True rms converters - Page 1
Forum section: electronics / general

author: Vgkid
author_rank: Super Contributor
timestamp: June 29, 2016, 05:11:40 am
content: Since I do nothave a stable ac voltage source I thought that I would build one using the schematic from the LTC1968. Since they are only a few offerings from LT/AD/Maxim. This would increase the odds that others would have dealt with them. My own multimeters(Fluke 87/34401) have these chips inside. There fore, they should be pretty good.That is not what I'm after. What I would like to know is how stable are these chips(day,months,yearly) drift rates, and what is the tempco of these units. I will be using a fixed frequency, and voltage.

author: tszaboo
author_rank: Super Contributor
timestamp: June 29, 2016, 08:36:30 am
content: I've used them. They are some 0.1% accurate. Not the most accurate things in the world, but for AC it is very good in fact. I definately prefer them over any heat transfer RMS or those pseudo RMS things from Analog devices. They do some delta-sigma magic in them, but that is OK. Very important, use a good quality foil capacitor on the output.

author: TiN
author_rank: Super Contributor
timestamp: June 29, 2016, 10:33:09 am
content: QuoteWhy so both ways? I think more detailed reasoning could help others to understand better too.

author: e61_phil
author_rank: Frequent Contributor
timestamp: June 29, 2016, 11:02:09 am
content: Quote from: TiN on June 29, 2016, 10:33:09 amI'm also interested in any answer to this question. I've bought a Fluke 540B thermal transfer standard for AC measurements some time ago..

author: ebclr
author_rank: Super Contributor
timestamp: June 29, 2016, 11:10:27 am
content: Another interesting circuithttp://www.linear.com/solutions/1594

author: tszaboo
author_rank: Super Contributor
timestamp: June 29, 2016, 11:13:27 am
content: Quote from: TiN on June 29, 2016, 10:33:09 amWell, they are simple to use. You only need to select one capacitor and scale the input voltage to 100-1000mV. For AD's solution you need a bunch of high value, stable capacitor which themselves could cost more than the IC.It is more accurate, i would say 2-5 times. And it is tiny. Price was also better. The old AD chips are like 20 euros. And then there is the fact, that it is differential input and output. Fits better to a system, and you can possibly avoid a level shifting, decreasing the errors. And it is linear technology.

author: MosherIV
author_rank: Super Contributor
timestamp: June 29, 2016, 04:50:28 pm
content: HiI am very new to the forum and I am also well out of my depth when it comes to precision stuff but......Surely any oscillator, now matter how precision, low noise, low thermal drift etc the op-amps and resistors are, the capacitors are the weakest things in the circuit as far as stability goes?Any oscillator based on RC oscillations is going to drift horribly over time and with temperature - or am I missing something?Surely, an Xtal or a silicone based oscillator is going to give better temperature and aging stability?What I am not sure about when using an Xtal is how much the aging of the Xtal is going to affect the amplitude of the oscillation.As far as I have seen, most silicone oscillators (or DDS) do not quote stability for the parts, so how good or bad are they for creating stable AC waveforms?I do not understand why 'heat transfer' is relevant to RMS voltage measurement. Heat transfer is a measure of power, not a measure of root mean squared of amplitude.Could someone explain please?I have been thinking about how to make an AC voltage standard for hobby use. So knowing the answer to these will be helpful.Thanks.

author: Kleinstein
author_rank: Super Contributor
timestamp: June 29, 2016, 06:58:30 pm
content: The heat transfer units compare the power of an AC voltage to the power of an DC voltage. In the application to stabilize an generator it does not matter if you keep the voltage or the power constant.There are also quite stable capacitors, so it is possible to build a relatively stable RC oscillator. Not as stable as a crystal one, but still reasonably stable (e.g. 0.1 %) in frequency. In the shown circuit the caps are only responsible for the frequency - the amplitude is set by the extra loop that uses the RMS converter. So the amplitude can stay stable not matter how much the caps are drifting. If you really need a stable frequency too, one could add PLL stabilization of the frequency. As the wien bridge type oscillator gives a good quality sine, there is no real need for a true RMS - simple full wave rectification could do the job as well.DDS chips might have a specification on how stable the DAC is. Together with the filter, this is what mainly sets the amplitude stability at low frequencies. For low frequencies like 50/60 Hz one might choose a separate DAC and µC/FPGA instead of an integrated DDS chip, and can thus chose a DAC with specified drift and maybe more resolution than typical low end DDS solutions. Still the filter could be a challenge. So even with a DDS generator one might chose the way of amplitude measurement and control loop.Crystal drift is usually quite small and thus less of an issue with such an reference voltage source.

author: barry14
author_rank: Regular Contributor
timestamp: June 30, 2016, 11:26:36 pm
content: The RMS voltage of a waveform, by definition, generates the same power in a resistor as a DC voltage of the same magnitude. Thus one way to build an RMS voltmeter is to compare the power dissipated in equal resistors by the input voltage and a variable DC voltage. When the power is equal (as measured by the temperature rise for example), then the DC voltage is equal to the RMS voltage of the input waveform. This method, which was used by Hewlett-Packard, is capable of wide bandwidths and is completely independent of the input waveshape.

author: johansen
author_rank: Super Contributor
timestamp: July 01, 2016, 06:46:31 am
content: Quote from: barry14 on June 30, 2016, 11:26:36 pmalmost sounds like something that might arrive in my email, given enough time.

author: MosherIV
author_rank: Super Contributor
timestamp: July 01, 2016, 12:23:52 pm
content: HiA big thank you to Kleinstein and barry14 for explaining things to me.I am still not sure that using power to measure RMS voltage is valid but in thinking about it, real world physics mean that even the integral (capacitor reservoir with bleed capacitor) method still takes current and therefore power.I had a look at how commercial units do it (Fluke 5200a).https://www.eevblog.com/forum/repair/fluke-5200a-ac-voltage-calibrator-teardown-and-repair/Sounds like they too use some kind of RC oscillator. Then amplify it and buffer through transformers. Finally rectify back to DC to compare with stable DC reference and use offset to adjust freq and voltage of original AC oscillator.This is more complicated than I would like to do in build my own AC reference.My concern with using Xtal is a) thermal and aging drift, even though I have found 10ppm partsb) how pure is xtal sine wavec) xtal p-p voltage is small - around 0.6Vp-p - amplifying this up 20 times - will this introduce noiseWill using a silicone based solution (eg the old 8038 oscillator) be as stable as an Xtal?It will produce larger output voltages. However, it is now obsolete. Not seen any replacement for it.DDS devices are too complex to use for my liking.Silicone oscillators all seem to be replacements for XtalLooks like it will have to be RC oscillator around an Op-Amp unless anyone has a better idea?

author: Kleinstein
author_rank: Super Contributor
timestamp: July 01, 2016, 04:13:14 pm
content: An RC oscillator will never be as stable as a XTAL when it come to frequency, but xtals are only fixed frequencies at about 32 kHz and up. So they are good to get a stable frequency for something like an DDS or PLL or a frequency counter. It depends on the oscillator circuit how pure or not the sine will be. In principle the Xtal can give a pure sine at that one frequency. The low amplitude and thus amplitude noise is not a problem, it's not that small.The ICL8038 or XR2206 RC oscillators give about a 1% distortion on the sine, so not very pure. Also stability in frequency and amplitude is not that great. The good thing is that it is easy to adjust the frequency over a large range. So for a reference source one would need stabilization in amplitude and maybe the frequency.The Wien bridge type oscillators are relatively difficult to adjust in frequency and usually need a amplitude stabilization even for basic operation. The sine quality can be really good (usually < 0.1 % distortion). So one would need amplitude stabilization and maybe frequency stabilization (with a PLL referenced to a XTAL).DDS is very stable in frequency (about as good as the XTAL / clock), good purity (better than 8038 but usually not as good as a good Wien bridge) and the amplitude can be quite stable (depending on the DAC). So one does not need a PLL and may get away without an amplitude regulation.For higher frequencies (e.g > 10 kHz) one may need the amplitude feedback or measurement, as it's difficult to impossible to make the amplifier the essentially zero output impedance at higher frequencies - so one would need some way to take the output impedance into account.For not so high frequencies, there is also the option to do RMS conversion with an ADC sampling the AC waveform and doing the RMS conversion in software. This is how high end DMMs line the 3458 do it. This way can be more stable than analog RMS converts. One might even use relatively cheap chips made for electricity meters for this purpose.

author: timb
author_rank: Super Contributor
timestamp: July 02, 2016, 12:48:12 am
content: Quote from: DiligentMinds.com on July 01, 2016, 04:17:14 pmThere's a few good Jim Williams app notes on this subject. He was really into thermal RMS conversion for awhile. In fact, LT actually made a thermal RMS chip! It contained the heater coil/NTC pairs and everything! Shame they no longer make the chip.

author: Gyro
author_rank: Super Contributor
timestamp: July 02, 2016, 08:45:31 am
content: There's a recent thread on thermal converters:https://www.eevblog.com/forum/metrology/diy-precision-ac-rms-to-dc-transfer-standard/msg920360/#msg920360

author: Vgkid
author_rank: Super Contributor
timestamp: July 07, 2016, 11:51:44 pm
content: Since it appears that a lot of meters use the AD637. How well is there short term stability?I plan on making a pretty stable source using one, a stable V-ref(possibly divided down, or ovenized), and the classic WBO*.**That is, if I don't get lucky with a 510A, currently 0 for 2.* Other techniques might be implemented, it is the easiest.** A poor mans fluke 510A...

author: David Hess
author_rank: Super Contributor
timestamp: July 08, 2016, 01:50:57 am
content: You do not need RMS conversion to stabilized the amplitude of a relatively pure sine wave source; rectification and averaging is enough and a bunch of the Linear Technology application notes show examples. RMS conversion is needed where the source waveform is undefined.Thermal converters have the advantage of dealing with wide bandwidths and high crest factors and it is a shame Linear Technology discontinued their LT1088 integrated thermal RMS converter.As far as the accuracy and drift of integrated RMS converters, check the datasheets.

author: ManateeMafia
author_rank: Frequent Contributor
timestamp: July 08, 2016, 11:15:07 pm
content: I know it defeats the idea of DIY, but if you want to buy a calibrated MJTC for a project, it can be bought from NIST.https://www-s.nist.gov/srmors/view_detail.cfm?srm=6002ahttps://www-s.nist.gov/srmors/view_detail.cfm?srm=6002b

author: Vgkid
author_rank: Super Contributor
timestamp: July 14, 2016, 12:46:14 am
content: I will most likely revisit this thread in the future. So anyone is welcome to post any related info in this thread.Why is this on hold...A 510A is in my future.

author: Vgkid
author_rank: Super Contributor
timestamp: July 21, 2016, 06:32:48 am
content: Looking into the spec sheets of both my 3456A, and 34401A. I noticed one thing, that the tempco's of the ac converters are rather large. 0.008%+60Counts (3456A) , and 0.005%+.004%(34401A). Interestingly enough the 34401A uses the ad637J, while the B/K variants are actually specced better.* Will need to look into the 34401A's manual more, I haven't had to fix that meter yet...Currently both are hooked up to my 510A...

author: Kleinstein
author_rank: Super Contributor
timestamp: July 21, 2016, 08:21:29 am
content: There is more than just the RMS converter chip that influences uncertainty in the AC ranges. It's also the amplifier / range switching part with frequency compensation parts. Further the specs are more like upper limits, not best estimates or even real estimates on how much the meters are really drifting. Sometimes the engineers are more conservative, as it takes more testing to give confidence to tighter specs, but more testing alone does nothing to improve the meters itself.