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LM2907-N: LM2907 0V input, non 0V output

Gary Perez
Prodigy 110 points
Part Number: LM2907-N

Hello, 

Here is my dilemma,  when applying a low signal (near ground), my output of the LM2907 is not zero and the voltage varies, 

I am using the tachometer with minimum component. 

My voltage supply is 12.5V

R1 = 100K

R2 = 10K

C1 = 0.1uF

C2 - 1uF

the device is very linear and works from a frequency input of 3 - 30Hz. But when a low ground signal is applied, my output is not at zero. 

Thanks, for the help. 

-Gary

  • 0
    TI__Guru* 93105 points

    Hello Gary,

    The lower output level VOL of the LM2907-N behaves as described in Note 2, located just  below the 7.5 Electrical Characteristics table, on Pg. 6. This is what Note 2 states:

    (2) VOH = 0.75 × VCC – 1 VBE and VOL = 0.25 × VCC – 1 VBE, therefore VOH – VOL = VCC / 2. The difference (VOH – VOL) and the mirror gain (I2 / I3) are the two factors that cause the tachometer gain constant to vary from 1. Therefore, if VCC is set to 12.5 V, VOL will be about 2.43 V.

    If you can follow the LM2907-N with a comparator, then you could have a near 0 V output on the low end and VCC (of the comparator) on the high end. If you need a comparator suggestion I can help.

    Regards, Thomas

    Precision Amplifiers Applications Engineering

  • 0 in reply to Thomas Kuehl
    Prodigy 110 points
    Thank you for ther help!
  • 0 in reply to Thomas Kuehl
    Prodigy 110 points

    Hi Thomas, 

    It seems I am not getting linear results when a frequency from 15Hz down to 0Hz. 

    From 15Hz to 30Hz, the voltage output is a clean 2.5V to 5V. 

    15Hz down to 0Hz, there seems to be a 200mV offset. Would have an average output of 2.2V  instead of 2V. 

    The results are a little worse when the Amplitude of the input sine wave is decreased. 

    My frequency input is not exactly centered around 0V.  Would this matter?

    Thanks for any advice and tips, 

    Gary

  • 0 in reply to Gary Perez
    TI__Guru* 93105 points

    Hi Gary,

    Please make sure that you have selected the components external to the LM2907-N for best performance over the intended input frequency range. Pages 2 through 6 of the Applications Note 162 are especially informative. If you haven't seen the applications note, you can find it here:

    http://www.ti.com/lit/an/snaa088/snaa088.pdf

    Once you have setttled on your design and if you find the performance still isn't satifactory, then post the circuit diagram showing the input conditions, the supply levels and output load.

    Regards, Thomas

    Precision Amplifiers Applications Engineering