Do I need to use a OP Amp in this application?

B

BrendanP

Senior Member
I've been away from the forum for a while, I had to catch up with work around the house. But the next project beckons....

I'm using the a FlexForce sensor from Tekscan to measure weight. pdf here.

http://www.tekscan.com/pdfs/FlexiforceUserManual.pdf

In essence the sensor is a flexible pcb, as pressure on the sensor increases resistance decreases.

The TekScan manual speaks of using a opamp (MC34071PG) in a drive circuit in conjunction with the sensor.

I have been using the sensor with picaxe's ADC in a potential divider setup on the breadboard. It sems to work ok.

Is the opamp essential?

As far as I can see I should be able to write/develop a simple program using a series of known weights to calibrate the sensor according to the resistance across the Flexforce. I could debug ADC result and make note of that value and the weight on the sensor and then incorporate that into my program.

Any comments/ observations welcome.
 
fernando_g

fernando_g

Senior Member
For improved accuracy, the answer is yes.

A picaxe's ADC input should be driven -ideally- from a source whose impedance is below 10k for optimum accuracy.

From what I read on the provided datasheet, the sensor's resistance range varies from 20k @ full load to 20M @ no load. Therefore the linearity and or accuracy should improve substantially, more so under low-load conditions.

Opamps are plentiful, even rail to rail ones.

If you don't want to deal with a negative voltage supply, you may still use the Opamp as a non inverting unity gain buffer for your potential divider.
 
B

BeanieBots

Moderator
Use an op-amp!

As pointed out by fernando_g, even in a simple pot divider circuit, the impedance is technically too high for direct ADC connection.
If you look at the "condcutance" curve, it's almost linear. Using the sensor between op-amp output and a virual earth would give an almost linear voltage/force voltage output.
 
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