Connecting two opamps to a single ADC channel - Page 1
Forum section: electronics / beginners

author: Aleksandar B.
author_rank: Contributor
timestamp: January 12, 2013, 09:05:28 pm
content: Hi there, I am a mechatronics student from Serbia,and at the moment I am designing a modular main board for autonomic robots.I am having some trouble with the analog inputs module. The basic idea of the whole main board is to be as simple as possible, since it is intended for educational and practical purposes for younger students. The practical part is intended for making a robot which we do on one of our courses,and since it is meant to be easy for use I have designed the analog input part so it has the possibility to use both analog sensors which give a current as a output or a voltage. So for the sensors that give current I have made a current to voltage converter with a opamp,and for the sensors that give voltage I am using a different opamp that has a gain from 1 to 11. You select which opamp you want to use with a jumper. So the problem is with the outputs of the opamps. They both go to the same input pin of the MCU,and I don't know how to connect them so they are not shortened one to the other.Some solutions that I am thinking about are:-Use a jumper at the output also ( but I am looking for a more elegant solution)-I was thinking of some solutions with diodes,but I think they will cause problems with the signals since they are non linear components.-Multiplexers are not an option since all of the pins of the MCU are used for other stuff so there isn't a way to generate the selection signals.On the pictures left side is the basic idea,and on the right side is the solution with diodes that I am uncertain about.Please bear in mind this is my first project,so there can be,and probably are some rookie mistakes.Also any kind of ideas and tips are welcome.

author: SeanB
author_rank: Super Contributor
timestamp: January 12, 2013, 09:38:39 pm
content: Use one opamp. Make the 250 ohm resistor and the 10K resistor jumper selectable, and if you use a 3 pin jumper block then depending on jumper position you have an opamp with a unity gain and a 250 ohm input resistance ( 100k resistor will do a minor error which can be ignored) in one position and in the other you have a voltage gain with high input impedance. You switch the lower part of the 2 resistors.

author: Aleksandar B.
author_rank: Contributor
timestamp: January 12, 2013, 10:55:24 pm
content: Thank you very much. It's such a simple solution. It didn't even come to my mind to try and solve it with 1 opamp

author: w2aew
author_rank: Super Contributor
timestamp: January 13, 2013, 01:19:37 am
content: Quote from: Aleksandar B. on January 12, 2013, 10:55:24 pmIf your gain/offset requirements change that make the single opamp solution impractical, then use the two opamps and put the jumper at the opamp outputs instead of the inputs.

author: SeanB
author_rank: Super Contributor
timestamp: January 13, 2013, 08:54:21 am
content: Going into a micro that is using relative changes to do detection, gain and offset will have little effect on it. If using the current option you just adjust the pot to minimum resistance as part of the setup instructions, it will have minimal effect then. Just use a single rail micro that has rail to rail inputs and output with a low offset.