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LM331: LM331 has random output when the input is 0V

Part Number: LM331

Hi team,

My customer is using LM331 to convert a input voltage to square wave output. The schematic is as below:

AIin is the input to LM331 and Fout is the output of LM331. We found that when AIin is connected to GND, Fout waveform has some random pulse output. Please see the waveform as below:

Purple:LM331 input AIin

Yellow:LM331 Fout pin

Blue:Optocoupler output AIin*

Green: NA

Zoom in single pulse on Fout waveform as below:

As you could see, the LM331 in stable @0V. But LM331 will output some random pulse. So could you please help look into this issue? Thanks.

Best regards,

Wayne

  • Hi Wayne,

    some component values are hard to decipher. Can you help please?

    Kai

  • Hi Wayne,

    From your schematic, here the V-to-F control parameters I extracted. 

    Rt=3.3 kOhm, Ct = 10 nf

    RL=100 kOhm and Rs = 14 kOhm

    Based on the transfer function, the max Vout frequency is approx. 20.3kHz at Vin=10V. 

    Please remove R1 of 47 ohm. since you do not have Offset adjustment. ( Please ignore the comment. 47 Ohm resistor in series with the CL capacitor provides hysteresis, which provides the better linearity in the comparator circuit, according to section 9.2.1.2). 

    Also, I was wondering if you use any decoupling capacitors at 10V of power rail. C10 or CL capacitor is recommended to use mylar type. 

    Please monitor the 10V supply rail and capture what the voltage rail is doing when unwanted pulse is triggered. 

    I am trying to find the simulation in this part. If I have it, I will post the simulation. 

    Best,

    Raymond

  • Hi Wayne,

    to achieve a good linearity down to 0Hz is very hard to realize for a voltage to frequency converter ranging from 20kHz to 0Hz, because the oscillation period becomes longer and longer and, finely, infinitely long. This "0Hz singularity" should be avoided in any case. Consequently, I do only know from industrial LM331 circuits which do not go all the way down to 0Hz.

    If you carefully look at figure 14 of datasheet of LM331 you will find this:

     

    So the datasheet recommends the operation of this circuit down to only 10Hz and not 0Hz. Reading a bit between the lines, this simply means: "To prevent issues avoid going all the way down to 0Hz." :-)

    Kai

  • Hi Wayne,

    We haven't heard from you in a while. We hope the issue was resolved. I am going to close thing thread now, please reply to reopen if you have any additional questions / information.