Diffrential amp design - Page 1
Forum section: electronics / projects

author: ali_asadzadeh
author_rank: Super Contributor
timestamp: January 23, 2020, 07:30:41 am
content: Hi,I have an old design and it's working perfectly, it's used in an Electromagnetic Flow Meter, the input sensor differential potentials is in uV ranges.Now I want to redesign it with newer and cheaper parts, because I want to increase the performance, and use lower priced parts, also It would be very beneficial if i could do it in a single supply config and also If I could do it fully differential and remove the GND from the input sensor, currently I'm using AD620 part, But for the new design I prefer using normal op-amps or use Chinese parts! any Ideas are highly appreciated. If I could do it under 1$ for total op-amps it would be perfect.

author: moffy
author_rank: Super Contributor
timestamp: January 23, 2020, 08:10:17 am
content: Because of the micro volt signal level, I would go with a true differential amp as your AD620, or risk getting swamped by low cmrr. But you don't need two, the second can be just a straight opamp.

author: Zero999
author_rank: Super Contributor
timestamp: January 23, 2020, 08:38:43 am
content: Quote from: moffy on January 23, 2020, 08:10:17 amYes, it's not possible to do this with a cheap op-amp, not without spending more on precision resistors. Accurately measuring microvolt signals is incompatible with low cost.

author: magic
author_rank: Super Contributor
timestamp: January 23, 2020, 08:43:14 am
content: You can eat your cake and have it too: buy AD620 from AliExpress

author: ali_asadzadeh
author_rank: Super Contributor
timestamp: January 23, 2020, 08:57:32 am
content: we have made a lot progress in the industry, why stick to some old and pricey tech!Can a few these zero-drift Precision OpAmps come in handy? Like GS8333https://lcsc.com/product-detail/Precision-OpAmps_Gainsil-GS8333-TR_C431326.html

author: moffy
author_rank: Super Contributor
timestamp: January 23, 2020, 09:37:30 am
content: Quote from: ali_asadzadeh on January 23, 2020, 08:57:32 amThe problem is not the opamp, but if you do it without a diff amp it's the precision and cost of the resistors over temperature to get your CMRR to good levels. They tend to be more expensive than the true diff amp.

author: OwO
author_rank: Super Contributor
timestamp: January 23, 2020, 09:53:42 am
content: I looked for cheap instrumentation amplifiers a while ago and didn't find any. There are probably 10 different Chinese op amp manufacturers and you can find everything from 500MHz op-amps to low offset op-amps, but it seems no one has ever heard of instrumentation amplifiers. I think it's probably because trimming the resistors is expensive. Some of the datasheets even suggest you build an instrumentation amplifier using op-amps and discrete parts. It depends what kind of price break you are getting on the AD620. If it's close to digikey prices then it's still far cheaper to do it with discrete op-amps and 0.1% resistors. Do the most obvious thing and keep all of the gain in the first stage. This way your total CMRR is the second stage CMRR multiplied by the gain.

author: ali_asadzadeh
author_rank: Super Contributor
timestamp: January 23, 2020, 10:10:37 am
content: Quote0.1% resistors are super cheap right nowSome values for referencehttps://lcsc.com/product-detail/Others_Viking-Tech-AR03BTCX1000_C319953.htmlhttps://lcsc.com/product-detail/High-Precision-Low-TCR-SMD-Resistors_100KR-1003-0-1_C122538.html

author: Warhawk
author_rank: Frequent Contributor
timestamp: January 23, 2020, 10:32:09 am
content: Quote from: magic on January 23, 2020, 08:43:14 amYeah, 100% genuine

author: Zero999
author_rank: Super Contributor
timestamp: January 23, 2020, 12:14:44 pm
content: Quote from: ali_asadzadeh on January 23, 2020, 10:10:37 amNo, that's not good enough. The AD620 has a CMMR of 130dB, which is much better than you'll get with 0.1% resistors.The circuit you posted at the start of the thread is nearly twice as expensive as necessary. The second AD620 is being used as a non-inverting amplifier and should be replaced with ordinary op-amp. Keep the AD629 for the first stage and increase the gain so the second stage can be unity gain. This will both improve the performance and reduce the cost.To reduce cost further, you might be able to omit the second stage, by reducing the impedance of the first filter, although that would require larger capacitors, which cost more. There's also no point in using 1% E96 resistor values such as 169k and 30k1 for the filter, because the capacitors probably have a wider tolerance than 5%. Use the nearest E24 values: 160k and 30k.EDIT: The total gain of your circuit is 71.3k so it will need more than one amplifier. Two amplifiers with a gain of roughly squareroot of the total required gain, will do.

author: SiliconWizard
author_rank: Super Contributor
timestamp: January 23, 2020, 02:08:16 pm
content: Quote from: Warhawk on January 23, 2020, 10:32:09 am

author: technix
author_rank: Super Contributor
timestamp: January 23, 2020, 05:19:36 pm
content: There might be some ADC with true differential inputs and has enough resolution natively. If that is the case, scratch the whole front end and have your sensor hooked directly to that ADC and call it a day. Maybe put a can over that, but anyway use as short an analog section as possible.As long as the noise is under control, systematic biases and nonlinearity can be trimmed out afterwards in software. You don't even need to know exactly what kind of nonlinearity you may have, just assume the inverse transfer function is a polynomial (actually the first few terms of the function's Taylor series, but that is usually enough since you have a limited range anyway) and grab a few calibration points so you can work out the coefficients.

author: Marco
author_rank: Super Contributor
timestamp: January 23, 2020, 07:01:37 pm
content: Quote from: OwO on January 23, 2020, 09:53:42 amThat's no excuse, the transconductance based differential amplifiers like MAX4208 don't rely on resistor matching.

author: MasterT
author_rank: Frequent Contributor
timestamp: January 23, 2020, 07:20:04 pm
content: Quote from: Marco on January 23, 2020, 07:01:37 pmProbably, they can't produce instrumental amp due copyright issue.

author: Zero999
author_rank: Super Contributor
timestamp: January 24, 2020, 12:32:24 pm
content: Quote from: Marco on January 23, 2020, 07:01:37 pmThe MAX4208 is very noisy compared to the AD620, around ten times as bad, so completely unsuitable for μV level signals.https://datasheets.maximintegrated.com/en/ds/MAX4208-MAX4209.pdfhttps://www.analog.com/media/en/technical-documentation/data-sheets/AD620.pdf

author: Marco
author_rank: Super Contributor
timestamp: January 24, 2020, 04:29:48 pm
content: That's not really the point, noise is not inherent in the architecture, AD8129 has 4.5 nV/√Hz for instance ... the architecture can target any niche in-amps are targeted at. So if Chinese manufacturers want to build differential to single ended amplifiers without trimming, it's there.

author: ali_asadzadeh
author_rank: Super Contributor
timestamp: January 25, 2020, 07:03:18 am
content: I know that we should do something,if we do not do something, we are just limiting ourselves to the high tech giants to drink the juice out of our pockets with out dated parts at high costs!

author: Zero999
author_rank: Super Contributor
timestamp: January 26, 2020, 11:34:29 am
content: Quote from: MasterT on January 23, 2020, 07:20:04 pmSince when has that bothered Chinese companies?Quote from: Marco on January 24, 2020, 04:29:48 pmI admit, I'm not familiar with transconductance based differential amplifiers. Do they have high input current noise? The AD8129 has ten times the input current noise, of the AD620, at high frequencies: 1pA/√Hz vs 0.1pA/√Hz at 20kHz and much higher 1/f noise, so at 10Hz, so nearly 20pA/√Hz vs just over 0.2pA/√Hz. I know the AD8129 an RF part and not optimised for low frequency operation like the the AD620, so we're not comparing like with like.Do you know of a part more suited to the original poster's application?Quote from: ali_asadzadeh on January 25, 2020, 07:03:18 amWhat do you think we need to do?The semiconductor manufacturers are not ripping us off. Some things, such as low noise, low offset, high CMMR, high accuracy, DC amplifiers are technically difficult to achieve and are costly to make. New technology has made many things cheaper, such as achieving higher speeds, more complex designs on the same piece of silicon, but not everything has progressed at the same rate. Basic laws of physics can often get in the way, which gives us diminishing returns in certain areas such as low noise design.I've already told you how you can nearly half the cost of your design: replace the AD620 on the right hand side of the drawing with a cheaper, ordinary op-amp.EDIT: Corrected decimal point on resistor value, in schematic.

author: 741
author_rank: Frequent Contributor
timestamp: January 26, 2020, 07:48:24 pm
content: "INA333 Micro-Power (50μA), Zerø-Drift, Rail-to-Rail Out Instrumentation Amplifier"

author: Zero999
author_rank: Super Contributor
timestamp: January 26, 2020, 08:10:32 pm
content: Quote from: 741 on January 26, 2020, 07:48:24 pmIt's noisy though, compared to the AD620 at 50nV/√Hz, vs 9nV/√Hz, which I think could be an issue as they're dealing with µV level signals.

author: MasterT
author_rank: Frequent Contributor
timestamp: January 26, 2020, 10:00:47 pm
content: Quote from: Zero999 on January 26, 2020, 08:10:32 pmWhat I see, ad620 noise goes up to 300 nV in the sub-Hz frequency. Application has filter at 0.4 Hz bandpass, so only this band matters. I'd agree that AZ OPA is better option in this case, since noise flour stays flat for them. Logically, AZ circuitry just "null out" low frequency noise and becomes a winner.

author: Zero999
author_rank: Super Contributor
timestamp: January 26, 2020, 10:54:48 pm
content: Quote from: MasterT on January 26, 2020, 10:00:47 pmThat's what you might think, but the data sheets say otherwise.https://www.analog.com/media/en/technical-documentation/data-sheets/AD620.pdfhttp://www.ti.com/lit/ds/symlink/ina333.pdfInput voltage noise from 0.1Hz to 10Hz is 1μVPPfor the INA333 and 0.28μVPPfor the AD620, at gains above 100, which the original poster's design has. The only advantage the INA333 has, noise wise is lower input current noise at 2pAPPvs 10pAPP, so might be better if the input is high impedance, but it's only rated to 5.5V single supply and the original posters design is +/-5V.

author: GigaJoe
author_rank: Frequent Contributor
timestamp: January 27, 2020, 12:24:07 am
content: I'm wonder ... if alternatives was observed ... as an example from the first page of AD620 PDF ?like AD8221 ?or replacement by regular 3-opamp schematics, with much wider choice for a regular opamp ...

author: MasterT
author_rank: Frequent Contributor
timestamp: January 27, 2020, 12:30:32 am
content: Quote from: Zero999 on January 26, 2020, 10:54:48 pmThe problem with DS is that there is no 0.01 - 1 Hz line, and looking into similar IC from TI TLV333 , where noise spec is split into 0.01 - 1 and 0.1 - 10 ranges, I see 0.3uV and 1.1uV, that's more than x3 times.Right, it's just analytics, but if I go father into <0.5 Hz than difference with ad620 where line is sharply rising would be more obvious. Theoretically, I don't know if China have any part numbers, very unlikely anyway.Regarding voltage, OP was going to redesign circuits and why keep same voltage?

author: OwO
author_rank: Super Contributor
timestamp: January 27, 2020, 02:50:20 am
content: Oh I didn't realize it's using a gain > 100. In that case a discrete design with 3 opamps can achieve >100dB CMRR easily because the first stage provides 40dB of common mode rejection.

author: ali_asadzadeh
author_rank: Super Contributor
timestamp: January 27, 2020, 06:27:17 am
content: Thanks for the good feedback, I want to replace it completely, so I'm not bond to any part or voltages, Just something came to my mind,Can I trough it all out and replace it with a decent 24Bit differential ADC, with an internal gain of 128 or more? something used for wight scales but with better common mode rejection. since I just want to measure a uV signal between two probes technically.I want to achieve a good reading, either trough discrete op-amps, or better AMPs or another ADC, for Now I'm using AD620 and LPC1758 ADC to read the values, I know the LPC1758 ADC sucks, and it's not good enough, But we have made lot's of progress in recent MCU specially from ST and NXP,

author: OwO
author_rank: Super Contributor
timestamp: January 27, 2020, 06:38:20 am
content: depends on your requirements:- CMRR- Gain- Noise floor (input referred)- Input common mode voltage range- Input differential mode voltage range- Input impedance- BandwidthIn some cases cheap opamps will do. I don't think abusing the differential input of an ADC is a good idea though. I've never seen CMRR specified on an ADC. I also haven't seen an ADC (apart from audio ADCs) that have that much built in gain.

author: Evan.Cornell
author_rank: Regular Contributor
timestamp: January 27, 2020, 06:26:47 pm
content: Quote from: OwO on January 27, 2020, 06:38:20 amA specific use-case, but ADS1232 (http://www.ti.com/product/ADS1232) has built-in gain of 128. OP hasn't said bandwidth required, so I have no idea if this would be suitable for the application or not.

author: Zero999
author_rank: Super Contributor
timestamp: January 27, 2020, 09:37:54 pm
content: Quote from: OwO on January 27, 2020, 02:50:20 amSorry, I can't see how, without using very well-matched resistors, which would be expensive, unless there's something obvious I've missed.Quote from: MasterT on January 27, 2020, 12:30:32 amYes auto-zeroing amplifiers theoretically have no 1/f noise below a certain point, but they do have more noise at higher frequencies, thanks to the interference from the oscillator. The data sheets don't seem to be showing it though, because as you said, there's no 10mHz to 1Hz data.Quote from: ali_asadzadeh on January 27, 2020, 06:27:17 amWhat's wrong with the circuit I posted? It did get two likes after all.You could replace the AD620, with the INA821, which is better in nearly every way and slightly cheaper, according to Mouser. It also has the same gain equation, so the resistor values wouldn't need changing.http://www.ti.com/lit/ds/symlink/ina821.pdfI can't say much about the ADC, as it's not something I've done before, but having a stable, low noise, voltage reference is very important.

author: ali_asadzadeh
author_rank: Super Contributor
timestamp: January 28, 2020, 06:32:00 am
content: QuoteThe Sample rate of the signal is around 500SPS and the signal is an alternating signal in the 30Hz range, it has a shape like thisQuoteIt's a good Idea and thanks for sharing it, But maybe we could achieve something cheaper at the end of the day.

author: Zero999
author_rank: Super Contributor
timestamp: January 28, 2020, 11:32:14 am
content: What's your budget? How many are you making? Generally for small volumes, it often works out cheaper, to use more expensive components, due to reduced design and assembly times.As I said above, the INA821 is cheaper, than the AD620 and better too.

author: SiliconWizard
author_rank: Super Contributor
timestamp: January 28, 2020, 07:22:53 pm
content: Quote from: Zero999 on January 28, 2020, 11:32:14 amAgreed.Quote from: Zero999 on January 28, 2020, 11:32:14 amYep, it's less than half the price, has better noise and BW specs, half the power consumption, lower input bias current... looks like a no-brainer here?

author: Marco
author_rank: Super Contributor
timestamp: January 28, 2020, 08:19:16 pm
content: INA821 seems like a no brainer, even with the BJT input it's not that far off from integrated JFET solutions. Cheap enough and you aren't going to do substantially better without going discrete.

author: ali_asadzadeh
author_rank: Super Contributor
timestamp: January 30, 2020, 06:28:36 am
content: QuoteWe will produce 20K units, so the lower the price the better.

author: Zero999
author_rank: Super Contributor
timestamp: January 30, 2020, 08:07:34 pm
content: Quote from: ali_asadzadeh on January 30, 2020, 06:28:36 amAccording to Digi-Key, the INA821 is $2.489 each, vs $5.60 for the AD620, in quantities of 2500.https://www.digikey.com/product-detail/en/analog-devices-inc/AD620ARZ-REEL/AD620ARZ-REELTR-ND/617756https://www.digikey.com/product-detail/en/texas-instruments/INA821IDR/296-INA821IDRTR-ND/10448327Use the circuit I posted previously with the INA821, instead of the AD620. The op-amp doesn't have to be anything special, because the common mode rejection and raising the signal well above the op-amp's noise floor, is done by the INA821. If you have a higher voltage rail than 5V available or can live with the output being limited to +3V, the OP07 is a reasonable choice and is cheap too. The output can be helped up closer to +V by using a pull-up resistor or current source (this can be as crude as a single PNP transistor), at the cost of extra parts and current draw.https://www.digikey.com/product-detail/en/texas-instruments/OP07CDR/296-14626-2-ND/555699NOTE:The current source is pretty crappy, because it relies on the transistor's hFEwhich is not a well controlled characteristic and is temperature dependant, but that doesn't matter, since all that's required is a pull-up of around 1mA or so.I chose the BC857B over just the BC587, because its gain can only range from 180 to 460, rather than 120 to 800 and it was the cheapest PNP BJT on Digi-Key, at a cent each.https://www.digikey.com/product-detail/en/rochester-electronics-llc/BC857B/2156-BC857B-ITTR-ND/11512248Of course if you're buying in quantities of 20k, then you want to go directly to the manufacturer, rather than a distributor, such as Digi-Key.

author: ali_asadzadeh
author_rank: Super Contributor
timestamp: January 31, 2020, 11:52:34 am
content: Thanks Zero999, so the BJT would allow for a 5V output swing?

author: Zero999
author_rank: Super Contributor
timestamp: January 31, 2020, 07:51:25 pm
content: Quote from: ali_asadzadeh on January 31, 2020, 11:52:34 amIt won't allow the output to completely reach 5V, but it'll get within a couple of hundred mV or so. Obviously it won't be able to swing to -5V. Oh and don't bother simulating this trick. It invariably doesn't work with most SPICE op-amp models because they don't accurately replicate the behaviour of the output stage.That raises another point: what's the input voltage range of the ADC? Presumably it can't go below zero, yet both the circuit you posted at the start of the thread and mine will output negative voltages.

author: ali_asadzadeh
author_rank: Super Contributor
timestamp: February 01, 2020, 06:02:05 am
content: The ADC input range is 0-3.3V

author: Zero999
author_rank: Super Contributor
timestamp: February 01, 2020, 01:51:45 pm
content: Quote from: ali_asadzadeh on February 01, 2020, 06:02:05 amOk then the PNP pull-up isn't needed, but what happens when the op-amp's output drops below zero?

author: ali_asadzadeh
author_rank: Super Contributor
timestamp: February 02, 2020, 06:25:00 am
content: There is a Reverse Biased Diode parallel to the ADC input