Analog signal linearization

[jebediah]

Hi,
I need to linearize voltage coming out of a potentiometer(voltage divider). Any ideas?

Thanks.

 

[ScottyUK]

A potentiometer is two resistors. Resistors are inherently linear.

What do you want to linearise? Please expand and clarify your requirement.

 

[nbucska]

If the POT is linear, the voltage is proportional to the
position (angle if rotary )

<nbucska@pcperipherals.com>

 

[felixc]

Depending on what is the impedance of your interface circuit, you may need nothing, or maybe a high impedance buffer. You may have more concerns about protecting your circuit for noise and spikes if the potentiometer is remote from your circuit.

 

[jebediah]

The potentiometer has 0.5% nonlinearity. I need to decreased the nonlinearity to 0.1%.

One way would be to have a lookup table using an EEPROM, but would it be more space efficient if I did it analog?

 

[buzzp]

Standard pots do not increase resistance linearly when turning the pot. I believe this is what the poster is referring too. The only way I know to do it is to use look up tables as stated or use a linear pot.

 

[Chuckles159]

When they say a pot has 0.5% linearity that means that one sample could be 0.5% high at a given point while the next sample could be 0.5% low. It could also mean say 0.5% high at 33% rotation and 0.5% low at 66% rotation. A lookup table is not very useful unless you are going to calibrate each one separately.

If you need 0.1% linearity you need to pay more, a lot more, for the pot.

 

[ScottyUK]

I think for the level of linearity you are looking for, you should consider one of the synthesised 'pots' available from the likes of Dallas Semiconductor or Analog Devices. These have excellent linearity and fine adjustment, and will offer a better solution than a lookup table for less effort. Some of these pots are have very good performance as a three-terminal pot, but are not so well defined as two-terminal variable resistances.

The pot you have is already pretty good, by the way. You will spend a lot of money to get 0.1% out of what is essentially a mechanical device.

 

[nbucska]

You can get .1% with a 10 bit DAC. Why do you need so good
linearity by the way ?

<nbucska@pcperipherals.com>

 

[felixc]

I can guess that your potentiometer is used for instrumentation, like a displacement transducer. Many transducers, like LVDTs exhibit an output/input graph shaped like an &quot;S&quot;, which can be easily &quot;linearized&quot; with a polynomial equation to improve the precision of the sensor. On a potentiometer, the &quot;input vs output&quot; curve may not look at all like an &quot;S&quot;. A look-up table may be the only way to go, with individual calibration, as Chuckles159 mentioned. Be careful with repeatability, it may ruin your efforts to improve the precision of the pot.

 

[lcsjk]

Look in digikey for the Bourns 3540 series. This is a 10T pot with standard linearity of .25% with an optional version of .1% Call Bourns to buy the tighter version. Not cheap, but will cost less than design time.

 

[jtsamuels]

When you say &quot;linearize&quot; the voltage coming from a potentiometer do you know what type of potentiometer you have? There are 3 basic types: Linear, Audio Taper and Log Taper. The Log Taper is sometimes referred to as a &quot;Power&quot; Taper. So first off you should make sure you are using a &quot;Linear&quot; taper potentiometer if you want a voltage out the is linearly related to the angular displacement of the shaft.

Audio taper is used as you would guess in audio applications to compensate for our hearing response.

Log or Power Taper is used to provide a response that is related by the LOG of the applied voltage.


So if you want a Linear output from the pot make sure you have started with a linear taper. You would be making your task more difficult to be using an Audio or Log taper pot and trying to linearize that.

From this point on your application can help define the style of pot you can use. Will this pot be adjusted only one time? Will the end user be adjusting it several times? Is this a control input? A reference setting (if so then I assume you would have measured the output)? A feedback sensing element? etc. etc..


For example, many precision pots are available with 0.1 or 0.05% linearity (Spectrol) for which you will pay in much dinero! These are usually 10-20 turn, wire wound, helical brushed pot. Now if you want to do this only once and not pay huge prices then you should use the previous suggestion of the Dallas EPot or one of the precision CERMET Film pots available from VISHAY (formerly Dale). The have a variety that shouls work for you.


Good luck,
Jim