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Am i analyzing this circuit correctly?

Stree

Dec 24, 2009
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Dec 24, 2009
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I want to create a simple anenometer like this for low speeds:

Circuit diagram:
http://archive.electronicdesign.com/files/29/1978/figure_01.gif

The authors explanation is

http://electronicdesign.com/article...s-and-a-microcontroller-simplify-anemome.aspx

Im not sure if i analyzed this one right so here it goes!

When deriving the output equation for the op amp of the circuit above, i found that it is approximately (R7 / R5) *A * V_ref where A * V_ref = DAC output. So the circuit basically attempts to keep R7/R5 = 1/A due to the comparator right after the op amp output.

However, the amount of time for the pulse skip modulation to be on and increase the resistance of R7 is dependent not only on the ratio but also on the temperature because at lower resistances the resistance of R7 needs to be increased less in order to maintain the ratio. If at ambient temp both were 500 ohms, then R7 would need to be heated to 1k ohm, 500 ohm difference, while at 100 ohms there will only be a 100 ohm difference to keep a 2:1 ratio.

I'm just a little confused because the author states that the voltage divider cancels out ambient temperature effects. I suppose it might at very small ambient temperature changes, but at larger variations of temperature, say from -50 to 0 celsius, temperature would need to be taken into account, right?
 
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