Op amp for use as a buffer for a voltage reference - Page 1
Forum section: electronics / projects

author: cape zoloh
author_rank: Contributor
timestamp: January 23, 2019, 01:29:05 pm
content: What do I need to look for, which parameters are most important, and which parameters are of less importance?I'm looking through the Analog Devices "catalog", and there is a lot to choose from.Zero drift, Precision, Low noise, Low input bias current, High speed, and so on. In which direction should I look?What I need is a voltage that is as stable (or as close to) as the voltage reference output. High output current is not needed.Thanks!

author: MiDi
author_rank: Frequent Contributor
timestamp: January 23, 2019, 01:54:01 pm
content: This depends on the type of voltage reference and how much degradation the buffer is allowed to introduce.If you are not fussed about cost/overengineering, a good chopper like ADA4522 or LTC2057 could be used.Main things to look at are long term stability and temperature coefficient of offset voltage.The maximum offset voltage could be a point, depending what is tolerable.

author: cape zoloh
author_rank: Contributor
timestamp: January 23, 2019, 02:07:48 pm
content: Thanks alot for the reply. I'm actually looking at the ADA4522 right now. When building only one piece of something, the cost of overengineering isn't necessarily going to break the bank, so, why not? Its fun!

author: Gibson486
author_rank: Frequent Contributor
timestamp: January 23, 2019, 02:36:30 pm
content: You want a low offset voltage. As mentioned, choppers can be used. They have low offset that are not choppers as well, but they get pricey (not that choppers are cheap either). Also, there are ways to fix the offset issue (look at the input current spec), but if price is not that big if an issue, why bother?

author: ArthurDent
author_rank: Super Contributor
timestamp: January 23, 2019, 04:01:26 pm
content: If you check the Metrology sub-forum "Ultra Precision Reference LTZ1000" you will see 100 pages of stuff related to voltage references, some with buffer/amps. I use a buffer/amp with my aged LTZ1000ACH to convert the 7.08003 reference voltage to 10.00000 volts using an LTC2057 or ICL7650 opamp and transistor inside the feedback loop so there is no concern for drift. The precision divider resistors determine the output voltage and the opamp isn't that critical.

author: David Hess
author_rank: Super Contributor
timestamp: January 23, 2019, 05:50:05 pm
content: References are usually so much more noisy than operational amplifiers that amplifier noise becomes less important.If you are going to filter the reference, then input bias current of the operational amplifier becomes important to get reasonable capacitor sizes. This suggests the use of low noise JFET parts in these applications although low input bias current bipolar parts like the LT1008/LT1012/LT1097 can be competitive here or even better in high temperature applications. Chopper stabilized parts have an awful lot of noise above 10 or even 1Hz so they are not apanacea.Operational amplifier offset voltage and offset voltage drive become less important at higher reference voltages. Chopper stabilized operational amplifiers are overkill for almost all applications compared to precision parts. Even at 1.2 volts, 1uV/C is only 0.83ppm/C and a precision non-chopper stabilized part can be 10 times better than this. Some JFET precision parts reach this level.If you are looking for something cheap, then the multi-sourced OP-07 is going to be difficult to beat.All of the above show why you often find "non precision" operational amplifiers like the 301A or 741 used to buffer references. Except in the highest precision applications, reference precision dominates so they are all that is required.

author: MiDi
author_rank: Frequent Contributor
timestamp: January 23, 2019, 06:10:34 pm
content: Quote from: David Hess on January 23, 2019, 05:50:05 pmDo the ADA4522 and LTC2057 have awful noise in your opinion?You are right, choppers have some drawbacks like input charge injection and noise depending on output current.

author: David Hess
author_rank: Super Contributor
timestamp: January 23, 2019, 08:09:30 pm
content: Quote from: MiDi on January 23, 2019, 06:10:34 pmThey are improved over older chopper stabilized operational amplifier but not as good as the better precision bipolar and JFET parts. Practical circuits take this into account and limit chopper stabilized amplifiers to low frequencies where their advantage of low 1/f noise becomes relevant.The question of charge injection is an interesting one. I have been doing an informal review over the past week and all of the application examples I have found use chopper stabilized amplifiers with low AC input impedance. Their low input bias current does *not* make them suitable for the same circuits that other FET and low input current bipolar parts are used for. You might see them used to buffer a reference in a high resolution voltmeter but they would *not* be used for buffering the high impedance input alone.

author: iMo
author_rank: Super Contributor
timestamp: January 23, 2019, 08:18:16 pm
content: FYI- I've been messing with the LM399 and using OP07. My OP07 does about +0.7uV/degC. The LM399 does more

author: spec
author_rank: Frequent Contributor
timestamp: January 24, 2019, 01:47:45 pm
content: The OPA191 is worth a lookand for more bandwidth, the OPA192They are both £2.50UK from DigiKeyUKhttp://www.ti.com/lit/ds/symlink/opa191.pdfhttp://www.ti.com/lit/ds/symlink/opa192.pdfThe TSX711 is now my goto opamp (£1.00UK)https://www.st.com/content/ccc/resource/technical/document/datasheet/eb/ec/a6/63/9a/ee/45/ad/DM00108580.pdf/files/DM00108580.pdf/jcr:content/translations/en.DM00108580.pdf

author: Gibson486
author_rank: Frequent Contributor
timestamp: January 24, 2019, 04:00:14 pm
content: Quote from: David Hess on January 23, 2019, 05:50:05 pmI can agree with this. I used a chopper for a low pass filter into a 32 bit ADC once....that was a horrible idea...

author: iMo
author_rank: Super Contributor
timestamp: January 24, 2019, 08:23:40 pm
content: Quote from: Gibson486 on January 24, 2019, 04:00:14 pmThere is a thread on the chopper's noise with some measurements:https://www.eevblog.com/forum/metrology/low-frequency-noise-of-zero-drift-amplifiers/25/It seems the are 1:1000 differences, imho.

author: Gibson486
author_rank: Frequent Contributor
timestamp: January 24, 2019, 10:01:01 pm
content: Quote from: imo on January 24, 2019, 08:23:40 pmI think it depends on the application as well. It is one of t hose things that where your answer will be application dependent.

author: MilkmanCDN
author_rank: Contributor
timestamp: January 25, 2019, 02:37:33 am
content: Hey there,If you're not drawing a bunch of current from the reference (steady-state or high-frequency), then you don't really need a buffer. If however, you're driving something like a SAR adc reference input, you may in fact need a buffer due to the high-frequency 'pulse(y)' current draw from the data converter.Two options:1. TI makes a reference with integrated buffer (REF6x series)2. Here's a good app-note on how to design one yourself with external op-amps:-http://www.ti.com/lit/ug/tidude2a/tidude2a.pdfGood luck!

author: Doctorandus_P
author_rank: Super Contributor
timestamp: January 26, 2019, 08:06:00 am
content: You will need a very high speed opamp to buffer your DC voltage.Really?How did that even get on your list to consider?Voltage references are all about stability, not accuracy.For an opamp to buffer that, you want the same sort of specifications.Therefore precision and offset voltage are relatively unimportant.Stability is everything though.If offset voltage is large, and it changes with temperature or age the opamp is useless for this.On the other hand. If offset voltage of an opamp is very low it can still be acceptable even if it changes 30% with temperature / aging / other.I did not look closely at the pdf linked by MilkmanCDN, but manufactures such as TI have loads of application notes on designing circuits with their components. Find some datasheets of voltage references (from different manufacturers) and then look for application notes for those voltage references.

author: David Hess
author_rank: Super Contributor
timestamp: January 27, 2019, 03:53:23 am
content: Quote from: imo on January 23, 2019, 08:18:16 pmYou can actually trim parts like the OP-07 for zero TC. Or get a bunch and grade them yourself for the lowest TCs; the metal cans were nice for this because they were easier to write on using a Sharpie.Quote from: Doctorandus_P on January 26, 2019, 08:06:00 amSometimes this is required to recover from charge injection at the driven input but there are other ways to handle this.QuotePrecision includes high open loop gain to prevent variation of the output with temperature and provide better load regulation. The two go together because open loop gain depends on thermal coupling between the output and input transistors. The same layout which improves this also improves offset voltage and offset voltage drift.

author: iMo
author_rank: Super Contributor
timestamp: January 27, 2019, 08:07:48 am
content: Quote from: David Hess on January 27, 2019, 03:53:23 amWith the trimpot at pins 1 and 8 to a none-zero offset? To add an NTC into the trimpot leg?http://www.ti.com/lit/an/sloa045/sloa045.pdf

author: David Hess
author_rank: Super Contributor
timestamp: January 28, 2019, 04:29:03 am
content: Quote from: imo on January 27, 2019, 08:07:48 amOne of the innovations with the OP-07 is that trimming the offset voltage to zero also trims the offset voltage drift closer to zero. In theory the offset voltage drift could be trimmed this way while ignoring the offset but this is probably a bad idea. Just trim the offset voltage to zero to improve the offset voltage drift. Incidentally, this is where the warning about not using the trim pins to correct external offsets comes from; doing so will ruin the offset voltage drift.I do not know if any earlier precision operational amplifiers had this property, except the OP-05 which does not really count, but many or all later ones do.

author: MilkmanCDN
author_rank: Contributor
timestamp: January 28, 2019, 06:36:48 pm
content: Quote from: Doctorandus_P on January 26, 2019, 08:06:00 amThere are cases where this is true. Buffering a precision SAR or delta-sig ADC for example when binary weighted capacitors are switched onto the reference node. During each acquisition, there is a short period of time that high-current is drawn from the reference and a droop of a few LSB's can't be tolerated as it'll impact accuracy. The term 'very' is a little bit vague as it applies differently depending on who you speak with. However, heavier reference draw is typically in the 10's to 100's of micro-seconds. There's a graph on the front page of the REF6050 describing this. Unless you need 14/16/18-bit+ accuracy, optimizing the reference may not be required.

author: David Hess
author_rank: Super Contributor
timestamp: January 29, 2019, 09:47:13 pm
content: Quote from: MilkmanCDN on January 28, 2019, 06:36:48 pmWhat is really required is a low AC output impedance which can be provided by either higher bandwidth or by adding a shunt capacitance across the output and some operational amplifiers are designed for this. Driving the signal input is a different matter because a shunt capacitance can only be used in low bandwidth applications.

author: tcatman
author_rank: Newbie
timestamp: August 25, 2023, 03:18:38 am
content: Do VREF opamp buffers need decoupling?like should I place 100nF caps close to and connected at VREF opamp power pins to ground??

author: David Hess
author_rank: Super Contributor
timestamp: August 25, 2023, 09:52:43 pm
content: Quote from: tcatman on August 25, 2023, 03:18:38 amIn the example given, they have the same decoupling requirements as a fast settling operational amplifier, so yes, they require decoupling.