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Rate estimation using analog methods

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gary36

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I have to compute the rate of incoming analog signal(0-10V) in V/min, the range is is 10mV/min - 500mV/min using only analog methods. I want to know the design of practical differentiator circuit that will do this job effectively
 

Capacitor in series, resistor to ground. Measure the voltage across the resistor. The current in the capacitor and hence voltage dropped across the resistor are proportional to the rate of change of input signal.

Brian.
 

Hi,

I don´t think a purely analog solution gives precise results with this low V/t rates.
At least it will be very difficult, expensive and prone for leakage current errors.

But any simple microcontroller with built in ADC will work.

Klaus
 

Why do you say analog will be error prone, is it due to large capacitance values? In my design I have to do pure analog and no digital
 

Hi,

I´ve already exlplained it:
and prone for leakage current errors

****
I can´t find a specificaton about your expected accuracy.
Did you do some calcuations on your own?

Depending on your specifications...
* an electrolytic capacitor may fail because the leakage currents
* a ceramics capacitor may fail because it´s unlinearity (depending on ceramics type, temperature range, expected precision, and your signal voltage range)
* thus I assume you need a good film capacitor.

The higher the capacitance value, the higher the current, the lower the external error influence.
Thus if you use a good 100uF foil capacitor ( wich costs 10 ... 100 times of a microcontroller)
and you have a 10mV/min rate and you want the leakage current error to be less than 10% then the total leakage current needs to be less than 2nA = 0.002uA.

Total leakage current = Capacitor leakage current + PCB leakage current + wiring leakage currents + measurement amplifier input current + all other leakage currents....
now there may be flux residuals, humidity that changes with daytime, temperature....

Not impossible, but you need (to be) a good electronics designer.

Klaus
 

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