Voltage range switch - how not to blow my ADC - Page 1
Forum section: electronics / beginners

author: DanioIO
author_rank: Contributor
timestamp: March 08, 2014, 03:33:20 pm
content: Hi,Im trying to make a simple voltage meter from 0-50V, what is the simplest way (but not affecting my precision) to make a range switch. Im using a 2.5V ref for the ADC. On the input i have PTC, high voltage resistor and a varistor to protect from to high input voltage. Next i have a simple voltage divider which change the 0-50V range to 0-2.5V. I would like to change my range automatically not using any rotating switch.

author: sync
author_rank: Frequent Contributor
timestamp: March 08, 2014, 05:02:58 pm
content: Please post your schematic. What ranges and precision do you what to achieve?

author: fmaimon
author_rank: Unknown
timestamp: March 08, 2014, 05:07:03 pm
content: https://www.eevblog.com/forum/beginners/how-do-multimeters-auto-range-without-blowing-up-their-analog-switch/msg386559/#msg386559https://www.eevblog.com/forum/projects/caddock-1776-c48-odd-resistor-dividor/msg386609/#msg386609

author: DanioIO
author_rank: Contributor
timestamp: March 09, 2014, 10:28:52 am
content: After reading some post i decided to use a resistor ladder switched by analog multiplexer. But every analog switch needs to be supply with the maximum voltage that it can switch, that means if i would like to switch max 40V analog mux must be supply by min. 40V.And the other problem is how to switch the mux fast enought not to blow the ADC?

author: Rerouter
author_rank: Super Contributor
timestamp: March 09, 2014, 10:33:49 am
content: you put input protection on your ADC, and scale it so it cuts in just below the top of the range, e.g. for a 12 bit dac, you get your 4000 counts, have the protection near 4070, and have it switch range above 4000 (with some hysteresis)also i like this approach on grounding the ladder as it makes switching much much easier (mux to ground) while the high input impedance means that the protection will never let it see more than the ADC ref voltage,https://www.eevblog.com/forum/projects/caddock-1776-c48-odd-resistor-dividor/msg386609/#msg386609

author: Zero999
author_rank: Super Contributor
timestamp: March 09, 2014, 09:06:32 pm
content: What's wrong with the divider method?The Caddock 1776-C48 is expensive but it can be built from discrete, albeit with inferior tolerance but some of those value can be a pain to obtain but you can make them. The 1.11111M resistor can be substituted with 1M5 and 1M2 in parallel and the other value should be fairly obvious.Another option is an op-amp with a multiplexer to select different resistors in the feedback loop but you need to be watch out for the leakage current through the multiplexer.The example below will be able to measure both positive and negative voltages biased around the voltage reference of 2.5V. The gain is set by Rf and Rin, for higher voltages the gain can be below 1 and for lower voltages, it can be much higher.The protection diodes may not be needed if the internal clamping diodes in the mux can handle the current.

author: uwezi
author_rank: Unknown
timestamp: March 09, 2014, 09:19:32 pm
content: Quote from: Hero999 on March 09, 2014, 09:06:32 pmMake that 15M and 1M2 in parallel

author: Zero999
author_rank: Super Contributor
timestamp: March 09, 2014, 11:17:01 pm
content: Oh, yes, got the decimal point in the wrong place!