# Frequency to Voltage Converter LM2917 Circuit Setup

#### simply_me

Joined May 6, 2010
50
Hello,

I'm attempting to convert a received signal frequency to voltage for a comparison/subtractor with the sent/original signal (I've a linear ramp oscillator). From the datasheet of the LM2917 f/v converter I know that Vout= Fin* Vcc*R1*C1.
If my original signal is about 4.2V (Vp-p), to which Vout should I design the circuit?
Ultimately, in the comparison stage, I need the voltage difference between the original signal and the one received (maybe I should have a subtractor?) in order to measure distance.

Thank you.

#### SgtWookie

Joined Jul 17, 2007
22,210

#### simply_me

Joined May 6, 2010
50
Hi SgtWookie,

Thank you for the link, I've actually looked at it before I posted (since I'm using Multisim, and it doesnt have the spice model). My question is different. I'm asking to which voltage should I design my circuit to be? I have a circuit from the datasheet that I think will do the job (Frequency to Voltage Converter with 2 Pole Butterworth Filter to Reduce Ripple). As stated above, I'm just not sure to what voltage should I design my circuit considering my later intentions (comparator or subtractor).

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#### SgtWookie

Joined Jul 17, 2007
22,210
Better read the notes in the Spice model.

Russ Kincaid, who made the base macromodel for the 2907, put a note in there about saturation at Vcc/2. I don't have any of these IC's, so don't know how accurate the simulation will be compared to the real thing. I do know that if your output gets too high in voltage, strange things happen that would not happen with a real device.

The real 2917x has a 7.5v Zener diode in it. That puts Vcc/2 at 3.75v. I suggest that you plan on making your circuit so that the output from the 2917 does not go higher than 3.75v for your input frequency range.

I don't know what else to tell you off hand.

Download LTSpice, and try out the models.

#### simply_me

Joined May 6, 2010
50
That might be a little problematic, since my output sawtooth wave is about 5.40V. In order to compare/subtract the two Vout from the LM2917 must be greater than that no?

#### SgtWookie

Joined Jul 17, 2007
22,210
You could use a voltage divider for your other input, or amplify the output of the 2917 using another opamp.