Help me choose a differential amplifier

Forum 2005-2010 (read only) Hardware Interfacing
1

Hello All,

I would like to use a differential amplifier to increase the effective resolution of a potentiometer. The pot can only move through 90 degrees of its 270 sweep due to mechanical restrictiions, so the measured value varies from around 400 to around 600 and I want to increase this. I have been able to increase this a bit by lowering the reference voltage, but I don't really want to do that as I am measuring other inputs as well that need the full 5v reference voltage.

I know that this is possible, but there are millions of different op-amps available and i just don't know where to start.

I am polling the pots (there are 4) every 10milliseconds.

HELP!

2

A low-voltage single-rail op-amp is going to help - I've recently got some LM358's which I think are quite a good spec for 5V use.

3

A low-voltage single-rail op-amp is going to help

That sounds lovely.... what's that then. I really know nothing about op-amps. Truly nothing, zip, nada, zero. I am an op-amp virgin.

4

A different opinion:

You can make a nice Differential amp with some resistors and a rail to rail TS272 op amp.

5

You can make a nice Differential amp with some resistors and a rail to rail TS272 op amp.

Could I possibly press you to tell me how? Assume I am an idiot. My analog electronics knowledge is very poor.

6

May I ask a couple of questions?

How much more resolution do you want?
Are they good quality potentiometers?

I admit I tend to use low-cost components, and I've found pots are often electrically noisy, so increasing resolution might be amplifying noise.

Just an observation.
GB

7

Are they good quality potentiometers?

Well they were 28 years ago when they were new, and they have been very well looked after but I could change them if they turn out to be scratchy. They are in an RC transmitter I am converting.

At the moment I get a voltage difference of about 2v from top to bottom of the swept range (which is less than the total range of the pot) and I would like it to be more, maybe 3 to 4 volts. A full 5 volt sweep would be great, as that would give me a resolution of 1024, but I don't think that is realistic for, as you pointed out, I would also be amplifying the noise.

If someone could show me a basic circuit and tell me which of the 15 billion op-amps to use along with the calculations for the gain then I can probably figure the rest of it out.

Thanks

Chris

8

I am not a certified electronics engineer, so I bow to those who are. Having said that, I don't think there is anything nasty about this.

These article explains how an inverting operational amplifier circuit works.
http://www.national.com/an/AN/AN-31.pdf
or
http://www.swarthmore.edu/NatSci/echeeve1/Ref/SingleSupply/SingleSupply.html
and

or

As you can probably see. The circuit you are looking for is the first, basic inverting amplifier.

Essentially, the gain is the ratio of voltages across the input and output. That is set with two resistors.

The golden rules of Op Amp behaviour with feedback (from "The Art of Electronics") are:
1 The output attempts to do whatever is necessary to make the voltage difference between the two inputs (+ and -) zero.
2. The inputs draw no current.

So if the + input goes to 1V, and the ratio of the two resistors is 1:2, then the output will go to 2V.

Negative feedback is a way to set the gain. You just need a gain of roughly 2. Further, the badwidth is very low, it's only the rate that you can twiddle the potentiometer, so almost any op amp should work.

I think you'd like an op-amp that works at 5V, and ideally are capable of rail-to-rail voltage swings (so that you can get as much amplification as practical).

The paper at swathmore recommends TLCV2772 or TLC2774 because they are rail-to-rail.

I had a look at Farnell, I found about two dozen on the UK site:

The Art of Electronics (TAoE) recommends using a pair of resistors with a reasonable value to keep the output impedance reasonably low, I.e. the sum of the two resistors in this case would be about 10K-100K. The Arduino analogue inputs likes to see in output impedance of the previous stage (the output of the op amp) of 10K or less. On the other hand, you'd like the two resistors to be about 10x bigger than the input potentiometer, so that they don't load it.

If you are okay with something a bit less fancy, TAoE recommends TLC27M2A (or related op amps).

At Farnell the TLC27M2A is very cheap http://uk.farnell.com/jsp/search/browse.jsp?N=0&Ns=PRICE_PLS_006_PRICE1|0&Ntk=gensearch_001&Ntt=TLC27M2A&Ntx=mode+matchallpartial
and starts as cheap as 0.31 GBP.

HTH
GB