Analogue Sensors - Calculate the Nonlinearity Introduced by a Load or Pull Down Resistor

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Introduction: Analogue Sensors - Calculate the Nonlinearity Introduced by a Load or Pull Down Resistor

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.

Step 2: The Math

I was actually working at the time on this Analogue Wind Vane project, which is basically a continuous potentiometer with a small 'dead zone' at north. Initially, the project was plagued with those familiar noisy readings until I looked at the resistor more closely.

Firstly, I plotted the actual resistance curve in Microsoft excel, which is the total resistance created by the combination of the wind vane and the load ... and I called it 'Total (RT)'. Initially, I was disappointed as I could not actually see any curve at all, so then I changed both the axes to log10. Hey presto! I can see a curve! (The blue curve on the left in the diagram above).

The total resistance is given by this well known formula:

RT = (Rw * R L ) / (Rw + RL)

• where:
• RT is the total resistance
• RW is the wiper resistance
• RL is the load resistance

OK, so far so good - not too complicated?

Next, I wanted to see how my new fancy curve looked side by side with a boring straight linear curve, just as if the results were not actually a curve at all, but a straight line. The formula for this is:

RLIN = (Rw * RTMAX)/ RWMAX

• where:
• RLIN is the hypothetical linear resistance (the 'imaginary' resistance)
• RW is the wiper resistance
• RTMAX is the maximum value of the total resistance
• RWMAX is the maximum value of the wiper resistance

This is all well and good, but where are we going to get the value of the total resistance from? I really thought this was going to be easier than this, but then realised that we've already calculated this value above, it's just the maximum value of RT . But just for clarification, here is the formula:

RTMAX = (RWMAX * R L ) / (RWMAX + RL)

• so, now, if we substitute out RTMAX we get:

RLIN = (RW * RTMAX) / RWMAX = (RW / RWMAX) * (RWMAX * RL) / (RWMAX + RL) = (RW * RL) / (RWMAX + RL)

Now we can plot our linear 'curve' (in red) and see if it's noticeably different from the curvy curve ..... and yes .... as long as we cheat by using log10, we can see the difference. If we open up the excel file, we can change the value of the load resistor to something stupidly small and get some pretty crazy curves produced.

Finally, I realised that we could then subtract the non linear curve from the linear curve and get a final outcome: the actual non linearity, or the 'difference' between the linear and non linear results. This is the pretty blue curve on the right and is given by:

RDIFF = RT - RLIN = (RW * RL) / (RW + RL) - (RW * RL) / (RWMAX + RL)

This equation could be reduced further, but the arduino nano is already going to struggle with some of the big numbers produced (we're working with 16 bit ADCs), so we need to help it along a little bit.

Eventually, the formula gets translated into arduino code:

```////////////////////////////////////////////////////////////////////////////////////////////////////////
// Non linearity calculations:

load2 = load / 1000 * maxSensorValue;

long  sensorValuez = sensorValue/5;
long  maxSensorValuez = maxSensorValue/5;

long  a = 5*((sensorValuez * loadz) / (sensorValuez + loadz));
long  b = 5*((sensorValuez * loadz) / (maxSensorValuez + loadz));

Serial.print("a = ");
Serial.println(a);
Serial.print("b = ");
Serial.println(b);
outputValue = sensorValue + (a - b);
//////////////////////////////////////////////////////////////////////////////////////////////////////   ```

You'll see that the code is more 'clunky' than the formula and I've had to divide by 5 to reduce the size of some of the numbers. But it works!

Hopefully, I've now also proved not only that maths can be fun but also relevant to the real world? But what did all this work actually achieve?

If we remember from earlier on, the recommended minimum load resistor was 100K, but if we reduce these ohms we can make the readings significantly more stable near the dead zone. I tried lots of different permutations and, using the above formula to negate the non linearity, I ended up using a super accurate (+-0.1%) 30K resistor and still got good readings at due south, which, according to our excel diagram, is where most nonlinearity will occur.

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12 Discussions

Very nice.
I dont have much knowledge on analog measurements and electronics, and I don't have an idea if it would be technically feasible, but my first idea would be not to use a single high qualtity potentiometer with a dead zone but instead a set of two, probably better three less accurate ones and calculate the direction by ratio values. Would this make sense?

Yes that has actually been done but increases the torque so much that the instrument is not sensitive enough any more. Also, it's a pain to code!

May I ask why you've used the two reistors beside the potentionmeter in first place? The potentiometer sets a voltage at a fairly low impedance which is then read by the ADC with a high impedance input. As long as the voltage across the potentiometer is stable the readings shouldnt shift much.

Usually the ADC spits out highly inaccurate values when a high impedance
source, say about 1MEG or higher, allows electric or magnetic fields to
couple into the conductor between said source and the ADC.

So I was curious about your solution and checked both, the adafruit page and the datasheet, and couldn't find any information on a load resistor. Where did you find the recommendation of using a 100k resistor? Can you expain why you've designed the circuit in this particular way?

There's a 10K resistor in series on the wiper to protect it as the wires on this particular unit are very thin. 10K was reduced to 8.25K, with no obvious benefit.

The 100K filters out noise as otherwise the pot acts as an antenna when it is in the 'dead zone'. I've seen this happening and the noise is reduced by using pull down resistors such as this one.

Readings anywhere other than the dead zone seem to be pretty stable.

This is the actual device here: https://www.instructables.com/id/Analogue-Wind-Vane-With-Self-Calibration/

Ok, I get the your reason for using the 10k resistor. However, if you stay within the the operation voltage range of the ADC it is no requirement as the imput impedance (= resistance) is very high on it's on (typ 100MEG). Goning beyond the ADCs maximum operation voltage can cause significant cureent flow in which case it is absolutly neccessary. Better be safe then sorry!

In which region did you experience the "dead zone"?

Yes, this particular sensor is very expensive so protection is necessary, just in case!

The sensor is effectively a continuous potentiometer with no end stops, so the dead zone is the small space where the tracks end and start again inside the pot. I call in 'no man's land' as that's where everything goes to s**t.

I am working on another alternative - basically a magnet rotating over a chip - and am trying to turn the quadrature signals into absolute linear ones - no dead zone - nearly there!

I see, the sensor you've been using can turn 360° and so there must be a place where the wiper isn't in contact with the metal and causes the input to float. No wonder the ADC goes crazy. Thanks for the clearifictaion, I haven't thought about that!

I see, the sensor you've been using can turn 360° and so there must be a place where the wiper isn't in contact with the metal and causes the input to float. No wonder the ADC goes crazy. Thanks for the clearifictaion, I haven't thought about that!

Good when using one sensor But still when dealing with ADCs in arduino boards , analog reads can be horrible sometimes . Especially when using more than 2 analog sensors . The ADC values fluctuate for each sensor . I dig deep into this issue through experiments and found that its because of loading effect on arduino Vcc and then something called " ghosting effect " . Loading effect can be tackled to some extend while the latter one is difficult to deal with .Especially when low impedance sensors like Lm35 and high impedance sensors are used together .Check it out

Hey thanks Am. I've got a high impedance sensor in my next project (Using a Tunneling MagnetoResistance chip) so I'll be careful not to use it on the same bank of ADCs as other stuff. Version 2 of the weather station project will have 3 arduinos connected by I2C in it to help avoid this sort of problem.