Using the LM331 V-F Converter

[Fowkc]

Hello all,

First time posting. I've got a fair bit of hobby electronics experience, but I'm running into a wall as far as this particular project is concerned.

I'm using an LM331 Voltage-Frequency converter, set up as per this diagram from the datasheet:


Only differences: my Vs is about 12V, and my input is 0 to 5v, not 0 to 10v.

The trouble I'm having is that at low input voltages there is no frequency output at all (just 0v). Ideally I would like a few kHz at 0v. I can see that it's going to be something to do with bias current or the "optional offset adjust" bit, which doesn't seem to do anything in my circuit. I don't have a -Vs though, and I don't know what value of potentiometer would be best to use.

Long story short, I'm stumpted. Can anyone render assistance? I'm aware I could just add a constant offset to the input voltage, but from what I can gather from the datasheet that shouldn't be necessary, as it quotes a 10Hz minimum frequency for the circuit I have.

[audioguru]

Hi Fowkc,
Welcome to our forum. ;D
The circuit goes down to zero Hz with a 0V input. The linearity is spec'd from 10Hz to 11kHz.
You don't need a negative supply nor the offset adjust pot since you don't want zero Hz.
Just bias the input and mix the input voltage with resistors so that when your source voltage is 0V then the circuit gets a few volts and the frequency is what you want.

[Fowkc]

Ah yes, that makes sense. Thanks.

I was hoping not to need yet another op-amp, but ces't la vie...