0-10V or +-10V to 3.3V ADC - Page 1
Forum section: electronics / beginners

author: PCBdesigner
author_rank: Newbie
timestamp: February 28, 2022, 04:34:11 pm
content: I need to interface some different signals into a 3V ADC, I will not have to monitor more than one signal at a time.Some input signals are 0-10V and others are -10 to +10v.I have one ADC and one input connector.I need to scale the input signals to 0-3V but to utilise the full scale resolution I would like to switch between the two, the switch can be a physical switch or something controlled with software (analog switch for example)Does anyone have any resources or examples for what I am after?

author: Kleinstein
author_rank: Super Contributor
timestamp: February 28, 2022, 06:20:07 pm
content: 10 V ro 3 V is a divider by a factor of some 3.4.+-10 V needs even more divider and an offset. So this this could be the inital divider plus another resistor to 3 V at the ouput.

author: ledtester
author_rank: Super Contributor
timestamp: February 28, 2022, 06:53:37 pm
content: Here is a good video on how to condition a signal using an inverting opamp:#328: Circuit Fun: Op Amp Signal Conditioning - a Practical Example -- w2aewhttps://youtu.be/OlhN7ADpKdsUsing an inverting amplifier will invert the range -- so +12 will be mapped to 0 and -12 to 3.3V, for instance.

author: PCBdesigner
author_rank: Newbie
timestamp: February 28, 2022, 08:58:38 pm
content: Something like this, I will add a BAT54s on the output to protect the ADC if the analog switch is selected for bipolar but a unipolar signal is on the input.

author: ledtester
author_rank: Super Contributor
timestamp: February 28, 2022, 11:13:22 pm
content: What ADC are you planning to use? You might be able to remove the external multiplexer since the ADC pins of a microcontroller are already multiplexed and a lot of ADC chips have multiplexers built in.

author: PCBdesigner
author_rank: Newbie
timestamp: March 01, 2022, 09:16:59 am
content: I am using the internal ADC in a xmega256, I might omit the analog switch if I end up with enough adc pins as you suggest.I just want to make sure that my circuit is a good way a of doing it and I am not missing something obvious

author: Kleinstein
author_rank: Super Contributor
timestamp: March 01, 2022, 01:34:01 pm
content: There should be additional protection if the OPs get a supply higher than the supply of the µC.One does not really need the OPs and exra analog switch, though the OP can help with the protection in case there is a negative signal, but the input still in 0-10 V mode. It is enough to switch the resistor to the +3 V level with a switch that is low enough that the swtich resistance is not relevant. A low level P_channel MOSFET can be sufficient. After all the ADC in the µC is only a 12 bit one with more limited accuracy.A buffer OP (powered from the same supply as the µC) may still be usefull to limit the voltage. This way a ot of range voltage would not effect the µC.The protection diodes in the µC can withstand quite some current, but when there is current slow from an excessive input, adjacent pins (e.g. other ADC channels) on the µC may show odd behaviour.

author: RoGeorge
author_rank: Super Contributor
timestamp: March 01, 2022, 02:02:12 pm
content: Quote from: PCBdesigner on February 28, 2022, 04:34:11 pmI've seen a shift and scale level in the input stage of an ADC from a MAX10 FPGA, in a devboard called DECA, from Terasic. They usedINA159.See the capture for page 29 of 134 in the attachment, from the user manual of the devboard:https://www.terasic.com.tw/cgi-bin/page/archive_download.pl?Language=English&No=944&FID=3ac35e74e46b2fa89c4ee85345ff8df2

author: Ian.M
author_rank: Super Contributor
timestamp: March 01, 2022, 03:00:41 pm
content: If the 3V range limit isn't critical, switching in a single pullup by changing an I/O pin from input to output high may be sufficient. Try a 6.98K:3.01K potential divider and a 2.26K switched pullup, which will give you about 3.15V on the high end for -10V to +10V in.