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LM2907-N: Max Output Voltage isn't Outputted at Max Frequency

Anthony Montalbano
Prodigy 50 points
Part Number: LM2907-N

Hi TI forum, so I'm designing a circuit to take rpm and convert it into a voltage to be read by an Arduino. I have create a circuit using the lm2907 IC but when I use an input frequency of 420 hz the max oltage is only 3.62 V. When I did the calculations for R1 and C1 I used a max frequency of 420 hz and a max Vo of 5 V. I don't fully understand the equations for how to find R1, C1, and C2. My circuit does work because when I change the input frequency the output changes in value but never gets higher than 3.62 V. I will include a few pictures and a schematic to give a visual representation of my design. If you need any other details please ask because I would really like to figure out my problem so I can move forward in my development. Also this might not be super important but I'm a second year computer engineer so I might use incorrect terminology and I apologize for that. Thank you for any input!

I also used another forum post to attempt to fix my problems but it didn't make anything better. Link

Specifications

Vin = 5 v

Vout = 0-5v

My Calculated Values

C1 - 80 nF

R1 - 27,777 ohm (I'm using 28k because its close enough)

C2 - 1 uF (I choose this based on the example circuit given in the data sheet)

  • 0
    Guru 140200 points
    Hi Anthony,

    I would increase the supply voltage.

    And why is the resistance at pin 5 (120R) so small? I would increase it to 10k.

    Kai
  • 0 in reply to kai klaas69
    Prodigy 50 points
    The resistance was calculated by one of the equations that says the output current should be less than 50 mA at your input voltage. So I calculated the resistance to be ~100 ohms. I have tried changing it but it doesn't change my output voltage.

    I can try changing the input voltage but my system has a 12 volt supply and the arduino can only take 5v on the input. Can I have the output voltage be from 0-5v while the input is +12v?
  • 0 in reply to Anthony Montalbano
    Guru 140200 points

    Hi Anthony,

    the LM2907-N needs a minimum supply voltage of 6V. But it's better to have a bit more. So, just try a supply voltage of +12V and look what happens. Keep the arduino unconnected for this test. You can later reduce the output voltage by the help of a voltage divider.

    Make the LM2907-N work first. Modify it to fit the arduino in a second step. Divide and conquer!

    Kai

  • 0 in reply to kai klaas69
    Prodigy 50 points
    Ok, Ill try increasing the input voltage. I won't be able to test it until tomorrow so until then I'll recalculate the external component values and get back to you tomorrow. Thank you!
  • 0 in reply to kai klaas69
    Prodigy 50 points

    So I tested using 12 v for vcc. I recalculated the value for R1, RL, C1 and the new values are 67k ohm, 10k ohm, and 34 nF. When I tried the circuit the max Vo is 10.6 at max frequency. I'm lost now. I thinking I should be okay with less than 12 on the output. Is this normal for the lm2907?

  • 0 in reply to Anthony Montalbano
    Guru 140200 points
    Hi Anthony,

    yes, 10.6V is ok. Have a look at the functional block diagram in section 9.2 of datasheet: Between pin 9 and pin 5 there are two pn-junctions plus some other stuff. Makes at least 1.4V voltage drop. And 12V - 10.6V is also 1.4V. So, everything is fine. The LM2907 is a dino under the analog chips, designed at a time when nobody expected the chips providing rail-to-rail performance...

    Kai
  • 0 in reply to kai klaas69
    Prodigy 50 points
    If that is how it's designed then I can work with that. Is there a newer version of a chip that does frequency to voltage? If not then thank you for your help. It was much appreciated!