mahfuzmatyaacob

Thanks for sharing it, but the circuit u posted is for generating a linear ramp from a constant source. My question was to change an exponential decay voltage to a linear increasing voltage. The dischrage is in exponential, so in order to linearized it, i have to have a circuit that have a -ln function. PIC would be my choice, but logarithmic amp could also do. Its juz that it might b in terms of analog, where u cant program to a certain logarithmic function like PIC does.. Thats what im thinking

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-Mahfuz-

JimB

Am I missing the point here?

I understand that the idea is to create a linear ramp from an exponential decay.

But why? What is so magical about the original exponential decay?

If you need a linear ramp, then make one using the appropriate techniques.

This makes as much sense as saying:

"I have a steam engine and want to convert it into a diesel engine"

Just my two Ringits worth.

JimB

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Experience is directly proportional to the value of the equipment ruined.

ronsimpson

You want an exponential input and a linear output. When the voltage is low the gain is low and then the voltage is high the gain should be very high. When the slope is positive and the reverse of that when the slope is negative.

On the negative slope, as the voltage approaches zero the gain approaches infinity. This is a recipe for instability. On the positive slope side the gain is infinity at the reference voltage. It will oscillate.

This sounds like a school project not a real world thing. If you want a linear ramp, make one.

mahfuzmatyaacob

Linearized ramp is to be used as a test fixture in embedded A/D test. That is why it need to be converted from exponential decay voltage (capacitor discharge to be exact). So, the circuit its just a converter, converting the exponential to linear ramp, so that it can be use in A/D testing.

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-Mahfuz-

ronsimpson

I think we all understand your desire for RC exponential to linear ramp.
Most of us know how to make a linear ramp.
None of us know how to make exponential to linear with out errors.
With “analog computers” there is +, -, log, x^2, AxB and some more. Log is not very accurate. Squared is ok. How good do you want your ramp? As the exponential approaches Vref or GND the error will get bad.

Search “analog computers” and see what functions there are. Then using these functions try to get from exponential to linear. I would use EXCEL to do the math. You could use spice but I like EXCEL better for math.

I think you are not getting an answer, just questions because this is not simple, accurate or temperature stable. AND a linear ramp is so easy to make.

Roff

I thought you were working on some sort of school assignment, where the goal was to linearize an exponential ramp. Now you say it is just for A/D testing!
So why can't you use a current source charging (or discharging) a capacitor, which generates a linear ramp you can use for A/D testing? I have done this. it works.

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Ron

Papabravo

Possibly becuase a fixation on a particular solution inhibits clear thinking and thoughtful consideration. But what do I know I'm just a graduate engineer sloping into retirement.

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We never have time to do it right; but we always have time to do it over.

ericgibbs

hi,
As you say, its a case of 'thinking inside the box'.

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Eric
"Good enough is Perfect"

PIC tutorials:
Nigel's: www.winpicprog.co.uk/
Gramo's: www.digital-diy.net/
Bill's: www.blueroomelectronics.com/