New favourite opamp for measuring equipment - AD8605ARTZ - Page 1
Forum section: electronics / projects

author: ddavidebor
author_rank: Super Contributor
timestamp: September 04, 2019, 11:17:26 am
content: Hello guys,I want to share with you a component that I have decided to adopt in a broad range of products (scientific equipment for physics).I was looking for an opamp with as many of the following characteristics as possible:RRIO (Rail to Rail Input and Output)offset voltage under 100uV max 50uV typicalinput bias current < 100pAoutput current >40mA with 1V dropunity gain stablestable with capacitive loadscontinuous output short circuit without damagecheap to source in Shenzhenavailable in single packagesunder 40 cents @ 100pcssecond source availableThe target product is a wide variety of measuring equipment where thus opamp should be used in a series of standard analog circuitry, such as:input buffersprecision comparatorsinverting and non-inverting amplifiersfiltersprecision rectifiersvoltage clampsac amplifierscurrent sourcesNow, I did not think i would find something like this... but indeed there is, the AD8605 and it's Chinese pin compatible equivalent TP2311 available for around 0.3$ and 0.22$ respectivelyThe technology Analog Devices used to achieve such performance is pretty interestinghttps://www.analog.com/en/design-center/landing-pages/001/digitrim-technology.htmlThis little buddy does the work of laser-trimmed opamps at a much lower price

author: Kleinstein
author_rank: Super Contributor
timestamp: September 04, 2019, 12:38:02 pm
content: The data of this OP don't look very impressive. In the graph the CMRR looks good at high frequency - not sure if this is true.The offset voltage as a function of common mode voltage looks relatively poor, indicating poor CMRR (some 70dB) even at low frequency.Rail to Rail input is a two sided thing: they usually have a cross over region, where the input changes between input stages. So in this case there is a range of some 0.5 V (e.g. 3.3-3.8 V with 5 V supply) that is no usable for precision applications.

author: OM222O
author_rank: Frequent Contributor
timestamp: September 04, 2019, 01:03:21 pm
content: The high output current (40mA is on the large side for an op amp!) contradicts the precision requirements.Also no bandwidth limit was mentionedA lot of physics instruments run at high frequenciesThe capacitive load requirement is also odd ... non of the applications you listed require high capacitive loads.anyhow, my personal go to op amps for precision work are the MAX4238 and the OPA188 / OPA187. they are a bit pricy (>1$ even at volume) but have been very decent and robust parts.

author: ddavidebor
author_rank: Super Contributor
timestamp: September 04, 2019, 03:01:25 pm
content: Quote from: Kleinstein on September 04, 2019, 12:38:02 pmThe cross-over region is an excellent point. I was not able to get it to act out in simulations, do you have a suggestion on how i would be able to better test this behaviour?

author: ddavidebor
author_rank: Super Contributor
timestamp: September 04, 2019, 03:13:10 pm
content: Quote from: OM222O on September 04, 2019, 01:03:21 pmIn my case bandwidth requirements are very limited. Many of the instruments I design stop at a few hundred KHz. Not really in the counting photons business for now (though I do have one light speed measuring apparatus in the work that works at 2GHz :-) - well, will I hope. )For the capacitive loads... peak detectors have a "hold" capacitor inside. When using it as an analog output buffer (I often have many analog outputs as everyone prefers a different datalogger and big resellers like to drop their favourite one in the kit as well) and there is the capacitive load of the wire and possibly datalogger input.I also like and have used the MAX4238 as my go-to chopper opamp.High output current has been very useful to me to simplify the feedback loop of current generators by removing the mosfet/bjt. Yeah i guess i could use a different opamp for that, and indeed had. Trying to slim the bom helps the wallet with the more low production number equipment.

author: OM222O
author_rank: Frequent Contributor
timestamp: September 04, 2019, 03:46:45 pm
content: for peak detectors, I think you should consider dropping the capacitor value and invest some time in learning how to minimize leakage current using guard tracesyou can easily get away with 100pF or at worst case scenario 1nF with proper designas for the current sources ... ah man! it hurts to hear you're using the op amp to directly drive the loadthe "feedback loop" is only 2 resistors and a capacitor. doing the AC analysis of it is also fairly trivial, shouldn't take more than 10 to 15 mins in ltspice! I would also encourage you about reading a bit more in that department.If you're really tight on the budget, it's fine, but I think a doubling or even tripling the cost of op amps (from 0.4$ to 1.2$) isn't going to reduce the profit margins by much, especially since you mentioned working in lower volumes!

author: 2N3055
author_rank: Super Contributor
timestamp: September 04, 2019, 03:56:41 pm
content: Quote from: ddavidebor on September 04, 2019, 03:01:25 pmOne way to check for problems is to sweep common mode voltage on inputs and look at offset voltage..Also you could inject small AC signal with DC offset and look for distortion while varying DC offset. When you hit the knee distortions will rise..

author: ddavidebor
author_rank: Super Contributor
timestamp: September 04, 2019, 04:26:25 pm
content: Quote from: OM222O on September 04, 2019, 03:46:45 pmCapacitors have leakage current too.The load is indeed part of the feedback loop. Suggesting to follow your own advice, I endorse the following:The Art of Electronics 3rd Edition, by Paul Horowitz and Winfield HillQuote from: OM222O on September 04, 2019, 03:46:45 pm...Quote from: OM222O on September 04, 2019, 03:46:45 pmThe load is part of the feedback loop... May I recommend to you The Art of Electronics 3rd Edition by Horowitz and Hill ?Quote from: OM222O on September 04, 2019, 03:46:45 pmIf there is no need for it then it's 0.8$ wasted (times 20 times 20pcs) that's 300+$ that is better spent on plane tickets to go meet resellers and customers in person.Money adds up pretty quickly. And the circuit are not made of only opamps. Were I to design instruments with this little regard to budget we would be long out of business.

author: ddavidebor
author_rank: Super Contributor
timestamp: September 04, 2019, 04:30:18 pm
content: Quote from: 2N3055 on September 04, 2019, 03:56:41 pmMmh. Tried that in spice without seeing any knee... have to do some tests IRL.

author: Kleinstein
author_rank: Super Contributor
timestamp: September 04, 2019, 06:56:52 pm
content: The input stage cross over is not part of the Spice models. If at all the different noise levels may be included. The not the not so good CMRR in the cross over region and possibly different offsets on both sides are more like uncertainties with ideally 0. Similar OP models usually have zero offset.Especially when cost sensitive, the choice of OP depends on the circuit.The AD8605 is fast and low cost with a relatively good offset for a CMOS OP. So it can be a good choice for some applications. However there are many low cost CMOS OPs for a 5 V supply to choose from.At least at mouser the AD8605 is not that cheap - more like relatively expensive.For the use in a peak detector the more critical leakage is usually the diode(s). Fast diodes (e.g. 1N4148) tend to have quite some leakage, while low leakage diodes are usually slow. How critical leakage is, also depends a lot on the needed hold time. Capacitor leakage should be less of an issue, it's more about leakage on the board, parasitic capacitance and the settling of the OPs that could cause errors with fast peaks.

author: magic
author_rank: Super Contributor
timestamp: September 04, 2019, 07:23:09 pm
content: TI has similar technology they call e-trim.Some part numbers: opa376, opa191, opa192.I don't remember their specs and have no idea if they are suitable for your use, but I remember very good offset specs not far behind auto-zero chips.The weakest part is as usual the rail-to-rail input stage. It frankly is a gimmick.BTW, here's the figure you are looking for from AD8605 datasheet.As you can see, offset voltage wobble in the input crossover region can exceed 100µVpp.

author: David Hess
author_rank: Super Contributor
timestamp: September 04, 2019, 08:26:13 pm
content: Quote from: ddavidebor on September 04, 2019, 11:17:26 amA rail-to-rail input conflicts with precision because of crossover effects unless a boost supply is used and there are some parts which do this internally.QuoteTaking advantage of low offset voltage for precision in non-inverting applications means also having a high common mode rejection ratio which the AD8605 lacks. The power supply rejection is commensurately low as well.AD brags about low noise at 8nV/√Hz and 2.3uVpp for the AD8605 however contrasting this with a real precision part like the LT6010 (1) which is 14nV/√Hz and 0.4uVpp (!) reveals that the AD8605 has very high flicker noise which is typical of CMOS parts.The AD8605 is not a precision operational amplifier despite its trimmed offset voltage. I would consider it more of a general purpose part and it may be completely adequate for your needs. Above I am just pointing out the more obscure issues I would consider.QuoteHow output current conflicts with precision because of self heating. Precision applications must minimize change in power dissipation with signal level. I would be leery of operating a part close to its maximum current rating if reliability is desired.Quote from: ddavidebor on September 04, 2019, 03:01:25 pmDoing a cross plot of the offset versus common mode voltage on an oscilloscope will reveal the problem. National Semiconductor published some application notes about it and Bob Pease discussed it.(1) The LT6010/1/2 S/D/Q are modernized versions of the LT1008/LT1012/LT1097 S/S/S which are themselves descendants of the LM308 and LM11 but with rail-to-rail outputs and might be considered "universal" precision operational amplifiers. They are useful as a benchmark but do not really meet your other requirements.

author: David Hess
author_rank: Super Contributor
timestamp: September 04, 2019, 08:28:51 pm
content: Quote from: magic on September 04, 2019, 07:23:09 pmI do not know how to find them in their selection guides but TI even makes some dual input stage rail-to-rail input parts with a 3rd input voltage which sets the transition region so you can move it away from your input signal level.

author: ddavidebor
author_rank: Super Contributor
timestamp: September 05, 2019, 01:20:38 am
content: Quote from: Kleinstein on September 04, 2019, 06:56:52 pmDamn... the future is not yet today...Quote from: magic on September 04, 2019, 07:23:09 pmI mean TBH I'd be happy enough with 500mV to the positive rail as long as I can get everything working on 5V, that would cover almost all applications I do.Thank you for the picture. I only found it in black and white. If I am not mistaken by the late hour, using it in an inverting configuration with a 2.5 ref voltage (what I use for pretty much everything) that would be fine. But indeed it makes less sense for non-inverting configurations.The opa376 is indeed very interesting. The offset is pretty very low up until VCC-1.3V and it is available with the same package and pinout for 1$.BUT after VCC-1.3V it gets in the mV range, while the AD8605 has 80uV typical with 300uV max over the whole supply range.cons: has even worst CMRRThe TP2311 I mentioned earlier after further investigation appears to be a clone of the OPA376.Also, the OPA2376 costs 1.2$, thus let's say it's a 60 cent opamp.Curiously, the TP2311 chinese opamp claims much better CMRR than bothhttps://datasheet.lcsc.com/szlcsc/1907250931_3PEAK-TP2311-TR_C116439.pdfTo be trusted? mmh...Quote from: David Hess on September 04, 2019, 08:26:13 pmThank you for all this insightful information. Indeed the noise figures looked a bit misleading.

author: Kleinstein
author_rank: Super Contributor
timestamp: September 05, 2019, 06:32:05 am
content: For the CMRR data it is a little had to tell if this is to trust. It is not impossible to get very good values if something like a bootstrapped cascode is used. The very low THD value also suggest good CMRR. So it is not only that one number.By the way: the CMRR value for the AD8605 also look a little odd - it looks better than graph for offset via common mode voltage.

author: magic
author_rank: Super Contributor
timestamp: September 05, 2019, 09:05:22 am
content: Quote from: ddavidebor on September 05, 2019, 01:20:38 amIt's weird. On one hand, they say 106dB typical at 2.5V. On the other, their CMMR vs input voltage plot shows 150dB at 2.5V and CMRR vs frequency says 160dB at DCAnd they are very careful not to say anything about CMRR at the input stage crossover point.Quote from: Kleinstein on September 05, 2019, 06:32:05 amTHD+N is only specified at 1Vpp at unity gain, though.Quote from: Kleinstein on September 05, 2019, 06:32:05 amWell, the difference between offset at 0V and at 5V is about 50µV. Dividing it by 5V we get box method CMRR of 100dB typical

author: ddavidebor
author_rank: Super Contributor
timestamp: September 05, 2019, 10:32:45 am
content: Quote from: magic on September 05, 2019, 09:05:22 amI've written 3PEAK out of curiosity... let's wait and see...