Pro Analogue - Ensuring Accurate Measurements over Temperature - Page 1
Forum section: electronics / projects

author: dannybeckett
author_rank: Contributor
timestamp: October 24, 2016, 09:55:41 am
content: Hi Guys,This is quite an open ended question, and I'm being vague on purpose. Please no replies with "give more details of circuits" - there is no circuit at the moment.I have a project on which involves measuring the impedance of a material using a sinusoidal signal at 18MHz. I plan to feed a stable excitation signal into the material (known f & V), and then determine Z by measuring the current flow. I need to achieve great stability and accuracy over a wide temperature range (-20 degrees to +120 degrees). All of the electronics in the circuit are going to be exposed to this range of temperature. I don't want the analogue measurement to be affected by resistors changing value etc as ambient temperatures change.At the moment, current techniques involve profiling the measurement boards over temperature, then normalising the effect out via firmware look-up tables. What I'd like to achieve is a reading stable enough over temperature that this step is not required. And so, my question:To any experienced analogue design engineers out there, what techniques do you employ to ensure accurate measurements over temperature? My current thinking is as follows:> Achieve a rock-solid excitation frequency by using a low ppm xtal fed through a DDS> Use something like a 2.5V voltage-reference IC that's stable over temperature to ensure a stable excitation voltage> Currently unsure how to measure current with such a high excitation frequency, with stability over temperature. A very fast transimpedance amplifier with fast rectification and filtering?Any hints/tips/help you guys can throw my way is greatly appreciated.Dan

author: tszaboo
author_rank: Super Contributor
timestamp: October 24, 2016, 10:10:34 am
content: Please at least finish you sentences.

author: dannybeckett
author_rank: Contributor
timestamp: October 24, 2016, 10:12:18 am
content: Quote from: NANDBlog on October 24, 2016, 10:10:34 amWasn't it quite obvious I was mid-post and updated the thread by accident?

author: tszaboo
author_rank: Super Contributor
timestamp: October 24, 2016, 11:08:02 am
content: Quote from: dannybeckett on October 24, 2016, 10:12:18 am« Last Edit: Today at 11:41:54 by dannybeckett »« Reply #1 on: Today at 11:10:34 »

author: Kleinstein
author_rank: Super Contributor
timestamp: October 24, 2016, 04:04:08 pm
content: Operation up to 120 C is quite challenging. Fast DDS chips often run quite hot by them self. So operation at 120 C could be difficult. If it is a fixed frequency, one could directly use a 18 MHz crystal oscillator and maybe filters to go from square to sine.I would not count on a really stable amplitude of the generator. It might be better to do measurement of the sample and a reference resistor in a fast sequence. This case the main critical parts would be the switching, the reference resistor and the linearity of amplitude measurement. If need the amplitude one could do a loop to adjust the amplitude.If noise is critical I would consider phase sensitive rectification or an ADC to sample the 18 MHz signal. But again a fast µC to run at 120 C is hard.

author: voltsandjolts
author_rank: Unknown
timestamp: October 24, 2016, 04:38:23 pm
content: Sounds like a challenge and there will be errors, the question is what accuracy do you really need? You might need to do some experiments to determine that.Does it really have to be an 18MHz measurement?Try to keep the measurement ratiometric and don't depend on the stability of a voltage reference. Perhaps try to feed the driving signal to a known impedance and the test sample simultaneously, then measure the ratio of the unknown impedance to the known impedance. Then minimise the temp co of the known impedance.If you use an ADC, source the reference voltage from the known impedance measurement (should be fairly constant voltage) again to keep everything ratiometric. Temperature related offsets in the ADC should roughly cancel.

author: Kleinstein
author_rank: Super Contributor
timestamp: October 24, 2016, 05:47:36 pm
content: If possible if would consider using 13,x MHz instead of 18 MHz, there is a small range for industrial applications - so regulations on emission are less stringent there, but you have to accept a little more background.

author: Marco
author_rank: Super Contributor
timestamp: October 24, 2016, 06:22:21 pm
content: Couldn't you simply use a Schottky detector with a small current source load? Should be linear as far as I can see, the offset will not stable over temperature, but with online calibration who cares?

author: Cerebus
author_rank: Super Contributor
timestamp: October 24, 2016, 06:59:47 pm
content: You haven't given us any indication of your overall error budget. If you want a result to 5% you'll get a very different set of answers then you will if you want a result to 5ppm.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: October 24, 2016, 07:16:00 pm
content: Also besides error budget; cost budget, volume budget, production quantity; and how much we're getting paid for consulting on this.Tim

author: Someone
author_rank: Super Contributor
timestamp: October 24, 2016, 10:53:37 pm
content: Quote from: T3sl4co1l on October 24, 2016, 07:16:00 pmBut we can't pay for your time because that will use up the budget!On a slightly more serious note, having done this before you put all the measurement hardware in a less extreme environment (or even a temperature controlled room/space/volume) the cables to/from the DUT don't change with temperature enough to worry about.

author: dannybeckett
author_rank: Contributor
timestamp: October 25, 2016, 09:19:06 am
content: Cheers for the replies guys.Just to give you a little background - I'm trying to move away from profiling because with my experience this kind of circuit, the effect temperature has on the measurement changes depending on the current operating point of the device. This means that I've had to force the device to read 'x' via an impedance network, run it through the temp range, force to read 'x+10', run temperature cycle, force 'x+20' etc etc. Each temperature curve was different depending on what the device was made to read (worth noting at this point that the circuit was 100% analogue however). The implementation of this normalisation was a little tricky. If a newer, better designed circuit didn't exhibit this phenomenon, I'd be happy to keep the profiling step (involving only a single temperature cycle for the electronics to obtain their characteristics).So back to the thread -Kleinstein, Yeah 120°C is tricky. I may be able to re-specify the project to drop down the maximum ambient temperature to something like +110°C if I need to. In terms of phase-sensitive detection, I have had some experience in this area. Part of a project I designed a few years a go required the use of a lock-in amplifier. The frequency range I was dealing with was way different to the one I'm working with now though - 0.01Hz to 1MHz then, as opposed to ~18MHz now. I suspect the techniques for creating a lock-in at this frequency are quite different to the ones I employed previously. Any clues? I'm wondering about how to multiply your reference and signal accurately when they're at such high frequencies. Use an RF mixer? An ADC circuit that could operate at high frequencies would be really useful for this project. 100MHz+ sample rate would be great to achieve, however I'm only going to be using a PIC32MZ micro (252MHz core).Cerebus, ideally I'd like to keep within 1% (2% if this is going to prove too difficult) of the full range of measurement, whilst varying temperature from -20°C to +120°C. As mentioned - if the temperature normalisation of the sensors is simple (the effect of temperature on the measurement doesn't change depending on the load) then I'd be happy to obtain a profile and normalise in firmware also.voltsandjolts, 18MHz is important for the measurement to work - this is a part of the physical properties of the material we're measuring the impedance of (more specifically its dielectric loss).Marco, I've never considered a Schottky detector for this application. It's certainly something I could prototype and test, it may provide enough accuracy for my measurement.Two of you guys have mentioned ratiometric measurements, I think this a sensible path to explore.

author: JohnG
author_rank: Frequent Contributor
timestamp: October 25, 2016, 02:12:34 pm
content: It depends a lot on what impedance range you are trying to measure. Is the material lossy, low loss, or in-between. Is it mostly resistive, reactive, or in-between. How are you making contact with the material? Is it linear? Is the system exposed to outside interference?I'll second the question about accuracy as well.John

author: Marco
author_rank: Super Contributor
timestamp: October 25, 2016, 03:17:14 pm
content: Quote from: dannybeckett on October 25, 2016, 09:19:06 amMy thinking is that as long as you have a good stable current source load instead of a resistive load this should give you a constant offset regardless of amplitude. The only expensive components would be two low tempco resistors. Everything else relatively cheap.Then you just need a phase comparator, maybe a 4046? Dunno, the transition times seem a bit slow ... any asymmetry between high/low transition speed will hit the accuracy hard. There's the AD8302 and some ECL phase comparators, but not for automotive temperature range.

author: DaJMasta
author_rank: Super Contributor
timestamp: October 25, 2016, 05:04:08 pm
content: The first direction I'd look has already been hinted at... Do you really need the measurement circuit to be in the same conditions as the DUT? Like really, really?As far as I'm aware, that's always the best option of it's available. There are tons of high precision measurement devices that are shoved in a box with some form of climate control (insulated box, oven, etc.) and don't need particularly fancy designs because they keep most of the temperature swing (or other hazard) out.If that was viable, something like a double walled box and some heatsinks could let your device run long enough to take some readings if it didn't have to stay in the same environment, and if there are more normal ambient temperatures then normalizing thermals in the long term would only take a fan.Otherwise, if you can get your hands on some details for designing electronics for use in space, you may find some similar examples or explanations of the kind of issues that can crop up with extreme temperatures and maybe more importantly, extreme temperature swings.

author: Kleinstein
author_rank: Super Contributor
timestamp: October 25, 2016, 05:32:16 pm
content: If you want to go digital, there is no need to really have the ADC so fast. My guess is one could use a kind of sub-sampling. So take the 18 MHz signal and run the ADC at something like 16 MHz, but with a fixed phase relation to the driving clock. So it is the S&H stage that sets the speed, not the sampling rate.Phase sensitive detection in this range in using a kind of RF mixer. The advantage is that you get good linearity and usually a low offset and also the separation into an in phase an out of phase component if needed. This might be needed to compensate for parasitic coupling (which is hard to avoid) - so you could calculate back. Also having the phase could be important for measuring the sample to.The problem is more like building a fixed frequency vector network analyzer. My favorite type would be using a mixer with something like 17.5 MHz to bring down the signal to a more manageable range and use an ADC only than. For changing between the sample and a reference (likely also an capacitor) one might use a relay. One might also use a bridge setup, so that one only measures the difference between the sample and a reference capacitor. The good thing about the high frequency is that transformers are an real option. For generating the frequency one still has the options DDS, PLL or maybe just dividers in a fast µC (in you get one for that temperature).

author: Someone
author_rank: Super Contributor
timestamp: October 26, 2016, 02:49:19 am
content: Quote from: DaJMasta on October 25, 2016, 05:04:08 pmDown hole instrumentation for drilling/oil is the primary application for remote sensing in extreme hot environments, they have plenty of money to spend on consulting

author: nfmax
author_rank: Super Contributor
timestamp: October 26, 2016, 06:35:02 am
content: Quote from: Someone on October 26, 2016, 02:49:19 amNot any more, they don't!

author: dannybeckett
author_rank: Contributor
timestamp: October 27, 2016, 10:04:12 am
content: Unfortunately, the measurement device (& all electronics) are going to be installed in close proximity to the hot/cold material we're measuring! I did think about utilising a TEC module to control the temperature of the electronics, but it would be more hassle than it's worth. So yes, the measurement electronics will be reaching these temperaturesThe exact impedance of the material we're measuring is currently unknown to me (I know, I know), but a rough estimate is around 10-100M at low frequencies (1Hz to 1kHz) and approaching 1K at high frequencies (20MHz). It *may* be the case that I can lower the frequency of the measurement somewhat if it makes the design easier. I'm requesting an impedance analyser so I can first and foremost measure the precise impedance of our material under test, then calculate the frequency, sensitivity and dynamic range required of the measurement device.Cheers for the input

author: splin
author_rank: Frequent Contributor
timestamp: November 03, 2016, 10:13:53 pm
content: Quote from: dannybeckett on October 25, 2016, 09:19:06 amApologies - I meant to respond sooner but I got sidetracked and then forgot!You might want to look at the NXP LPC4370 series; 204MHz triple core ARM devices with a Cortex M4F plus two M0+ cores. It has lots of RAM and some interesting and powerful peripherals, including a very flexible reconfigurable serial interface peripheral, but its outstanding feature is an 80MSPS 12 BIT ADC - and all for less than $6. An external 80MSPS/12 bit ADC will probably cost more than double that on its own.On the downside they are ROM-less (use cheap QSPI Flash) and are only available in BGA packaging. They are rather slow running from serial Flash so you will need to execute high performance code from RAM. They are complex devices so potentially a lot to learn and I believe official support can be less than stellar, but Howardlong here has used them successfully - notably to implement a direct sampling radio receiver using that ADC and lots of DSP.If you were to use a high speed external ADC you would very likely have to use an FPGA to handle the data - I don't know of any microntrollers which can handle DMA GPIO to memory rates of 80MHz or more, though you might be able to interface a parallel ADC to a microcontroller with an external memory interface using a smallish CPLD or discrete logic.