Low offset OPA for REF?  - Page 1
Forum section: electronics / general

author: jonpaul
author_rank: Super Contributor
timestamp: January 06, 2022, 02:58:52 pm
content: Bonjour a tous,Returned to the 10.0000V REFs, tested several LT1001ACN8, considered OPA227.Seeking a better OPA, 8 pin DIP, lowest OS V, best OS stability, possible external trim of OS.In the circuit the OPA is taking the LM399 ~ 7V ref and using a precision divider to get 10.000,Oddly the offset V read with 34465A Keysight are huge!150..200 ...600 uVThe readings change if one swaps the leads around IN + and IN -.Looking at the Analog Devices parametric search, 100s of parst (no filter for package) Most are SMD.https://www.analog.com/en/parametricsearch/11094#/p2687=|1Any advise is appreciated,Kind Regards,Jon

author: exe
author_rank: Unknown
timestamp: January 06, 2022, 03:18:09 pm
content: I'm not a voltnut, but I'm eager to share my opinion that opa189 is considered to be one of the best on this forum. Since it's autozero, it would need low-impedance input (read needs capacitors). For AD devices, ada4522 and ada4523 are considered "voltnut-compliant", with the same remark about input impedance.As of offset issues, Idk what to suggest. But it looks like the problem is not with an opamp. My guess it's thermal emf, or oscillation, or RFI, noisy power supply, or all together.PS picture of setup would help. Just to make sure it's a bread-board setup, or reference is not in a socket

author: jonpaul
author_rank: Super Contributor
timestamp: January 06, 2022, 05:08:10 pm
content: Here is the board, I have only placed the LT1001 on machined pin gold plated socket to exchange.OPA input must not be loaded as the resistor divider will be wrong.Jon

author: Kleinstein
author_rank: Super Contributor
timestamp: January 06, 2022, 05:49:34 pm
content: The OPA227 like the older OP27 or LT1007 has quite some input bias current and possibly dirft of the input current and also related current noise. They are good for a low impedance, but allready with some 5 K the LT1001 can be better.We had that before: the OP177, OPA177 and OPA277 are quite similar to the LT1001. With the LM399 reference the OP is anyway the smaller problem. Even if the OP has some offset, the offset tends to be quite stable over time.New parts are mainly SMDAn AZ OP like OPA189, LT2057, MCP6V51 or ADA4522 do produce some EMI and should have some filtering at the input and output and the supply. The EMI part can cause reations from refected signals in the RF range, that can cause some unpleasant surprises. So unless there is a good reason to use them, I would avoid them. The AZ OPs are anyway mostely SMD. AFAIK there are 1 or 2 LT types still available in DIP, but they are relatively expensive and still like the filtering.

author: exe
author_rank: Unknown
timestamp: January 06, 2022, 07:12:19 pm
content: how do you measure offset?

author: Andreas
author_rank: Super Contributor
timestamp: January 06, 2022, 07:46:34 pm
content: Quote from: jonpaul on January 06, 2022, 02:58:52 pmThis might be a indicator that you measure influence of EMI instead of offset. (changeing parasytic capacitances from one to the other pin).Here a example of 24uV difference between input and output of a "zero offset" buffer due to EMI:https://www.eevblog.com/forum/metrology/ultra-precision-reference-ltz1000/msg846835/#msg846835Be carefully: you need a "capacitive isolation cirquit" to do this (see old data sheets) otherwise the buffer will oscillate.With best regardsAndreas

author: dietert1
author_rank: Super Contributor
timestamp: January 06, 2022, 07:58:03 pm
content: To connect test leads at the opamp input to do uV measurements, one should think about RF pickup on the test leads. What will happen to the reference if you feed RF? How will the opamp inputs respond to RF or voltage spikes?You could try two things: Maybe the test lead exchange difference goes away if you use two 1 KOhm resistors near the circuit to dampen RF from the test leads. And the reference could get a 50 Ohm + 100 nF RF snubber to Gnd.Another hint: Best practice is deriving the reference "zener" current from the 10 V output using a 3 KOhm resistor. If possible, the whole power supply should be derived from the reference voltage.Regards, Dieter

author: jonpaul
author_rank: Super Contributor
timestamp: January 06, 2022, 08:39:52 pm
content: Gentlemen! Monsieurs! Rebonjour,Mille mercis pour votre advices gentil!1/ Offset measurement was 34465A DMM and 50 cm gold plated bananas leas to probes! Yes lots of RF pickup....my mistake!2/ Shall try again wit some isolation R and also use the scope on the output to check for RF or oscillation.3/ Agree with Klienstein, forgot the influence of input bias current! Yes, the LT1001 is the best part, and the AZ parts are not useable here. I am ordering more and shall try several out as the PCB is gold pin socketed.Bon chance et bon Sante! \Jon

author: David Hess
author_rank: Super Contributor
timestamp: January 07, 2022, 12:25:28 am
content: The LT1001 and improved OP-07 type operational amplifiers are all about the same for noise and drift and nothing is better unless you use a chopper stabilized part. They are all better than the LM399 reference for noise and drift, so if you are having problems, then it is not the operational amplifier.