19 am
content: I'd start withthe TI zero-crossover range. Mind that some of those are choppers ("zero-drift") which may or may not be what you want. (I do not usually find choppers useful.) I think OPA320 is the "first line" zero-crossover part. They have an internal charge pump and so avoid the use of a second input pair entirely. This is not perfect (see datasheet figure 3), but good luck doing better without a chopper.If this is truly a precision buffer application (which it seems to be if you are talking 10uV), you may need to consider having supply rails bootstrapped to follow the input. This can kill off a surprising amount of distortion, and keep you away from any and all kinds of CMRR woes, but at the price of needing wider rails. If you already have those or can get them easily, do not ignore this possibility.

author: magic
author_rank: Super Contributor
timestamp: October 17, 2022, 05:23:38 am
content: The TI charge-pump chips come to mind like OPA365, but still only 100dB guaranteed CMRRNational had LMC6482 and similar which avoid switching to N-ch by manipulating Vgs of their P-ch input stage over common mode range, but you wouldn't even want to see the specsSeems you are left with auto-zero or generating a higher rail.

author: TheUnnamedNewbie
author_rank: Super Contributor
timestamp: October 17, 2022, 01:30:23 pm
content: Considering how rail-to-rail inputs are designed I think it is impossible to get flat offset. Unless someone makes a part with an internal DC-DC that moves up the rail voltage of the input pair up by 2 Vsat so you still keep everything in saturation at input supply...

author: 3roomlab
author_rank: Frequent Contributor
timestamp: October 17, 2022, 01:41:18 pm
content: Quote from: dobsonr741 on October 17, 2022, 05:01:51 amOPA x388 ?

author: dobsonr741
author_rank: Frequent Contributor
timestamp: October 17, 2022, 03:00:00 pm
content: And AD8628 comes back into the picture, after cross-reffing OPA388. On one read of the AD8628 data sheet the CMRR rate seemed limited, now I see it was under the 2.7V single supply.

author: exmadscientist
author_rank: Frequent Contributor
timestamp: October 17, 2022, 04:44:35 pm
content: Quote from: TheUnnamedNewbie on October 17, 2022, 01:30:23 pmThat is exactly what the charge pumps in the "zero-crossover" parts I suggested do for you.If AD8628 or any other chopper will work out for you, they are definitely the easiest way to meet your requirement. Just make sure you understand their care and feeding, especially the weird stuff:http://e2e.ti.com/cfs-file.ashx/__key/telligent-evolution-components-attachments/00-14-01-00-00-70-21-03/Chopper-Noise.pdfOtherwise, take a second look at OPA325, I guess....

author: dobsonr741
author_rank: Frequent Contributor
timestamp: October 17, 2022, 09:48:27 pm
content: Thanks all - awesome "new" development by TI on zero-crossover inputs. It seems the adoption started ~2017. Though I see the historical roots of the boosted CMOS input stage in LTC1152 from the early 1990s.I'm convinced I need continuous time op amp - so the AD8628 will rest in the drawer. Digikey has a bit of supply of OPA2325, just as TI direct.

author: magic
author_rank: Super Contributor
timestamp: October 18, 2022, 05:53:08 am
content: The TI design dates back to at least OPA365 from 2006.In OPA325 the advertised 114dB CMRR is only typical. Guaranteed minimum is 100dB or 10µV drift per 1V of input change. Twice that across full operating temperature.And there is still some weird glitch at Vcc-2V on these plotsIn particular, the darkest line swings almost 10µV p-p, which is your limit.Can you guarantee no real unit will be worse?Not sure if they lie about this being a zero-crossover chip or if it's some weird interaction between the input transistors and the substrate potential (which may be Vcc in CMOS) or whatever, but it seems quite real.

author: David Hess
author_rank: Super Contributor
timestamp: October 18, 2022, 06:19:07 pm
content: Quote from: magic on October 17, 2022, 05:23:38 amI used them for a long time, but they are not for precision applications. Their only real advantage is the lack of a sharp input offset voltage transition.Quote from: dobsonr741 on October 17, 2022, 09:48:27 pmI did not even know that Texas Instruments was making boosted operational amplifiers. The Linear Technology LTC1152 is what I was going to suggest. Note that chopper stabilized parts like the LTC1152 *are* continuous time. Chopper stabilized means that the chopper amplifier corrects the offset of the continuous time amplifier which is always connected to the inputs.An alternative which I have used in the past is to steal a clock signal from some other circuit, or make my own, and drive a diode-capacitor multiplier to boost the supply voltage to power a low power operational amplifier, however this has the disadvantage of allowing the output of a rail-to-rail output amplifier to move above the normal supply voltage which may not be acceptable, although it should be fine in a buffer application.