Hows much current does an ADC input need - Page 1
Forum section: electronics / projects

author: brownt
author_rank: Regular Contributor
timestamp: July 19, 2018, 01:58:00 am
content: How much current does an ADC input need to function correctly

author: BrianHG
author_rank: Super Contributor
timestamp: July 19, 2018, 02:25:52 am
content: Which ADC?What does the data sheet say about the input bias current?

author: coppice
author_rank: Super Contributor
timestamp: July 19, 2018, 02:31:46 am
content: Quote from: brownt on July 19, 2018, 01:58:00 amThat is not a meaningful question, as the input of most ADCs does not draw a steady current. You need to study the datasheet of the particular ADC, and understand the nature of its input circuitry, to understand the nature of its current draw. For example, the SAR converters in many MCUs have a sampling gate connected to the input pin. When that gate opens a small capacitor is connected to the input pin, and a short burst of current flows to charge this sampling capacitor. For the rest of the time the input current is extremely low.

author: brownt
author_rank: Regular Contributor
timestamp: July 19, 2018, 03:38:16 am
content: I see. the uC is a PIC18f26k40. And currently the 5 volts rail to one end of a pot, the wiper of which goes to the ADC, is resistor limited to 5mA. I was just wondering how low I could go. 5mA is not a problem, things are working ok, but "waste not, want not".

author: Bud
author_rank: Super Contributor
timestamp: July 19, 2018, 04:33:44 am
content: Google for ST application note "how to get the best ADC accuracy in STM32 microcontrolers". it explains how to calculate maximum resistance of the ADC driving circuit. That has to do with charging the ADC input capacitor quick enough to not affect the ADC accuracy at given sample rate.

author: JS
author_rank: Frequent Contributor
timestamp: July 19, 2018, 04:56:08 am
content: Ok, I see you started two threads on the same topic, I don't like that but I'm not a moderator so you are fine... Just a comment, but try not to do it as we already have one too many threads and it only makes harder to answer you and keep track of all the other posts.JS

author: HB9EVI
author_rank: Frequent Contributor
timestamp: July 20, 2018, 03:07:51 pm
content: As far as I remember from my PIC times, the adcs have around 10k impedance; like others already said, it's not a steady current, but depending on the needs of the circuit, it has to be taken into consideration

author: Siwastaja
author_rank: Super Contributor
timestamp: July 20, 2018, 03:49:46 pm
content: Put a 100n cap (10n is most often enough, but 100n is often a handy BOM reuse) from signal to ground directly at the ADC input pin. Provides RC filter for noise & the required low impedance for the ADC sampling. A great low-cost alternative to the "standard" opamp follower circuit, when you don't need high sampling rate (so that the cap has time to replenish through your high resistance between every conversion).Although they say that PIC ADC works acceptably with fairly high input impedance, many SAR ADCs don't work well at all with 10k input impedance. Often, well below 1k is desirable, hence the opamp buffer or the simple capacitor.