Inputs -  Resistive / 4-20mA  - Page 1
Forum section: electronics / projects

author: NMP
author_rank: Contributor
timestamp: August 03, 2016, 01:46:48 pm
content: Hello all,I have a Joystick with a Resistive Roller. It has a Impedance from 2.5K to 7.5K, a 4 to 20mA input its also needed.Researching quite a bit, ampop and ADC selection has some quite reading/learning side.Thought doing the attached circuit.The Roller it whould be applied some Voltage, 10V or so it whould be enough i think.Im not any kind of expert in this matter, i test it on a bread board withouth the ADC, with similar components and i think it will work.What do you think about it? whould you do it different?Thank you!

author: rs20
author_rank: Super Contributor
timestamp: August 04, 2016, 05:55:35 pm
content: 20 mA going through the 250 ohms of R24/R25 gives 5V, which is delivered directly to the input of the MAX11636. The internal reference for the latter is only 4.096V. This means that any currents from ~16mA to 20mA will be off-scale high as far as the ADC can see. Even if you are bringing it within range by doing some sort of differential measurement, it's very poor practice to have absolutely no margin for error like this. I'd significantly reduce (halve at least) R24/R25.In a similar vein, I'd be powering U19 with 5V, not 12V. Right now, 30mA to the input will cause 6V to be strongly delivered to the ADC, possible destroying it. As there's no point going even close to 5V (see previous paragraph), there's no need to have such a high supply.It's also completely unclear to me why you have two resistors in parallel there for the I->V conversion. If it's to get more accuracy, that's totally pointless because other errors in the system will dominate, and software handling the ADC values can compensate too, if necessary. Heck, it's only a joystick, it's not like the human hand is 0.1% accurate!All these pedantic criticisms aside, though, it'd probably work OK as you've shown it here.

author: NMP
author_rank: Contributor
timestamp: August 04, 2016, 10:43:43 pm
content: Hi rs20,Well, you unclear me a lot of doubts, thank you so much! Going to make the changes right away.The 4-20mA its for a inductive sensor, you are right, i insert the 2 resistors for accuracy, wanted to experiment the pratical difference between 1 or 2The joystick its resistive, 2.5K to 7.5K, i thought applying a voltage 5V, with a resistor divider, R27, i can get the resistiance of the joystick but only can have a 1V difference.for instance if i have 2.5K in INA+ of U22 i have 4.138. At maximum, 7.5K, U22 INA+ have 3V.Is a wheatsone bridge a bit overkill? Any ideias?

author: rs20
author_rank: Super Contributor
timestamp: August 08, 2016, 11:42:07 pm
content: Once again, setting the max value of INA+ of U22 above 4.096V means that you can't measure it single-ended. You should reduce R27 to bring the max value comfortably below 4.096V. Even if your full-scale swing is less that one volt, that isplentyof signal to work with for a low-frequency signal like a joystick position.Yeah, I do think a wheatstone bridge is overkill*, and since we're only talking about a joystick (an application in which enormous accuracy doesn't make sense), I'd say it's unnecessary -- but of course, if you're having fun, you can do whatever you want!* Wheatstone bridges are typically employed in situation where you're trying to measure 0.1% changes in resistance (e.g., strain gauge) with extreme accuracy. Since you're looking at a +/- 50.0% change in resistance, you're orders of magnitude away from needing it.