What is the use of True RMS-to-DC Converter IC? - Page 1
Forum section: electronics / beginners

author: matrixofdynamism
author_rank: Regular Contributor
timestamp: August 03, 2023, 12:09:48 pm
content: I came across a rather peculiar component when searching for something else. It is the AD737 "True RMS-to-DC Converter". The datasheet (https://datasheet.lcsc.com/lcsc/2304140030_Analog-Devices-AD737JRZ-5_C208624.pdf) does not give any applications apparently, maybe I am wrong. But really, why would someone need to use this type of IC? I can't think of any applications of it since I am a digital circuit person (FPGAs and Microcontrollers).

author: Siwastaja
author_rank: Super Contributor
timestamp: August 03, 2023, 12:17:39 pm
content: In multimeters where RMS values are desirable and only very low cost (for the accuracy) analog-to-digital converters with super slow sample rates are available, so that digital domain RMS calculation was impractical. Nowadays you can of course just sample at 10kHz or 100kHz or whatever and calculate RMS on digital domain, but multimeters are still manufactured using pretty old designs.On the other hand, usefulness of true RMS measurement in a multimeter is IMHO very much exaggerated. I have never found any use for the feature. I usually measure DC or sinusoidal AC, and when not, I want to see the waveform and/or more complicated math features and use a digital storage oscillosscope instead.

author: schmitt trigger
author_rank: Super Contributor
timestamp: August 03, 2023, 01:10:55 pm
content: Back in the day when accurately controlling large amounts of AC power required SCRs and phase control, a true RMS converter was necessary.Although phase control has fallen out of favor, one may still find distorted waveforms which require an accurate measurement.

author: tggzzz
author_rank: Super Contributor
timestamp: August 03, 2023, 01:57:52 pm
content: Quote from: Siwastaja on August 03, 2023, 12:17:39 pmNoise on a PSU rail or bench power supply output? Of course with ananaloguescope you can measure the RMS noise using the "tangential" techniquehttps://bestengineeringprojects.com/electrical-noise-measurement-technique/ or, especially, the Tek1502 battery chargerMemo to self: next time I have one open, try to estimate the crest factor.

author: matrixofdynamism
author_rank: Regular Contributor
timestamp: August 03, 2023, 02:10:30 pm
content: Do I understand correctly that these ICs are for the most part obsolete but still manufactured for some sort of backward compatibility?

author: tszaboo
author_rank: Super Contributor
timestamp: August 03, 2023, 02:13:34 pm
content: You can use it to do all kinds of AC measurements. For example impedance analyzers. ESR meters.It's easier on the digital side, more complicated on the analog side. Although doing high speed ADC measurements (accurately) is not easy either.Quote from: matrixofdynamism on August 03, 2023, 02:10:30 pmNot at all, they make it into new designs all the time.

author: bdunham7
author_rank: Super Contributor
timestamp: August 03, 2023, 02:46:32 pm
content: Quote from: matrixofdynamism on August 03, 2023, 02:10:30 pmAbsolutely not. Many if not most mid-to-high-end handheld multimeters sold today will have an AD737 or similar converter chip inside. The higher precision and much more costly AD636 was found in the recently discontinued Fluke 8845A and 8846A, among others, and is still used in some current competetive models. Bench meters seem to be switching over to digital sampling for AC as newly designed models come out, but these chips are all still in production and fairly widely used in products being made today.It isn't as easy to replace these TRMS chips with ADC-based solutions as it might seem. The better ones have usable BW of >1MHz and meters using them commonly have very good specified accuracies up to 100-300kHz. They also can deal with crest factors of 5 or 6, which means that an ADC solution would have to not clip until at least 4X the specified range. That means you need two more bits and at least 14-16 ENOB to get equivalent performance. So a replacement ADC-based system might need 18 bits at 1-10MSa/s or more to really provide equivalent performance, at least with a sequential sampling system and not subsampling.The place where I find TRMS to be the most useful and necessary is when looking at AC current in mains or mains-derived power systems. Although the voltage waveform at the socket and after transformers may be reasonably sinusoidal, the current waveform is often grossly non-sinusoidal and the RMS value of this is important to such considerations as transformer heating, wire sizing and fuse blowing. In a linear DC PSU the AC current through the transformer secondary will typically be a multiple of the DC average current at the output and having a TRMS meter allows you to measure this directly. TRMS current measurement can be important in many other situations as well, including things as simple as 'hmm, why is this breaker tripping?'

author: Terry Bites
author_rank: Super Contributor
timestamp: August 05, 2023, 04:15:37 pm
content: If there wasn't significant demand still, they wouldn't make them. They turn up in all kinds of instrumentation and control systems not just DMMs.There are more modern devcies that use digital techniques. eg LTC1966, and wideband analog types to over a Ghz.You'd likely find a few of those in every modern cell tower.

author: radiolistener
author_rank: Super Contributor
timestamp: August 06, 2023, 06:51:44 am
content: It is used everywhere where AC voltage measurement is required. For example it is used in DMM for AC RMS voltage measurement.Quote from: matrixofdynamism on August 03, 2023, 12:09:48 pmdigital measurement of RMS voltage requires pretty high ADC bandwidth (about 10 MHz and higher) and FPGA computational resources, such a digital solution will eat a lot of current and is not suitable for a low power mobile divices such as DMM.

author: Kleinstein
author_rank: Super Contributor
timestamp: August 06, 2023, 07:31:26 am
content: Quote from: radiolistener on August 06, 2023, 06:51:44 amThe wanted bandwidth depends on the application. For mains use there is no need to go very high in the BW. E.g. power meters use some 15 kHz BW. Some DMM with digital RMS only have some 1 KHz BW.Digital RMS can also use subsampling, especially with pseudo-random dithering in the sampling time and thus get a BW higher than the sampling rate. It is still true that the digital RMS solution tends to be higher power. No need for an FPGA - a moderate µC is sufficient and there are also integrated chips for power metering that include digital RMS in addition to the power readings.The analog RMS solutions also have there weak points:The BW goes down with lower amplitude - this leads to some nonlinearity toward small signals.Compensation for internal noise power is very tricky and this again a linearity limit for small signals.It needs a balance between response speed and true RMS performance to low frequencies (e.g. below 30 Hz). Usually the response, especially a step to 0 reading is a bit sluggish and much below mains frequency the response in no longer actually RMS.Nonlinear analog circuits also tend to be temperature sensitive and may show drift.With improved ADCs and µCs there is a tendency to use more digital RMS and less of those old style analog solutions for the lower frequency part.