Analog input for ADC with multiple potentiometers noise

Question

I am making a circuit that should read the values of many (4-16) potentiometers in a sequence, using one analog to digital converter. My approach is, as shown on the image below, to feed 5V in steps to each potentiometer, while the others are grounded (using a shift register.)

First I started with the classic layout, with one diode at the output of the potentiometers. This was giving me a lot of noise in the ADC output. I also tried to buffer the output with an opamp as a voltage follower. I thought that this will solve the issue, but to my surprise there was still a lot of noise.

Then I experimented with a layout like the image on the right. Again I am feeding 5V in steps to each potentiometer, while the others are grounded. This variant is giving me a lot more steady reading with almost zero noise.

I am not very experienced in eletronics though, and I don't know if this is a valid approach. I feel like it's a bit of a heresy. For now I am sticking with it though, because it works well, although the number of transistors required is high.

What do you people think, did I mess up the op amp buffer, or maybe loose wiring on the breadboard? I made sure my wiring was good but who knows. Does the schematic on the right look like a heresy to you?

I added the opamp buffer version to clarify what I did. Should this improve the noise in theory? For the record, I know about the way to do it with mux (have done it before), but I am trying this as an alternative, because it will benefit the device I am constructing in other ways(not relevant to the dicussion)

Answer 1

The circuit on the left is producing noise, and probably incorrect voltages to what you want, from the diodes. The diodes are not passing sufficient and consistent current into your ADC so their drop is varying through junction noise.

You don't specify part numbers for your ADC or the source of the row of drivers at your schematic top. I'll assume for now that this is all from a microcontroller with an internal ADC and GPIO pins driving the potentiometers.

The input impedance of your ADC will be very high and therefore draw almost no current from its input source, let's say 10 uA. That's borne out by the right-hand circuit producing almost no noise. That means the diodes in the left-hand circuit are passing a very tiny current. Putting in a op-amp buffer in will leave the same problem, as the op-amp also has a very high input impedance.

The underlying problem there is that you're seeing the diodes as 'magic one-way wires' and not designing based on their operating characteristics.

You should see the noise reduce if you connect, say, a 47K resistor across the ADC input pin and GND. This is not a solution, as it ruins the linear behaviour of your pot' and won't work properly when the pot' outputs low voltages, but it will demonstrate the point.

What you have in your right-hand circuit is not the neatest solution for it but at least your selected pot' provides strong resistances for the ADC pin to the voltage and to GND, while all other pot' paths are at very high resistance from the diodes and BJTs. That's why it has lower noise.

Proper analogue multiplexers would resolve the problem. There are plenty to choose from.

You don't specify any operating voltages. As an example of a mux, the long-standing CD4051 has 8 channels and a mux inhibit pin, INH i.e. an enable input. The below example circuit shows how to connect two to support 16 pot' inputs. Note that it won't operate at 3.3 V or pass rail-to-rail input voltages, it just illustrates the point. You'll need to work out your electrical requirements then choose a mux based on that.

(Modified form of Fig.25 from the Texas Instruments CD405xB datasheet (schs047i).

Answer 2

You are using Arduino Nano and the AVR it has requires source impedance for ADC input to be less than 10k for best results. But as the pots are not directly connected to ADC input, it does not matter.

The diodes also conduct in one direction only, so the ADC input may be pushed higher via diodes at low impedance, but will have trouble going lower as the impedancr is very high, so the diodes and buffer will just make noise worse, and buffer it to ADC input.