Op amp gain calibration setup advise needed - Page 1
Forum section: electronics / beginners

author: V_King
author_rank: Regular Contributor
timestamp: December 16, 2024, 02:11:46 pm
content: EDIT - question is about op amp gain calibration. I deliberately removed with this edit background information, because people are getting distracted with their answers. Question is about op amp gain calibration only. Thank you for sticking to the subject.----I am looking for a practical advise for a setup to calibrate op amp gain on very small board runs.The situation: the op-amp need 500x gain: 0-20mV input, 0-10V output. Output goes into ADC with 15bits of useable resolution.My two main questions are below, together with my thoughts:1. How to generate voltage reference? My thoughts - get a precision voltage reference IC, which would feed to a voltage divider and a buffer to output a stable voltage somewhere in a middle of a range, ie ~10mV. I understand engineers like absolute numbers, but I could see us working easily with random voltage within a required range, as long as it is stable, and then just multiply the voltage by 500, to know what voltage to look for in precision multi meter. For example 8.786mV in, x500= 4.393V out. Maybe there are off the shelf options for the voltage reference? I don't mind buying a "makers" version. Even if there is a drift, on a day we will be calibrating for those few hours the drift would be negligible?2. What multimeter to use? My thoughts: 15bit ADC input has a resolution of 0.3mV. So how many points do I need on a multimeter? Would 4.5 be enough, or do I need something higher? Farnell has good price 4.5 digit bench multimeters, with 0.1mV resolution at 5V input (https://uk.farnell.com/multicomp-pro/mp730889/dmm-bench-10a-1kv-50mohm/dp/3972198). That is 3x higher than ADC. I see Siglent has a 5.5 digit reasonable priced unit (Almost 3x though, compared to Farnell option). Would a cheaper one with calibration certificate be better than the more expensive one with more digits? We don't want to buy used equipment. Also, do test probes matter in such setups? Any other "traps for young players"?Thoughts relevant to the above are much appreciated, but wearing practical hat is important here. The setup above will just be used solely for the purpose above.

author: Overspeed
author_rank: Frequent Contributor
timestamp: December 16, 2024, 02:39:27 pm
content: HelloA gain of 500 ? is quite high but perhaps linked to your measurement or gage size / excitation voltage ? be aware of strain gage self heating.A important factor is the gain linearity and the bridge ''zero'' circuit so if that robust you can just calibrated on two point the ''zero'' no no load and the maximum load IF NO you need you need more points .You can calibrate the whole system as a scale by using a calibration weight OR the bridge by using a pre set resistor value to simulate the maximum load OR the amplifier by injecting a know voltageRegardsOS

author: mawyatt
author_rank: Super Contributor
timestamp: December 16, 2024, 02:56:41 pm
content: Maybe consider breaking the 500 gain into 2 op-amp sections, say 25 and 20. Dual precision op-amps don't cost much more than singles and trimming the overall gain with the 2nd op-amp should be much easier.Best

author: EC8010
author_rank: Frequent Contributor
timestamp: December 16, 2024, 03:05:52 pm
content: Surely all you need is an accurate 500:1 attenuator? Put that on the output of the amplfier and compare it to the input using any old meter. The accuracy of the source won't matter, nor the meter, just the attenuator. 0.01% resistors are readily available, so you should be able to make the attenuator quite cheaply. Job done.

author: Overspeed
author_rank: Frequent Contributor
timestamp: December 16, 2024, 03:19:57 pm
content: Quote from: mawyatt on December 16, 2024, 02:56:41 pmHelloI agree but bandwidth is also a parameters as a weighting scale and a supersonic blast generator for wind tunnel is not the mechanical systemI have used INA 110 and still use them with good results for mechanical testing up to 40 KHz with a low pass filter , the acquisition is a 16 Bits ADC at 500KHZRegardsOS

author: V_King
author_rank: Regular Contributor
timestamp: December 16, 2024, 03:23:13 pm
content: Quote from: EC8010 on December 16, 2024, 03:05:52 pmYou mean feed the amplifier output via 500:1 voltage divider into the amplifier input?

author: Kleinstein
author_rank: Super Contributor
timestamp: December 16, 2024, 03:52:44 pm
content: Ideally a DMS system should work ratiometric. So the reference for the ADC would be from the same source as driving the DMS bridge. So the ADC part often is kind of part of the system and only the combined gain is what matters. In many cases one would do the final calibration for the complete system anyway. So use test weights and the digital result and no DMM involved at all.When using a DMM as a reference the challanging part is measuring the low voltage of some 5 or 10 mV, so more the low level performance, not the percentage error in the best ranges.A voltage divider with known ratio for the input could be a solution, though this ideally still needs a check - maybe from resistance readings. The bridge input may need some extra care with the common mode voltage too. The DMM would read the voltage before the divider and after the amplifier.It depends on the system if the amplifer gain on itself is relevant or if only the combination of amplifier plus DMS drive part and ADC is relevent. This would than be more of a DMS replacement circuit (resistors and switchs) to test.

author: Overspeed
author_rank: Frequent Contributor
timestamp: December 16, 2024, 04:56:26 pm
content: HelloDepends what is the purpose , a calibration of ONLY the amplifier is perhaps not enough as there are several error factorLinearity of the amplifieramplifier offsetbridge zerobridge stability versus self heatingbridge resistor performances including tempcobridge itself linearity as a quarter bridge is far to be linearbridge voltage excitation voltage stabilityfrequency response as a DC calibration is not a pulse voltage calibrationa very important factor / parameter is the repeatability of the setupRegardsOS

author: nfmax
author_rank: Super Contributor
timestamp: December 16, 2024, 05:09:19 pm
content: Off the top of my head, I would look at using a known ratio voltage divider, such as the resistor array used when making a DVM (e.g. from Vishay) at the input of the OpAmp to be calibrated. The ratio doesn’t have to be 500:1, a 100:1 standard value would do. Feed it from a stable DC voltage, which doesn’t need to be accurately known, that gives a near full scale output from the OpAmp. Connect a 4-terminal bench DVM in voltage ratio mode, with the sense inputs connected to the OpAmp output, and the reference inputs connected to the divider input.That will give you a direct measurement of the DC gain of your OpAmp and the divider together: multiply by the known divider ratio to find the OpAmp gain.If you reverse the polarity of the input voltage and make a second measurement, you can measure and correct for the offset voltage, if necessary.

author: David Hess
author_rank: Super Contributor
timestamp: December 16, 2024, 06:06:24 pm
content: Quote from: V_King on December 16, 2024, 02:11:46 pmIs it gain of a single stage after the instrumentation amplifier, or does it include the instrumentation amplifier?When we did this, we used a load cell simulator to convert the excitation to a suitable precision voltage. If we did not have a load cell simulator, then we used a load cell with no load to generate a differential signal, and then added an offset resistor to the output to create a differential reference signal. For these I designed a precision 10 volt reference which could easily drive the excitation of a load cell, but any stable power supply could be used.My reference produced an output of 10.000000 volts with a 0 to 1 amp load regulation of less than microvolt, but this performance was a lot better than required.QuoteOur calibration stations used 4.5 digit multimeters, but these multimeters had 20 millivolt input ranges which is unusual now. That means using a 6.5 digit multimeter today if the smallest input range is 2 volts.The above is not the best way to do load cell and weighmeter calibrations. Better is to use ratiometric measurement of the load cell, and use the weighmeter itself as the meter because it should have better linearity than common multimeters.