Summary of Analogue Sensors – Calculate the Nonlinearity Introduced by a Load or Pull Down Resistor
Summary: Adding a pull-down (load) resistor to analogue sensor circuits can introduce significant nonlinearity and noisy readings at extreme positions. The author demonstrates this using a potentiometer, a 16-bit ADC, and a 100K recommended pull-down, showing how lower resistor values (e.g., 10K vs a 1K pot) worsen nonlinearity. The article explains visualizing and quantifying that distortion in Excel and derives a nonlinearity adjustment formula to correct readings.
Parts used in the Analogue Sensors – Calculate the Nonlinearity Introduced by a Load or Pull Down Resistor:
- Three-legged potentiometer (ground, 5V, wiper)
- 8.25K series resistor between wiper and ADC input
- 16-bit ADC chip
- 100nF capacitor from wiper to ground
- 100K pull-down (load) resistor from wiper to ground
- Power rails (ground and 5V supply)
- Microsoft Excel (for plotting and analysis)
Have you ever had that terrible feeling that adding a load resistor or ‘pull down’ to your sensor is messing up all your analogue readings?
Maybe you’re wondering why we’d want to spoil a perfectly good circuit by putting in a load resistor at all?
For many years I found that I would get strange, unpredictable, readings from my sensor related projects at the maximum and minimum locations when using analogue digital convertors (ADCs). I always blamed this on poorly designed micro processors and never for once thought that it might be my own circuit designs at fault ….. until now.
To use an analogy, when the sensor goes to maximum or minimum, it does not just reach a maximum point, but quite often actually falls off the edge of the world into a kind of no man’s land where it is then prey to all kinds of digital noise and other generally nasty things like Goblins and Elves. Anybody who, like me, who has blamed this on their arduino is totally forgiven!
The example I’m using here is a simple three legged potentiometer with a ground, 5 volt and ‘wiper’ connection.
Using the correct pull down resistor we can eliminate noisy readings from our projects ……. And ……. just to prove that math can actually be fun ………. I’ll tell you how I discovered the non linearity adjustment formula through diagrams and images.
Step 1: The Circuit
In the circuit above we have are reading a simple potentiometer through it’s ‘wiper’ arm through a 8.25K resistor and 16 bit ADC chip. Crucially, there is also a 100nF capacitor and a 100K resistor going to the ground rail from the wiper. We’re going to concentrate on the 100K resistor. 100K is the recommended value from the manufacturers of the instrument.
There’s nothing unusual about this circuit and it looks pretty boring until we look at what’s happening with the resistor in more detail.
To get rid of the noises (and the Goblins and Elves) we want the resistor to be fairly small in ohms – maybe 10K, but if our pot is, for example 1K, we’re going to get a massive amount of non linearity – see for yourself by opening the excel sheet in the next step.
Initially, I did not set out to discover any formulae – I just wanted to visualise the non linearity created by a load resistor in a sensor related circuit. I wanted to try and isolate the curve and plot it as a graph in Microsoft excel. It just seemed like fun.
Read more: Analogue Sensors – Calculate the Nonlinearity Introduced by a Load or Pull Down Resistor
- Why add a pull-down resistor to a sensor circuit?
To reduce noise and stabilize readings by providing a defined load to the sensor wiper. - Can a pull-down resistor cause measurement errors?
Yes, it can introduce nonlinearity especially when its value is not well matched to the sensor pot value. - What resistor values are discussed in the article?
The article mentions a recommended 100K pull-down, a possible 10K pull-down to reduce noise, and a 1K potentiometer as an example that can exacerbate nonlinearity. - How was the nonlinearity visualized?
The author plotted the curve in Microsoft Excel to visualize the nonlinearity introduced by the load resistor. - Does lowering the pull-down resistor always improve readings?
No; lowering the pull-down can reduce noise but may create large nonlinearity if the pot value is low relative to the pull-down. - What additional components are used to condition the wiper signal?
A 100nF capacitor is used from the wiper to ground for signal smoothing and noise suppression. - How does the author describe extreme sensor positions behavior?
The sensor at maximum or minimum can fall into a no man's land vulnerable to digital noise, producing unpredictable readings. - What was the purpose of the 8.25K resistor in the circuit?
It is used in series between the potentiometer wiper and the ADC input as part of the input network described.
