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LM331 Voltage to Frequency conversion problems.

Other Parts Discussed in Thread: LM331

I am trying to debug 10 boards that are implementing the LM331 as a voltage to frequency converter. My desired frequency is 63 kHz. On 5 of the boards the frequency is within +/- 2kHz which is okay for my application. However on the other 5 boards the frequency is more like 78 kHz. The only difference that I have found is the voltage at Pin 5 is about 0.5V higher than ones that are in spec. The voltages at all the other pins are the same. All my resistors  are 1% and the caps are 5%. I've tried changing Rt, Ct and the LM331 but the result is still the same. My schematic is shown except that Rs is now 10K and Rt is now 200 with a with a 2.6V input. Any suggestions as to why this is? Disregard the notes on the schematic. Thank You

  • Hello,

    Try replacing the 10k Rs with a 7.5k resistor in series with a 5k pot.  Using the pot are you able to adjust the frequency to the correct value?


  • That is not really a viable solution for manufacturing and doesn't really explain the large differences in outputs.

  • Hi Brady,

    I tried giving you a call, but I get a message that the number is disconnected.  Maybe I copied it down wrong?

    My suggestion for for putting a resistor and pot in the circuit was for trying to troubleshoot the problem.

    An even better method is to take a part from a bad board and put it on a good board and put the part from the good board on a bad board.  Does the problem follow the part or the board? 

    If it follows the board then you need to check that all parts are soldered correctly, no shorts, no open traces or connections, input voltage is clean and stable, supply voltage is clean and stable, and that the components are the correct value.

    If it follows the part then would be a good time to try the resistor and pot substitution.

    Are you using capacitors with low dielectric absorption such as NP0 ceramic capacitors?