# LM331 as F2V Converter

Discussion in 'General Electronics Chat' started by adrianolabrantes, Oct 10, 2012.

Oct 10, 2012
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I would like to know if the LM331 is suitable to convert the frequency of a rectangular waveform signal. I am just worried about the IC having trouble with signals that present many harmonic components.

Thank you for your time and I apologize for any noob mistake while posting.

Oct 10, 2012
3
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I also would like to know what is the maximum or suggested amplitude for the input signal coming in to pin 6 through the 470pF cap. Should it be Vs(+15V)? I looked for this information on the datasheet but could not find it.

Thanks

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3. ### ScottWang Moderator

Aug 23, 2012
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1. You didn't talk about the +Vs voltage of LM331.
2. How much voltages for +Vs of LM331 and input signal?
3. How much Khz of input signal?

The 470pf input cap will affect the input frequency, so you can adjust the cap to suit your need.

The better way is the amplitude of input signal equal to +Vs, if you connected +Vs to 15V, and you want the amplitude of input signal more less as 5V, the you can try this way:
1. disconnect the two 10K(pin6,pin7) from +Vs(15V).
2. Connecting the floating pins of two 10K to +5V.

4. ### Dodgydave AAC Fanatic!

Jun 22, 2012
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Oct 10, 2012
3
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The signal I am measuring will barely reach 400Hz, its amplitude is 15V (Vs). My biggest worry is getting a bad measurement due to such a low frequency. From the datasheet I understood that I will be getting a Vout ranging from 0 to 0.4V and I am planning to amplify that using an OP177. Do you think that might work out? Should the input cap be different from 470pF due to this low frequency measurement?

6. ### ScottWang Moderator

Aug 23, 2012
4,933
777
I used 403 hz from ne555 to drive a NPN transistor, and C Of NPN connected to a 10K to +15V.

And I used the signal from C of NPN output to 470pf and 10K, it will make Low level not too Low, so I change 470 pf to 102, it looks seems OK, and I still tried 102+470pf, 103.

By the way, you have to parallel a 1N4148 with 10K, the positive of 1N4148 connecting to +15V.

I don't have the LM331 to try, so you can try it as above described.