JFET VCR for linearizing sensor response

Thread Starter

shivasage

Joined Jan 12, 2020
12
Hello. I posted about my pet project a while ago. It's still ongoing.

I'm trying to create a non-inverting op amp circuit in which a force sensitive resistor (which I will press with my finger) controls Vout from 0 to +5V. It'll look something like this. With no pressure, the sensor has a large (>1M ohm) resistance. With a bit of pressure, it gets to 1M. With more pressure, it'll get down to ~50K. It is for a musical application so exact accuracy is not required.

With Vref = +0.5V DC and Rfeedback = 2M, I have managed to simulate a circuit in which, at zero force Vout = 350mV, at 1M resistance Vout = 750mV, and Vout saturates at 5V at around 74K. This is all good and well, the only issue is that the change in output is not linear (as expected). Vout sweeps the last couple of volts too quickly. Higher values for the feedback resistor yield a smoother curve, but the output reaches 5V too quickly.

So I wondered if the feedback resistor could be variable, i.e. a voltage controlled resistor, but this is completely new territory for me. Thus my question: Is it possible to use a JFET VCR to create a variable resistance in the feedback loop that is controlled by Vout? That is, as I push on the sensor and Vout gets higher, the feedback resistance gets lower, so the Vout does not increase as fast?
 

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ericgibbs

Joined Jan 29, 2010
10,042
hi shivasage,
Do you have a pressure versus resistance plot chart of the pressure sensor that you could post.?
If you have a datasheet or link for the sensor post that would also be helpful.
E
 

MrChips

Joined Oct 2, 2009
21,126
It would appear to me that what you need is a different kind of circuit, such as a logarithmic circuit.
Something like this:

1583155984171.png
 

Thread Starter

shivasage

Joined Jan 12, 2020
12
hi shivasage,
Do you have a pressure versus resistance plot chart of the pressure sensor that you could post.?
If you have a datasheet or link for the sensor post that would also be helpful.
E
Here's the datasheet, and the pressure / resistance curve from an integration guide PDF that required me to fill out a form before downloading:

caantitled.png
It would appear to me that what you need is a different kind of circuit, such as a logarithmic circuit.
Is it possible to do a logarithmic circuit with a non-inverting op amp?
 
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