Lab Grade Instrumentation Amplifier - DIY or Buy A Chip? - Page 1
Forum section: electronics / projects

author: t1d
author_rank: Super Contributor
timestamp: July 15, 2024, 02:28:34 am
content: My Background = Self-taught hobbyist with significant knowledge gaps, due to only studying what interests me. I have good skills... KiCad, ordering boards and soldering them up. I also have a really good hobbyist lab and can test most things.I love to DIY my own EE bench tools. A Three-Op-Amp Instrument Amplifier design looks easy enough. However, it appears that there are all-in-one ICs that have better spec's than what I would end up with, if I built it, myself. Of course, there is the joy of learning and the satisfaction of building. But, the circuit is so straight forward that I think I have learned what there is to know. IMO, a chip price of say $20 would be worth it, for true lab-grade spec's. So, DIY, or Buy, and Why? If buy, what chip do you suggest? It could include more advanced features/techniques, of course. Thanks!!!

author: Sensorcat
author_rank: Regular Contributor
timestamp: July 15, 2024, 03:00:52 am
content: If you enjoy to DIY the 3-OpAmp instrumentation amplifier, do it!If you need it for another project to work, as a component of that project, it is wiser to find an integrated instrumentation amplifier. For help which one to choose, you must provide detail on your requirements.

author: moffy
author_rank: Super Contributor
timestamp: July 15, 2024, 03:02:15 am
content: A brief answer is that an all in one chip will tend to have a high CMRR due to matched/laser trimmed components which would require expensive matched resistor networks to replicate. As far as a recommendation, what is it intended for? Low noise, low offset, large voltage range, low bias current, high input impedance? etc. etc. The recommendation depends on the use cases.

author: TimFox
author_rank: Super Contributor
timestamp: July 15, 2024, 03:30:20 am
content: If you need to match resistors from a batch, a good DMM in ohms mode can be trusted for equality of a pair to its resolution, better than the calibration accuracy. Also, it is easy to trim CMRR at DC by zeroing the offset with the two inputs connected to zero volts, then applying a reasonable voltage to both inputs (still connected together) and adjust a suitable trimpot in one resistor.

author: t1d
author_rank: Super Contributor
timestamp: July 15, 2024, 04:59:35 am
content: Excellent answers, everyone! Thank you.To answer Sensorcat and moffy, my use is just a nice, general purpose instrument amplifier to have on the bench for general purposes. I do not have a specific need in mind. I have moved into the nicer grade of hobbyist instruments. A Siglent SDM3065X (DMM), SDS1204X-E (Scope), SPD 3303X (PSU) and Rigol 1062Z (FG), etc. So, I would like for you to recommend a capable IC... I am on a retirement budget, but the chip doesn't have to be dirt cheap. $25USD might be an idea of what I can manage. But, cheaper than that is fine to... <grin>I appreciate your help and look forward to investigating your recommendations.

author: moffy
author_rank: Super Contributor
timestamp: July 15, 2024, 06:05:44 am
content: Without specs it is impossible to advise, at Digikey they range from $1 AU to over $500 AU here is a search selection:https://www.digikey.com.au/en/products/filter/linear/amplifiers/instrumentation-op-amps-buffer-amps/687?s=N4IgjCBcpgbB0QGMoDMCGAbAzgUwDQgD2UA2iAMwCsAHAJw2wgC6hADgC5QgDKHATgEsAdgHMQAX0IBaAExRQKSAICuBYmRBUWUkNLoLkUVepKRyEZhN3bEbKGHb3IsqrqaJBAE27SwABgh2LkgQEEIOAE82XG50bBRrIA

author: Kleinstein
author_rank: Super Contributor
timestamp: July 15, 2024, 07:17:48 am
content: There is no universal best amplifier, the same amplifier can be good for one job and awful for another. It really depends on the signal source to look at, which amplifier is good. It is more about voltage noise and offset voltage for a low impedance source and more about current noise and bias for a high impedance cource.For the more intermediate cases there are good off the shelf instrumentation amplifiers. For more unusual cases (especially need for "zero" drift or very low bias, but also a large voltage range) the separate build 3 OP-amp solution still makes sense.For the chips it also depends on the supply range. INAs usually have a somewhat limited common voltage range - a point sometimes overlooked.

author: Phil1977
author_rank: Frequent Contributor
timestamp: July 15, 2024, 09:49:06 am
content: DIY or buy is from my experience heavily dependant on the frequency range you need.If you have more or less static input voltages then DIY setups can quite easily be well calibrated to the required accuracy. You also get quick feedback during the calibration process if things are okay or not.If you have input signals with a higher bandwidth (100Hz or more) I´d strongly recommend to use a dedicated chip with matching specifications. It´s complex in the narrowest sense of the word to design amplification stages with matching frequency response. You definitely need simulations and very precise components for that.If you want to have an idea about the maximal frequency that can be handled as quasi-static you can look into the datasheets of the respective opamps. Usually you find a "closed loop gain against frequency" plot. There you can look for the curve that´s matching your maximum gain and then see where the curve is flat. You need to be safely in the flat zone so that you can rely on a purely static calibration

author: Conrad Hoffman
author_rank: Super Contributor
timestamp: July 15, 2024, 11:48:45 am
content: Commercial ones can be expensive and I've sometimes had less than stellar results using them. I'd DIY your own and then try a commercial one if you have special needs that yours can't do. There are a couple different configurations you can try and you can certainly use trimmers to get good CMRR.

author: t1d
author_rank: Super Contributor
timestamp: July 16, 2024, 12:06:14 pm
content: To each and all, thank you for the really great replies and information. You have pointed out a lot of type-of-use considerations that I did not know about. Wonderful! I like to learn...Quote from: Conrad Hoffman on July 15, 2024, 11:48:45 amConrad's suggestion seems to be the way to go, for me. I think I will try this one, just for fun: Any tips, tricks, or pitfalls to know of?Thanks! Cheers!

author: Kleinstein
author_rank: Super Contributor
timestamp: July 16, 2024, 01:12:14 pm
content: Unless one wants really high overall gain, the 2nd stage gain should be relatively low (like x2 or x5). The first stage gain improves the CMRR of the 2nd stage. So one wants much of the gain in the first stage and not in the 2nd.The is a small trap for the young players: don't use an OP-amp that is not unitiy gain stable in the input stage. One usually has quite some gain for the differential part, but not for the common mode part. This is enough for the system to oscillate with OP-amps that are not unity gain stable (e.g. OP37 / LT1037, LF357).I would avoid trimmers where no needed, like in the first stage.

author: TimFox
author_rank: Super Contributor
timestamp: July 16, 2024, 04:04:25 pm
content: If you need to tweak the CMRR at high frequencies, you could add a small fixed capacitor across R1 and a trimmer capacitor across R2, or across R6 and R7+R9.However, I agree with Kleinstein's suggestion about high gain in stage 1 and lower gain in stage 2.

author: Conrad Hoffman
author_rank: Super Contributor
timestamp: July 16, 2024, 06:13:38 pm
content: That's the classic circuit and works well. Capable of high gain, but follow the above recommendations. A hint- signal to noise is always established at the first stage of an amplifier. You can never improve it after that, only wreck it, so pay attention to that first stage!

author: t1d
author_rank: Super Contributor
timestamp: July 17, 2024, 10:38:58 am
content: More great information from everyone! Thank you! See my replies to quotes in the individual posts, below.

author: t1d
author_rank: Super Contributor
timestamp: July 17, 2024, 11:07:19 am
content: Quote from: Kleinstein on July 16, 2024, 01:12:14 pmHi, Kleinstein! So great to have your expertise! I am still enjoying the e-load that you helped me design.It is interesting advice about having the gain in the first stage, because that is somewhat non-intuitive. One would think that high upstream amplification might clip the downstream amplifier.I do not understand why the design has such high gain. If we are looking at a signal source that needs 1000 times amplification, isn't it so far down in the noise floor as to be unmanageable (in terms of reading it, particularly with hobbyist instruments,) even with the built-in differentiation function?If the second stage amplification would be better at x2-x5, might a third stage be useful to bring it up to say x10, or x100?I would like to have some means to adjust the gain. A pot would introduce the problems that pots have - temperature instability, etc. Mechanically switching set gain brackets is simple and reliable, but may lack needed resolution. Digital pots seem to work around such issues and are doable, within my skill set. Well, what I could learn and employ... Thoughts?Thanks! Next...

author: t1d
author_rank: Super Contributor
timestamp: July 17, 2024, 11:11:56 am
content: Quote from: TimFox on July 16, 2024, 04:04:25 pmGreat, Tim! It is easy enough to add auxiliary footprints to the board and populate these components, at a later date, if needed/desired.

author: t1d
author_rank: Super Contributor
timestamp: July 17, 2024, 11:28:13 am
content: As I am just learning about this particular circuit, I could use help with the specifics of what components to add, or change, to improve the circuit with the groups suggestions. Calculations (for Tim's caps,) what to place where, etc...Here is an interesting video. But, I have not finished watching the series.

author: t1d
author_rank: Super Contributor
timestamp: July 17, 2024, 11:41:36 am
content: What about breadboarding a proof of concept with TL081s and a LM741?What about using an 8-pin 10K resistor array, for better matching and thermal coupling. Three of the resistors for R1,2&6 and the remaining five in parallel for R3/2K1-1%? And a dual array for R4&5/100R-1%?

author: Kleinstein
author_rank: Super Contributor
timestamp: July 17, 2024, 01:29:30 pm
content: A resistor array makes sense. It is only the 2nd stage to effect the CMRR. The first stage resistors are for gain stability only.Quite often the 2nd stage is with a gain of 1 and thus 4 equal resistors (e.g. 4 x 10 K to 50 K depending on the supply / voltage range). An array of 7 or 8 may be an idea for the gain of 2 or 3 case (e.g. CM range more important than CMRR).

author: t1d
author_rank: Super Contributor
timestamp: July 20, 2024, 05:35:14 am
content: EDIT: While not considering the circuit correctly, I posted several comments that were just plain wrong. I have removed those, to try to prevent further confusion. I will have to try again when my health has improved.

author: Terry Bites
author_rank: Super Contributor
timestamp: July 20, 2024, 08:48:08 am
content: A regular differential amplifier design, assuming an ideal opamp and discrete resistors, CMRR≈ 20* log((1+Av)/4t)Where t is the per unit resistor tolerance. That means that the best you could get with 0.1% resistors is about 54dB. That still assumes that the amplifier has infinite CMRR which a real world opamp does not. Using an array reduces drift but only provides a very modest improvement in CMRR. CMRR≈ 20* log((1+Av)/3t)Seewww.eetimes.com/the-effects-of-resistor-matching-on-common-mode-rejection.Discrete INAs are best avoided because of these limitations. The 3 opamp design is pretty much a just a text book thing these days. Still highy instructive though.Compare the price of precision parts, tweaky bits and a precision opamp with the price of a decent INA eg an INA849. No contest!Consider the practicalities such as power supplies, gain control and input protection/ coupling.Here are a couple more CMRR tweaks that can be applied to the 3 amp design.

author: t1d
author_rank: Super Contributor
timestamp: July 20, 2024, 05:41:28 pm
content: Thank you, Terry. Your conclusions that a chip would have better spec's and economies is quite true. However, there seemed to be a conscience that it would be difficult to select a particular chip, without a well defined stated use.I have briefly scanned the 849 DS. I do not generally work with high frequencies. So, maybe this chip will work for me as a general bench amplifier. It can't hurt to give it a try.I wonder if there might be a demo board PCB in the documents. I am sure that the board layout is critical.A battery power supply seems advantageous... What are the needed considerations for selecting a single, or split, power supply design?Thanks for your help and participation!

author: Kleinstein
author_rank: Super Contributor
timestamp: July 20, 2024, 09:23:48 pm
content: The simple CMRR estimate applies to a simple 1 stage difference amplifier. With the 3 OP-amp design the CMRR can get better by the gain of the initial stage. The input stage amplifies the differential signal, but not the common moder signal. With high gain even the separate build version can reach high CMRR.Which INA fits really depends. The INA849 is an example with very low voltage noise, but also very high current noise. So it may be good, but could also fail with high impedance source resistance.Especially with battery supply one has to watch the input voltage limitations. INAs are usually not rail to rail input, but may need quite some headroom. How much depends on the type and gain. Especially with the very low noise types like INA849 a high supply can cause quite some power consuption.

author: Terry Bites
author_rank: Super Contributor
timestamp: July 21, 2024, 12:17:34 pm
content: Quite so. There are loads of types, none of which are ideal. Horses for courses.I'm guessing that a bench amp would largely be used as a low gain differential to single ended converter.T1d needs to define general purpose of course.

author: t1d
author_rank: Super Contributor
timestamp: August 01, 2024, 04:38:57 am
content: Thanks, Kleinstein and Terry, for your continued help and support!It has been pointed out several times that I must state very specific needs/uses, in order to be able to select a particular all-in-one chip. I have supplied the details that I know. Admittedly, that information has been insufficient. I just don't have that knowledge base... yet. All I can say is that I don't generally work with super-high frequencies. I work with the typical MCU oscillator frequencies, and maybe, someday, I will advance to the transmission frequencies for Blue Tooth and WiFi, in support of the MCUs. As for all the perimeters that are a trade-off, one for the other, I would say to split the difference in performance. I do know that I would uses it for various sensors - such as a strain gauge, or an earthquake sensor.I intend to continue to work/learn/have fun, within the subject, by breadboarding the 3-OA design that I mentioned earlier. See LT1002 Data Sheet/Pg 10. I will substitute the op amps that I have in stock... TL081s for the LT1002s and a LM741 for the LT1037. If it fails, I will learn why. If it succeeds, I will add the additional suggested tweaks.I had asked about using split supply - the advantage/disadvantages. I still need those details, please. I understand the battery power considerations.

author: Kleinstein
author_rank: Super Contributor
timestamp: August 01, 2024, 09:30:56 am
content: Having a dual supply helps with DC signals. The alternative, especially with battery operation can be a virtual ground.The circuit in the LT1002 data-sheet is a bit unusual: normally there is less gain in the 2nd stage and more in the 1 st stage. The circuit is kind of an odd adaption to the relatively slow LT1002.The TL081 and LM741 are not really good performance and also rather high in supply current. So it would be only a curde test on the bread board.For a strain gage it usually is about lower frequency performance and low impedance. So a suitable choice for the input could be OP-amps like ADA4522, OPA388 or OPA205. There are also suitable ready made INAs for this, as DMS are a rather typical application.Many modern ADCs have a differential input and one may not need the 2nd stage of the 3 OP-amp circuit or would consider the fully differential type for the 2nd stage (mainly for a large CM voltage range or high speed).The ready made INAs can have some adventage when it comes to power consumption, as the input amplifiers don't need to be full amplifiers in the integrated version.

author: Terry Bites
author_rank: Super Contributor
timestamp: August 01, 2024, 10:24:15 am
content: The 741 only exists because people buy it by accident!

author: Phil1977
author_rank: Frequent Contributor
timestamp: August 01, 2024, 10:47:45 am
content: Quote from: Terry Bites on August 01, 2024, 10:24:15 amAnd as a reference design for fake ICs

author: t1d
author_rank: Super Contributor
timestamp: August 01, 2024, 01:35:55 pm
content: Thanks, Kleinstein. My replies are in the quote...Quote from: Kleinstein on August 01, 2024, 09:30:56 amLots to consider! Thanks, again!

author: t1d
author_rank: Super Contributor
timestamp: August 01, 2024, 01:40:31 pm
content: Quote from: Phil1977 on August 01, 2024, 10:47:45 amThanks, Terry and Phil. What other jellybean models (that I might have in my stocks) would be better for this use application?

author: t1d
author_rank: Super Contributor
timestamp: August 01, 2024, 01:50:58 pm
content: EDITED@Terry =Is this the proper summation of  suggested tweaks? PictureCaltshould be Potalt. Potalt should be a panel variable resistor configuration, for on-the-fly gain adjustment? Cb should be a static trimmer. What type should C1 be? A static trimmer cap, or varicap?... The symbol is for a varicap. Thanks!

author: David Hess
author_rank: Super Contributor
timestamp: August 01, 2024, 04:03:08 pm
content: Quote from: t1d on August 01, 2024, 04:38:57 amIf strain gauge sensors are used in applications extending to DC, like weigh scales, then precision operational amplifiers need to be used. In the past, this meant parts like the bipolar OP-07, and its many descendants like the LT1002, explaining why these part numbers show up in examples. These parts have low input offset voltage drift, high common mode rejection, and high open loop gain.In the past chopper stabilized operational amplifiers were problematical in strain gauge applications, (1) and the OP-07 types were preferred, but modern chopper stabilized parts like the ADA4522 and OPA388 mentioned by Kleinstein should be fine. For Texas Instruments, the bipolar OPA205/OPA206 is an excellent bipolar choice, and the bipolar OPA210 could be used for higher speed and better accuracy at higher frequencies.(1) Old chopper stabilized operational amplifiers had at least two problems in strain gauge applications. In the critical 0.1 to 10 Hz frequency range, chopper stabilized parts had higher noise than bipolar precision parts; they were only lower noise at lower frequencies. They also tended to suffer from intermodulation problems when exposed to external signals, which appeared as excessive drift; I am not sure why this was the case, but I am suspicious of the three operational amplifier instrumentation amplifier if the chopper clocks were not synchronized.

author: t1d
author_rank: Super Contributor
timestamp: August 01, 2024, 05:20:58 pm
content: Hi, David. It is wonderful to hear from you and to have you join in. As always, your comments are on-topic, concise, precise and helpful! Thanks! I will consider those models of OAs. I posted a chopper INA video link, earlier. You might find that interesting. Cheers.

author: Overspeed
author_rank: Frequent Contributor
timestamp: August 01, 2024, 06:25:12 pm
content: HelloAD serie as AD8421 are interesting and easy to solder as thar SOIC 8 and also not really costly , AD8428 allow 2000 gain and can be set in parallel for ultra low noise application .after could be interesting to list / crucial crucial parameter as offset value / gain / max frequency / .... a universal amplifier do not exist and at least need voltage divider or selectable gaina amplifier for a strain gage scale in the 1/1000 grams is not an amplifier for ballistic test strain gageRegardsOS

author: David Hess
author_rank: Super Contributor
timestamp: August 02, 2024, 07:40:24 pm
content: Quote from: t1d on August 01, 2024, 05:20:58 pmWhen I worked in the load cell industry many years ago, integrated instrumentation amplifiers had worse performance than precision operational amplifier solutions. That may have changed, but I have had no reason to look into it.Quote from: Overspeed on August 01, 2024, 06:25:12 pmI have always wondered about the ballistic application, if I am understanding you correctly. A ballistic application does not require the same level of accuracy, and increasing bandwidth while maintaining accuracy is straightforward, so never saw it as a difficult problem. Faster precision operational amplifiers can be used with more of them cascaded for greater gain-bandwidth product. Gain-bandwidth product of a single precision operational amplifier can be increased by adding fast gain within the feedback loop at high gains.Even in the past amplifier error was only a small part of total error. Strain gauges themselves, especially when used in a load cell, have inherent accuracy limits. I designed a super precision strain gauge amplifier for test purposes, but there would never be a reason to use such an amplifier in a real world strain gauge application.

author: Overspeed
author_rank: Frequent Contributor
timestamp: August 03, 2024, 11:12:19 am
content: Quote from: Overspeed on August 01, 2024, 06:25:12 pmI have always wondered about the ballistic application, if I am understanding you correctly. A ballistic application does not require the same level of accuracy, and increasing bandwidth while maintaining accuracy is straightforward, so never saw it as a difficult problem. Faster precision operational amplifiers can be used with more of them cascaded for greater gain-bandwidth product. Gain-bandwidth product of a single precision operational amplifier can be increased by adding fast gain within the feedback loop at high gains.Even in the past amplifier error was only a small part of total error. Strain gauges themselves, especially when used in a load cell, have inherent accuracy limits. I designed a super precision strain gauge amplifier for test purposes, but there would never be a reason to use such an amplifier in a real world strain gauge application.HelloIn ballistic application there is some applications for strain gage or piezo sensor_ rocket motor thrust : a 10 KHz sensor is far enough as that a quite slow event_ barrel pressure usual test as chamber pressure / :muzzle pressure : 20 KHz sensor is enough ( as Kistler piezo ) with a data acquisition rate from 60 KHZ to 100 Khz_ detonic or instability propellant combustion : 100 KHz with a 500 KHz data rate acquisition is the rulestrain gage limit ( metal ring test ) is in the 250 KHz max . Piezo sensor are in the 20 / 25 KHz maxBallistic tests amplifier setup main target is to measure single event in a vert short period of timeMy original setup what base on a INA 110 amp op instrument ( 20 years ago ) now I have improved the amplifier boards by using AD8421RegardsOS

author: David Hess
author_rank: Super Contributor
timestamp: August 03, 2024, 01:09:32 pm
content: Quote from: Overspeed on August 03, 2024, 11:12:19 amFor small strain gauge load cells, dynamic performance is limited by the load cell and anything attached to settling times of milliseconds, which works well with weighmeters which are synchronized to the line frequency to remove power line interference. Sample rates are typically 10 samples per second at most.I had some interest in measuring chamber pressure because I do reloading, but never pursued it beyond acquiring a Thomson Center Contender.

author: Overspeed
author_rank: Frequent Contributor
timestamp: August 03, 2024, 02:18:22 pm
content: Quote from: David Hess on August 03, 2024, 01:09:32 pmHelloFrequency response of the complete system is the key factor , for scale purpose not need ''fast system'' / high frequency responseIf ou plan to play with chamber pressure measurement a single gage ( 1/4 bridge ) in 350 Ohms or even in 120 Ohms fed with a good low noise voltage source ( ref02 powered by using a 9V battery ) and a INA 110 as instrument amp with a OP2205 low pass filter ( 50 Khz ) allow to do a lot of work with a setup close to ballistic lab with the terrible of Kistler piezo sensor120 Ohm are easier as resistor in 120 Ohm are far cheaper that 350 Ohm one .You can use a bridge with temperature compensation is you shoot several rounds without to let the barrel cool down ( skin temp = ambient temp ) but I prefer software compensation base on thermocouple measurement.Note : be aware of large range pressure reading because 1/4 bridge have linearity problem and don't compensate Poisson Coeff .RegardsOS

author: David Hess
author_rank: Super Contributor
timestamp: August 03, 2024, 05:18:52 pm
content: Quote from: Overspeed on August 03, 2024, 02:18:22 pmI was using that as an example of the fastest "scale" application which can be expected.What we were actually doing was improving the common moving weigh scale, which would be used on on a conveyor system and have a settling time of 10s to 100s of milliseconds. On these systems, the weight has to stabilize before a measurement can be made. I came up with a surprising easy analog way to measure the peaks of the exponentially decaying ringing after weight is applied, and produce an accurate measurement within milliseconds and way before setting occurred. I do not know if it was ever implemented, and today it would be done digitally with a fast high resolution sampling converter and DSP.QuoteI was thinking a full bridge with one pair of diagonal gauges mounted at 90 degrees for better stability. Then the standard modulus compensation gauge for the steel type could be used in series with the excitation for temperature compensation of the sensitivity.QuoteColumn load cells have limited but still respectable accuracy because of Poisson's ratio and they do not bother to compensate for it.I think a larger problem is that shock loads tend to damage strain gauges, so proper mounting is critical. The peak strain levels might also be too large for reliable operation.

author: Overspeed
author_rank: Frequent Contributor
timestamp: August 04, 2024, 12:26:38 pm
content: Helloballistic testing as mechanical testing by using strain gage is a bit far different than scale measurement which have been designed to use strain gage :That existing system under test whatever that a gun , a gearbox casing , a frame are not designed to make an easy and efficient installation of strain gage so generally room is limited , surfaces can be complex and non symmetrical and strain gage installation very tricky .If you take the case of a gun barrel ( chamber pressure ) so at the barrel tenon level and a quite simple part which a thick wall cylinder .1/4 bridge is the most easy to install . 1/2 bridge follow , full bridge could be done but on some gun as shot gun or even a TC under the tenon barrel there the lock part so fit a full bridge is impossible , same on a revolver cylinder .There is also the problem of the gage length ( effective length ) which is linked to the sensibility of the setup and cost as quality brand strain gage are costly , Chinese generic are cheap like dust but specification are? and they are not available in all size .I link a doc on non linearity of the strain gage bridgeRegardsOS

author: t1d
author_rank: Super Contributor
timestamp: September 09, 2024, 07:34:41 am
content: Just a quick note to say...1) I have had to move to another room in the facility, due to ventilation problems. That isslowingprogress on this project. Yep, moving twice is not fun.2) I am in the process of breadboarding the circuit. It is close to being finished and I look forward to playing with it. I will share what I learn.3) That I have had a huge break-through, in my understanding of the circuit... Woot. woot, woot!!! Specifically, the workings of the gain of both the front end and output amplification. With the new knowledge, I scanned back through this thread, to get a better understanding of your comments. Things are making more sense, now. Yea!4) The project is still open and ongoing.As said, I have a clearer understand of the calculation of the two gains - front end and output. The determination of the various resistor values seems to revolve around the value selected for R3/8 (in most tutorials, this resistor is labeled "R2", or "Rg" as Tim used.) And, where only having unity gain in the output diff amp stage would introduce the least amount of noise, the modest gain that Kleinstein suggested is necessary, if one desires to have a little control over the AC and DC Common Mode Rejection Settings.Do I correctly understand the need for this particular gain?Thanks for your continued interest, support and help. As Buzz Lightyear says: "To infinity and beyond!" <grin>