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LM331: F-V Converter, for input frequencies below 100Hz

user1077783
Prodigy 40 points
Part Number: LM331

I would like to make the following using LM 311.
· We want to detect 0 to 100 Hz at 0 to 10 V.
· Circuit parameters
 Rs = 62 kΩ
 Ct = 0.1 uF
 RL = 100 kΩ
 Fin = 100 Hz
 Vout = 10 V
 Rt = Calculated  by the following equation: 300 kΩ
 Vout = fin × 2.09 × RL / Rs × (RtCt)

However, it was 2 V at 100 Hz, in the made prototype.
Why does it isn't 10 V at 100 Hz?

  • 0
    Guru 140200 points
    Hi,

    can you show us a complete schematic?

    Is Ct a high quality cap?

    Kai
  • 0
    TI__Guru* 93105 points

    Hello User1077783,

    Indeed it would be best if we can see a schematic for your LM331 F-V circuit design. Then we can see exactly how you are setting up the LM331.

    An often overlooked, and maybe under emphasized point in the datasheet is when selecting components for an LM331 application the value that Rs is set to is fairly critical.

    If you study the datasheet closely you will notice that in the 7.5 Electrical Characteristics table, Rs is set to 14 kΩ. Doing so achives a current source output current of 135 uA, which is about optimum. The 14 kΩ Rs value is mentioned or used in several of the datasheet sections, and the F-V figures 18 and 19, it is shown as a 12 kΩ resistor in series with a 5 kΩ potentiometer. I suspect the 62 kΩ resistor you selected for Rs is resulting in the different output voltage vs frequency output range than expected.

    I suggest setting Rs to 14 kΩ. Then, select the rest of the resistor and capacitor values to satisfy the Vout equation for the 10 V range, and 100 Hz maximum input frequency.

    Regards, Thomas

    Precision Amplifiers Applications Engineering 

  • 0 in reply to kai klaas69
    Prodigy 40 points

    Thank you for your reply.
    I attach a schematic.
    Ct is normal cap of ceramic.

  • 0 in reply to Thomas Kuehl
    Prodigy 40 points

    Thank you for your reply.
    I attach a schematic.

    It operates when changing the resistance value to the following.
    · Rs = 12 kΩ (<= 62 kΩ)
    · Rt = 56 kΩ (<= 300 kΩ)

    In the data sheet [8.3 Feature Description], the following is described.
    "The voltage at pin 2 is regulated at 1.90 VDC for all values of i between 10 μA to 500 μA."
    How do should I decide Rs resistance value?

  • 0 in reply to user1077783
    Intellectual 995 points

    Hi user1077783,

    Try to attach your schematic once again. I am not able to see your attachment.

    Regards,

    Harish KS

  • 0 in reply to user1077783
    TI__Guru* 93105 points

    Hello User1077783,

    Point noted regarding the 10 to 500 uA current source range mentioned in section 8.3. That indeed indicates the useable range for the current source, but doesn't relate how the selected operating current affects the the V-F, or F-V performance. The underlying point is Rs should be close to 14 kΩ for predictable LM331 datasheet performance. That is the Rs value I would use for a design.

    Regards, Thomas

    Precision Amplifiers Applications Engineering