maximum pot value for ADC - Page 1
Forum section: electronics / projects

author: brownt
author_rank: Regular Contributor
timestamp: July 19, 2018, 01:05:30 am
content: What am I looking for in a uC datasheet that tells me the maximum and minimum value that a pot connected to an ADC, can be?I using a PIC18F26K40

author: jpmkm
author_rank: Contributor
timestamp: July 19, 2018, 01:55:32 am
content: An ADC measures voltage, not resistance, so you should be looking for a voltage range on the datasheet. That chip should allow you to select the high end of the ADC to either be VCC or an external VREF input(max is typically around VCC, I believe). Minimum would be 0V(or a VREF- input, if used). In simplest terms, your range will be 0V-VCC. Doesn't matter if you are using a 1K pot or a 100K pot, so long as you are within the correct voltage range.

author: brownt
author_rank: Regular Contributor
timestamp: July 19, 2018, 01:57:06 am
content: I thought there might be some maximum value, relative to the impedance of the ADC input

author: BrianHG
author_rank: Super Contributor
timestamp: July 19, 2018, 02:24:29 am
content: Just use a 10k linear pot if you project is not battery powered.Use a 100k pot if battery powered.Use a linear pot.Additional exception if battery powered, instead of VCC, wire the + side of the POT to an output pin and program that output high, then pause, then sample the ADC pin connected to the wiper, then set the output pin low for the rest of the time to save battery power. This wont make sense if you are sampling continuously fast, you would just keep the IO pin high, and then set it low when placing the MCU into sleep mode.

author: brownt
author_rank: Regular Contributor
timestamp: July 19, 2018, 03:36:09 am
content: that's ok. I have been shown the section in the datasheet, it says "The maximum recommended impedance for analog sources is 10 k".

author: JS
author_rank: Frequent Contributor
timestamp: July 19, 2018, 04:53:58 am
content: Quote from: brownt on July 19, 2018, 03:36:09 amThe difference between a regular pot and other analog sources as sensors is that you don't care the actual voltage of the unloaded pot, those 10k would be to keep the error between the input impedance and the source impedance under a certain value, probably similar to a LSB, for a 10kΩ source and 8 bit converter you would need an input impedance of 2.5MΩ.Also consider that the output impedance of a pot changes with position and worst case is mid point at which it is Rpot/4, so for the 10kΩ pot you have 2.5kΩ. 25kΩ of a 100kΩ potentiometer isn't that far off for an application that isn't particularly high precision. If you would come to use a 2MΩ input impedance you will see some problems and won't be much left from your signal, if you were driving a high precision fast opamp which sample and hold is a small capacitance you probably (for sure) want a dedicated low impedance driver, data-sheet will probably show a suggested circuit as each ADC has it's own input characteristics.If you find yourself in a situation where the only specs you have are input impedance (Zi) and bit depth (N), to get the source impedance (Zs) you should do Zs<Zi/(2^N) to keep the error under 1LSB if that's your goal, you might be fine with 4LSB if you have a spare 16 bits when you only need 12 or if using a 24 bit ADC where you won't get the full 24 bits as meaningful.JS