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[on:]

Hello,

Having a conversation with someone today, they said that I should use an op-amp as a buffer ...putting the op-amp between the voltage divider circuit which contains the FSR and Arduino board.

Why is this?

I know that the unity gain op-amp configuration will eliminate loading effects. But beyond that sentence, I am not sure what is going on, and I hope someone can help.

Thanks.

Seán

[Reply #1 on:]

It isn't strictly necessary. It depends upon the value of your FSR and other resistor in the voltage divider. With effective source resistances above 10k ohms you are going to start to see A/D conversion imprecision (according to the AVR datasheet). Whether or not that matters to you depends on your application and how much precision you really need in your FSR measurement.

The op-amp will help there if you use large value resistors (much bigger than 10k) by providing a low source resistance to the A/D converter.

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[Reply #2 on:]

Hello,

Thanks for that ...Can I ask another question related to op-amps ...Is it okay to connect the -Vcc pin to Ground, or would think cause any problems? I only ask becuase, before I connect the op-amp in, I am getting the resulsy I would expect. I hook up the TL081 Op-Amp (http://focus.ti.com/lit/ds/symlink/tl081.pdf) and the results go up the left. I have set it up in a Unity Gain configuration with Pin 2 (inverting pin) connected to pin 6 (output) and Pin 3 (non-inverting) connected to the voltage divider containing the FSR, with Pin 4 (-Vcc) to Ground and Pin 7 (+Vcc) connected to +5V.

Thanks.

Seán

[Reply #3 on:]

It's OK to connect the -Vcc pin to ground but you will get the problems you are observing. That is because "regular" op-amps (like the TL081) cannot drive outputs all the way up to +Vcc or all the way down to -Vcc. That is, they do not have "rail-to-rail outputs" (+Vcc and -Vcc are called the supply "rails").

An op-amp like the TLV2371 will work better in this application as it has both rail-to-rail outputs and rail-to-rail inputs (a.k.a. RRIO).

The connections you describe should work if you use an RRIO op-amp.

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[Reply #4 on:]

Hey,

That's great, thanks!!

Seán