Designing Multiplier-Divider Circuits - Page 1
Forum section: electronics / projects

author: polarKaung
author_rank: Contributor
timestamp: July 27, 2022, 03:40:19 pm
content: Hello,This is a design question. In my project I need to be able to measure RMS without MCUs. I cannot also buy chips like AD637 or 636. We don't have them here. So, I decided to make my own. So, I landed on a topology that uses multiplier-divider circuit. Now, I can't buy multiplier chips as well. So I also need to design them as well. I do understand how they work, with log-antilog topology, also the math and stuffs. But I don't know how to adjust the dynamic range. My input signal to the multiplier is +- 15V sine wave.Any resources to help design these types of multiplier. The topology I found can be read in this article.https://www.digikey.com/en/articles/understand-how-to-effectively-monitor-signal-voltage-and-powerThanks a bunch.

author: moffy
author_rank: Super Contributor
timestamp: July 27, 2022, 04:45:22 pm
content: The MC1496/LM1496 is a classic Gilbert cell based multiplier which is composed of a relatively small number of transistors. The trick is to linearise the carrier input port and deal with offsets. The other way is as you mentioned log/antilog circuits, look for Jim Williams app notes, National Semiconductor app notes but you will need good matched transistors but you can use LM3046 in a pinch. But typically with log/antilog you need specialised temperature compensating resistors.

author: Kleinstein
author_rank: Super Contributor
timestamp: July 27, 2022, 05:00:56 pm
content: Building an RMS circuit from scratch (without special chips) is quite tricky, especially if good accuracy is aimed for. There is the option to build essentially the same circuit as in the TRMS chips from other parts, like discrete transistors / transistors arrays.The internal circuit of the AD636 and similar would be a starting point. Doing a simulation first is a good idea, though the more ideal parts make things easier.How critical / difficult the gain switching is depends on the required BW and accuracy. It can take quite some effect to use well compensated dividers with suitable capacitors in parallel to the resistors.The circuit has 3 parts that can be tested relatively separate: the input divider (maybe with gain switching), the rectifier (if the log anti log circuit is used) and the muliplier / divder.A frist point is finding the requirements (bandwidth and accuracy) and if actuall true RMS is needed.Just buying a relatively cheap Chinese DMM may be a real option.

author: polarKaung
author_rank: Contributor
timestamp: July 27, 2022, 05:37:18 pm
content: The application that I wanted to use that for is to process mains ac voltage. First, step it down and read rms and send that value to mcu adc.

author: Zero999
author_rank: Super Contributor
timestamp: July 27, 2022, 05:40:19 pm
content: It smells like homework.Check out the following thread. Please include a link as a reference in your write-up.https://www.eevblog.com/forum/projects/poor-mans-analog-multiplier/msg515819/#msg515819

author: polarKaung
author_rank: Contributor
timestamp: July 27, 2022, 05:54:13 pm
content: looks interesting. Well, it is a part of my project and I have been researching for a while. I experimented with log-antilog topology, but I had no idea how to adjust its input and output range. When finding rms, I have to do both multiplication and division as well. So I tried Multiply-cum-division circuits from a book. But that didn't pan out very well.

author: Kleinstein
author_rank: Super Contributor
timestamp: July 27, 2022, 06:05:51 pm
content: Quote from: polarKaung on July 27, 2022, 05:37:18 pmIf the result is anyway send to a MCU with ADC, I would still consider using the ADC in the µC to sample the 50 Hz and do a little more math. I did this with a 4 MHz AVR (mega48, with internal 10 bit ADC) and it works reasonable well - kind of a challange to get better analog with discrete parts. One does not need much math (the square and sum in real time and one square root for a result). The frequency is not limited by the sampling rate, but by the BW of the S&H stage. So the AVR reading at some 10 kHz can still do RMS to some 40 kHz (the ADCs S&H stage BW limit).

author: Terry Bites
author_rank: Super Contributor
timestamp: July 27, 2022, 07:13:36 pm
content: I assume that the LT1966 etc are also unavailable?Building an RMS core from discrete components is going to cost more in parts and long sad days than a cheapo DVM. But who knows if these cheapo DVMs are really true RMS?If you can live with slower response times (seconds), go old school and use a resistor-diode (or even thermocouple) thermal bridge. See section Ihttps://www.analog.com/en/education/education-library/rms-to-dc-application-guide.htmlor See LT1088 dtasheet for ideas.You'll need a pair of well matched diodes, transistors or thermocouples and a pair of well matched resistors. Tuned up correctly these schemes can be better than 1% accurate.seehttps://www.eevblog.com/forum/projects/anyone-ever-attempt-a-diy-lt1088/for mostly negative remarks on this idea. Good luck...

author: Kleinstein
author_rank: Super Contributor
timestamp: July 27, 2022, 07:58:18 pm
content: Some of the cheap DMMs use digital RMS, which works resonable fine and even has some advantages over a AD637 or similar (rather fast response). The main downside of digital RMS, especially with a SD ADC as in many DMM chips sets is the sometimes rather limited frequency response. For just mains frequency it is usually OK.One would still need to check the manual / specs if they are true RMS, or just RMS reading with a sine input from a scaled rectified voltage.

author: KellyGreené
author_rank: Newbie
timestamp: August 02, 2022, 08:22:02 pm
content: If you're looking for resources that'll help you with your design, you can check outhttps://www.pcbway.com/project/shareproject/1850V_Cockroft_Walton_Voltage_Multiplier___Bug_Zapper_Circuit.html.You'll also find a ton of shared projects that might come in handy later in the future.

author: moffy
author_rank: Super Contributor
timestamp: August 03, 2022, 12:31:12 am
content: Just remembered a wattmeter design by the maestro of electronics, Bob Pease:https://www.pa4tim.nl/?p=4751andhttp://www.introni.it/pdf/Bob%20Pease%20Lab%20Notes%20Part%205.pdfon page 12. It's about as analog as it gets.It is a mains power meter but shows how to multiply two signals together with minimal components.