Fully differential amplifier gain and mismatch trimming - Page 1
Forum section: electronics / projects

author: filssavi
author_rank: Frequent Contributor
timestamp: December 22, 2016, 11:12:40 pm
content: Hi, I'm in the process of designing an analog ADC frontend for a sensor, the signal is differential and weak, let's say 5 to 10 mVpp depending on sensor's sensitivity (sadly this sensor is specified as a design constraint and I can do nothing about that), since the signal is already differential my plan is to use a Fully Differential Amplifier (LTC6363) as an amplifierADC driver; to have the best signal possible at the ADC input my plan was to adjust the gain of this amplification stage, being this a low volume application I'm fine with having to manually tweak trimmers to get the desired gain.So is it ok if I use two trimmer in series with R1 and R2 so that I can set the gain as I want, also since I' need quite a strange ratio between the resistors that rules out COTS matched resistor networks, that leaves me with 2 open routes1) Get a custom one made from caddock Vishay or someone alike, but since I feel that it would be crazily expensive I have almost discarded the Idea, am I wrong it there any way to get custom precision resistor networks in low-ish volume (I can probably deal with a MOQ of 50-100 pcs not more) at a reasonable price (let's say 1-3€ each)2) just use normal .05 % resistors for R1 and R2, and a COTS matched network for R3 and R4, but since I'd like to avoid taking a 30 dB hit in CMRR (as for the datasheet) is it a good Idea to use trimmers to match the resistors value manually I know that this is a bit of a TempCo hell but could this work especially since this object wil work in a lab type environement (so T fairly constant when in use) and also I have a temperature sensor on board thus I could compensate for that

author: Conrad Hoffman
author_rank: Super Contributor
timestamp: December 23, 2016, 12:09:00 am
content: Best bet for fine tuning resistors is to use a bit higher value than you need, and then parallel it with a trimmer and high value series resistors as necessary to bring the value back down. Way more stable than putting anything adjustable in series with it.

author: KhronX
author_rank: Frequent Contributor
timestamp: December 23, 2016, 12:32:45 am
content: I was thinking something along the same lines. Maybe something like a 300k resistor parallelled with a 500k or 1meg trimmer, on one side, and a fixed 249k on the other?Quote from: Conrad Hoffman on December 23, 2016, 12:09:00 am

author: danadak
author_rank: Super Contributor
timestamp: December 23, 2016, 01:35:26 am
content: Some useful docs regarding CMRR.https://www.dropbox.com/s/plck7e95v7pw33c/CMR%20Analysis%20IA.pdf?dl=0Regards, Dana.

author: wraper
author_rank: Unknown
timestamp: December 23, 2016, 01:41:13 am
content: Do you really need differential output? Why don't just use instrumentation amplifier? And make differential output by other means if it is that important.Like here (programmable gain). But of course you could use parts where gain is set by resistors.

author: Marco
author_rank: Super Contributor
timestamp: December 23, 2016, 01:52:55 am
content: I've always found the use of matched resistors in differential chains and inamps questionable, it's not necessary to have resistor matching for CMRR. There are topologies to avoid it based on transconductance amplifiers, though I know of no fully differential amplifiers which do that.You could use Wraper's circuit with a MAX4208 though, 140 dB CMRR and you can trim the gain with a single pot.

author: wraper
author_rank: Unknown
timestamp: December 23, 2016, 02:01:33 am
content: Quote from: filssavi on December 22, 2016, 11:12:40 pmDo you really think it's a good idea to implement the hell of manual precision trimming at the production, with dubious stability in the future?Quoteif it was that simple.

author: wraper
author_rank: Unknown
timestamp: December 23, 2016, 02:10:38 am
content: Also, considering output voltage of the sensor, I'm somewhat dubious it will be happy with very low input resistance of your circuit.

author: exmadscientist
author_rank: Frequent Contributor
timestamp: December 23, 2016, 05:10:59 am
content: Sounds like a recipe for disaster. Fully differential amplifiers are not really precision devices, no matter what LTC likes to claim on the 6363's datasheet. They don't tolerate imbalances well at all, which isn't surprising but thedegreeto which imbalances cause issues may be. (TI has some good app notes on that.) They're also quite difficult to trim for good DC performance, as trimming circuits are themselves imbalancing and can get you into trouble fast if you're not careful.The LTC6363 is actually kind of an interesting device. It has good DC precision, but its bias current of up to 1 uA means it's not useful with high impedance sources. It has loads of gain-bandwidth (500 MHz GBP) but its slew rate can't back that up (75 V/us) and it's got a strangely low -3dB frequency (only 35 MHz). You'd think it'd be a fantastic general-purpose high-speed amplifier, but the low slew rate limits its utility and you don't end up getting much over the LTC6362. It's one heck of an ADC driver, though!The appropriate circuit for your use case depends a lot on your needed bandwidth. If your SPICE screenshot is accurate and your signal is at only 30 Hz, just throw your favorite instrumentation amplifier at the problem. You can follow it up with a FDA if you really need differential out. (Mind the DC bias currents, though.)

author: filssavi
author_rank: Frequent Contributor
timestamp: December 23, 2016, 08:00:03 am
content: Thanks to all of you who answered my question and since differential adc input is not really a necessity I think I will use a normal In-amp and a single ended converter, i could probably put up withquite a lot of manual trimming, since the device will not be mass produced, but I don't want to go down a massive rabbit hole trying to get something that is not really needed working well

author: David Hess
author_rank: Super Contributor
timestamp: December 23, 2016, 08:07:10 am
content: It sounds like you have a strain gauge or other low frequency bridge type transducer. Design considerations are significantly different for low and high frequency applications. If low frequency, consider something like an LT1124 configured as a 3 resistor differential amplifier. Other operational amplifiers will be better depending on the sensor's output impedance.If you have access to the bridge transducer's excitation, then the transducer's differential output can be converted to a single ended output by driving one of the excitation signals to balance the bridge producing a high CMRR without differential or instrumentation amplifiers and their matching requirements.The CMRR of the differential amplifier is not usually a problem. Gain error will convert some of the differential signal into a common mode signal but CMRR is still increased by the gain of the amplifier and later stages should not care.Like exmadscientist says, the Linear Technology differential amplifiers like the LTC6363 make great ADC drivers. Capacitive input fast sampling ADCs are very difficult to drive without degrading their AC performance and that is what these fast differential amplifiers are for.

author: voltsandjolts
author_rank: Unknown
timestamp: December 23, 2016, 08:29:12 am
content: If you have low bandwidth requirements and a 10mV Full Scale Output just connect the sensor directly to a sigma delta ADC. You can then do a digital calibration to sort out all the offsets in the system, no fancy trimming required. You say that you don't need many of these units, so maybe you could just use an ADC evaluation board with USB connection like this one:http://uk.farnell.com/analog-devices/eval-ad7194ebz/evaluation-module-adc/dp/2378293

author: filssavi
author_rank: Frequent Contributor
timestamp: December 23, 2016, 10:17:10 am
content: @David HessYou are exactly right the sensor in question is a load cell made with some sort of strain gage bridge arrangement (it's a commercial one, so I don't know what's inside), I was exploring the possibility of using a differential amplifier to drive the ADC but since that would be quite expensive and not that beneficial in the end I'll stay with the current working solution that uses a single instrumentation amplifier (AD8237) to provide the necessary gain and drive the ADC (which incidentally have it's own PGA, but since there is not enough gain i'll use it as a buffer and do all the amplification beforehand)

author: David Hess
author_rank: Super Contributor
timestamp: December 24, 2016, 02:17:50 pm
content: Quote from: filssavi on December 23, 2016, 10:17:10 amThe AD8237 is sort of a good choice. The important criteria are low offset voltage drift and low 1/f (flicker) noise. Based on this criteria, the LTC6363 is actually not a bad choice either but the AD8237 is better even if misapplied to drive a differential input.If your ADC has differential inputs, then an instrumentation amplifier is not the usual choice and the lowest cost option is to use a single ended non-inverting amplifier made from a precision operational amplifier and two resistors. This degrades the common mode rejection but a differential input ADC will not care and besides being less expensive, it also has lower noise because there are fewer active devices. Check out the 2nd to the last example on page 4 ofLinear Technology application note 43which discusses bridge circuits in general. Ground as shown is not actually ground but the other ADC input.For the highest performance when the ADC does not have a differential input, instead of an instrumentation amplifier I like the last example on that page but it requires access to one of the bridge's excitation signals. If the ADC has a differential input, then the 2nd to the last example using a real differential amplifier is better. I have done the later using a pair of operational amplifiers in the 3 resistor differential amplifier configuration.

author: wraper
author_rank: Unknown
timestamp: December 26, 2016, 12:07:49 pm
content: Quote from: David Hess on December 24, 2016, 02:17:50 pmThere are 2 issues with those. Second circuit requires flying capacitor which will cost as much as in-amp IC.The last one:QuoteYou cannot just directly connect output of the opamp to the strain gauge. So I don't see how those are more simple or cost effective solutions. Total cost unlikely to be lower than in-amp + diff-opamp solution.On the other hand, you can make ultra cheap strain gauge frontend based on AD8293. Although because of the fixed gain, additional amplification stage probably will be needed.

author: David Hess
author_rank: Super Contributor
timestamp: December 26, 2016, 02:28:56 pm
content: Quote from: wraper on December 26, 2016, 12:07:49 pmI was referring to second from the bottom on page 4 and not second from the top on page 3. It is too bad that the LTC1043 switched capacitor building block is so expensive.QuoteYou *can* connect the output of the operational amplifier to the strain gauge excitation if it has sufficient drive capability or if a high resistance strain gauge is used and I have seen this done, once even with 4 operational amplifiers in parallel, but in most practical implementations, a buffer will be used as shown in figures 5 through 10 and some others on the following pages.The requirements for the driving amplifier are not severe so a precision operational amplifier is not required. For instance operating in inverting mode does not require a high common mode rejection ratio.Despite the cost, this one is the least expensive of the highest performance solutions. The alternative I have used in the past is basically the one you suggest with the AD8293 except with a 4 operational amplifier low noise chopper stabilized differential preamplifier preceding the instrumentation amplifier. This has the only virtue of not needing to drive one of the excitation lines which will may not be possible in some applications.QuoteThe AD8293 looks like a nice medium precision alternative.

author: filssavi
author_rank: Frequent Contributor
timestamp: December 26, 2016, 04:54:38 pm
content: the AD8237 is used in a single ended fashon since the ADC supports both SE and diff inputs (4 Diff 8 SE, since doing a completely differential signal chain is somewhat difficoult and frankly out of scope for this project, I reverted to single ended ADC, that simplifies also greatly the system design since I can eliminate an external mux and use the input channels provided from the ADC.As for the AD8293 it is a really nice IC but unfortunately it has the disconfort of having fixed gain, it could probably be salvaged if it had a higher gain (like 4/500) since I could overamplify and then attenuate to get the range I want, but unfortunately it's gain is so low that even with the most sensitive load cell I have it still isn't enough to saturate the adc Input range so that as you said would need further amplification that would add noise, and I'd like to avoid it if at all possibleI also saw the switched cap circuit, apart from being quite expensive (I could probably get away with if it was the only choice), the real problem is that having never worked in practice with SC circuits (I studied them at uni but that's it) and I really don't want to bite more than i can chew

author: wraper
author_rank: Unknown
timestamp: December 26, 2016, 05:18:04 pm
content: Quote from: filssavi on December 26, 2016, 04:54:38 pmAs if you would set higher gain on in-amp itself, there would be less noise. Actually it is less noisy than AD8237, specified figures are before they get amplified, after all. Additional opamp won't add any significant noise by itself, as you would amplify only a few times, with a quiet large signal on the input. I use them connected directly to ADC BTW, 2mV/V load cell powered from 5V and about 1.6V reference voltage works perfectly fine for my non super precision needs (G160 version). The fact is, I get complete load cell frontend for less than one buck.