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I'm trying to use an LM331 in a frequency to voltage converter configuration.

LM331 block diagram: LM331 block diagram

I'm trying to implement this circuit from the datasheet for a supply voltage of 5V and an input frequency of 0-100 Hz.

fig18 circuit

Given my input signal frequency range, I'm trying to figure out how to set the full scale range of the frequency measurement. Is it as simple as setting the resistor values such that Vout = 5V at 100Hz, or do I need to do something else with the RC timing circuit?

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Page 3 of the data sheet says the the typical output range of pin 1 is 10uA to 500uA. This also means you can probably get a dynamic measurement range of 50:1. Because 500uA is the maximum current, you'd want to choose a resistor that doesn't cause the output to hit the 5V end-stops so if you reckoned 2.5V is the maximum voltage output when 500uA was being driven from pin 1, this equates to a load resistor value of 5kohm.

On the other hand, if you chose the circuit of figure 19 - the current output from pin 1 goes into a virtual earth op-amp circuit and therefore the restriction on impedance is of no importance any more - but obviously you have to obey the rule of not having so much gain in your op-amp circuit that the output hits the rails.

I think it's worth mentioning that although when operating as a voltage to frequency converter, a dynamic range of 100dB is attainable (1Hz to 100kHz) you won't be able to get anything like this when operating as a frequency to voltage converter (as mentioned above).

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  • \$\begingroup\$ Did this answer your question? \$\endgroup\$ – Andy aka Jan 5 '17 at 20:17

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