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Voltage to Frequency Convertor - output unstable

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PRAVEEN K

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I am designing a simple voltage to frequency convertor using LM331. I followed the same circuit which is recommended in its datasheet. But I found the output frequency varying(swings within two values) for a constant input voltage. I tried for different range of inputs but the result was same.

What may be the reason for this swing? Can any one help me?
 

maybe the chip problem? try to change one and test again
 

I don't see a V/F converter in the original LM311 datasheet. Do you actually mean varying frequency or a constant average frequency with two alternating pulse widths? The latter would be most likely by design, observed with many V/F converters.
 

What type of capacitors are you using?
 

I am using tantalum capacitors. The output frequency is varying but not abruptly, it only switches between two values. I tried no. of times with different ICs. I have attached the circuit diagram.
For my application it is undesirable that the output frequency vary for a constant input voltage. I need an output range of 10Hz to 10KHz for input 10mV to 10V( as mentioned in datasheet).

Thank U
 

And I am not using LM311...... its LM331 Precise Voltage to Frequency Convertor
 

I think, the precision V/F circuit may not work correctly with unsuitable Rt/Ct/Cf dimensioning. I don't think that you tantalum capacitors are used for Ct or Cf?
 

Sorry...there was some problem with measuring device, thats why the output switched. I reconnected the circuit , and fed a constant input voltage. Now I found that the output frequency is gradually increasing. In 2 hrs there is a deviation of 10Hz. All the compenents values are exactly as per the ckt diagram. But the output is not stable. Why it is so??
 

I use MF resistors and tantalum capacitors. If its values are varying with temparature, what can be done for temparature compensation?
 

The datasheet says
For the timing capacitor, CT, use NPO ceramic, Teflon®, or polystyrene.
Click to expand...
I think, polypropylene will also do.
 

The datasheet shows a stable mylar capacitor.
But the unstable circuit uses a tantalum capacitor that is extremely unstable.
 

I found the VCO out put constant with mylar capacitors..
 

PRAVEEN,

Hi..i am actually simulating the same circuit with yours. But i cant get anything for my output pin 3, Fout. Do you have any idea why this happen? What device you use to display the output? I need the answer asap.. Thanks =)
 

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