Looking for a very accurate voltage controlled PWM duty cycle chip or circuit - Page 1
Forum section: electronics / beginners

author: AQUAMAN
author_rank: Regular Contributor
timestamp: January 31, 2020, 09:48:42 pm
content: I have data from an NTC and RTD that I need to pass over an isolation.What I am thinking of doing a normal voltage divider for both and then turning this voltage into a duty cycle or frequency signal to pass through a digital optocoupler. Probably looking at about 10kHzIn designing this circuit money is my least concern, but footprint (both height and area) and accuracy are my main requirements. I need it to be very accurate and and I need the components height less than 2.5mm. So for resistors, caps, NTC and RTD I'm using 0603 or 1005 SMD.I have been looking at LTC6992 (https://www.analog.com/media/en/technical-documentation/data-sheets/LTC6992-1-6992-2-6992-3-6992-4.pdf)I like this chip, it's easy to use, small footprint, but I worry it will not be accurate enough. It says plus minus 4.5% accuracy in the datasheet. It is also only 0.1-0.9V control voltage so I think it makes my readings from NTC and RTD more susceptible to noise because I have to lower the sensing current to get it to this levelAre there any other ICs like this, or would I be better off trying to find op amps in small footprints - or would to do it all on a small micro controller like ATTINY85 by better?

author: jmelson
author_rank: Super Contributor
timestamp: January 31, 2020, 10:05:58 pm
content: Would a voltage to frequency converter be better? There can be some distortion of pulse widths when passed through optical isolators.Jon

author: iMo
author_rank: Super Contributor
timestamp: January 31, 2020, 10:14:10 pm
content: https://www.analog.com/en/parametricsearch/11101http://www.ti.com/isolation/isolated-adcs/products.html?pqs=paqs&familyid=2021#

author: ogden
author_rank: Super Contributor
timestamp: January 31, 2020, 10:25:42 pm
content: Besides mentioned isolated ADC solutions other way would be to use generic (i2c or SPI) SD ADC with bus isolator. Also there is "programmable SD ADC", msp430f2013. Any (discrete) voltage to PWM or frequency converter will be much less accurate than ADC isolation approach.

author: AQUAMAN
author_rank: Regular Contributor
timestamp: January 31, 2020, 10:58:51 pm
content: Quote from: imo on January 31, 2020, 10:14:10 pmHiThanks I'll have a look, but most of these are low input range of less than 1VI also have to send the reading over 1 metre since to the control unit

author: AQUAMAN
author_rank: Regular Contributor
timestamp: January 31, 2020, 11:01:28 pm
content: I've been looking into these alsoI've been simulating this relaxation oscillator in this PDFhttp://ww1.microchip.com/downloads/en/appnotes/00895a.pdf

author: pwlps
author_rank: Frequent Contributor
timestamp: February 01, 2020, 01:41:55 pm
content: Quote from: AQUAMAN on January 31, 2020, 10:58:51 pmYou also have purely analog isolation buffers like the ISO224 (+/-12V input) or for higher precision the ISO124 (+/-10V). This might save some space since no adc/transceiver is needed on the isolated side. This will be less precise than the isolated adc solution though.

author: Mechatrommer
author_rank: Super Contributor
timestamp: February 01, 2020, 02:19:54 pm
content: Quote from: AQUAMAN on January 31, 2020, 09:48:42 pmwhat accuracy are we talking about? how much delay you can accept? attiny10 is much cheaper than LTC6992, can take 5V directly, 16bit PWM and 8bit ADC, 1 bit error is 0.4% error, averaging can give better and stable output but more delay, you choice... pro: you can do many more than your V->PWM convertor, incl V->FREQ... con: you need to program it.