Max input impedance for a signal into an ADC (ADS1115) - Page 1
Forum section: electronics / beginners

author: mortrek
author_rank: Contributor
timestamp: March 27, 2020, 06:51:18 pm
content: I have a battery-operated project and it occasionally polls an ADC connected to the main power rail through a simple resistive voltage divider. As it is, it consumes a couple hundred uA (a few thousand ohms at the moment). I'm wondering how low an ADS1115 can go before the readings get unreliable.I believe the ADC (ADS1115) has a 6 megaohm input impedance spec for the configuration I'm using.datasheet:http://www.ti.com/lit/ds/symlink/ads1115.pdfFrom the datasheet: "Unless the input source has a low impedance, the ADS111x input impedance may affect the measurement accuracy."When it says "measurements accuracy", is the problem purely a result of the analog line's impedance becoming too large relative to the ADC's input impedance (simple issue w/ohms law), or is something more complex going on? What is the maximum impedance I could use before adversely affecting the reading more than a few significant digits? I am fine with a reading of precision such as 2.00V, as long as it is accurate.Also as a newbie, is there some way to contribute to the forum? I ask a lot of questions and I wish I could be useful, but I don't have the knowledge or confidence to really answer any questions.Thank you

author: atmfjstc
author_rank: Regular Contributor
timestamp: March 27, 2020, 08:49:08 pm
content: Quote from: mortrek on March 27, 2020, 06:51:18 pmAs with anything worthwhile in life, there are no shortcuts. Just stick around, ask the questions you want and be respectful. Over time (years) you will gradually gain experience in various fields and be able to guide newbies yourself.

author: David Hess
author_rank: Super Contributor
timestamp: March 28, 2020, 02:48:43 am
content: The ADS1115 is a delta-sigma converter so it continuously samples its inputs which causes a transfer of charge. From the outside, this appears as a relatively fixed resistance which depends on the sampling rate which depends on the gain of the input stage. Higher gain for more sensitivity yields lower effective input resistance.So the resistance of the source forms a voltage divider with the effective input resistance of the ADS1115 which changes the gain of the transfer function.An estimate of the allowable error can be made from the gain accuracy which could be as bad as 0.15% so worse than 1 part in 1000. Or in a precision application where calibration is done, the expected gain drift. 1 part in 1000 suggests a maximum input resistance of about 22k to 1k depending on gain. In a precision application, I would expect a source resistance of essentially zero ohms from the output of a precision feedback amplifier.

author: iMo
author_rank: Super Contributor
timestamp: March 28, 2020, 09:00:31 am
content: Quote from: mortrek on March 27, 2020, 06:51:18 pmIt basically says the input impedance of the ADS1115 adds up in parallel to the low side resistor of your divider.You may calculate the 1LSB resolution of your ADS1115 (in V) and compare that 1LSB value to the voltage difference at the divider when the ADS1115 is connected/disconnected to/from your divider.The resistance values of your divider should be such the difference is smaller than 1LSB.