Used the LM331 V-F? Please help....

[StudentSA]

Hi All...

Im using an LM331 as a simple Voltage to Frequency convertor however I am having some trouble.

The Input voltage signal is only producing a output frequency for 0 -> approx. 3.5V after which the output frequency stops (i.e. 0Hz).

My Vs is 5V and components where selected according to the formula in the datasheet for my required frequency range 0-30kHz.

If anyone has any experiance with this IC please could you shed some light on this issue.

Thanks
StudentSA

 

[ericgibbs]

Hi All...

Im using an LM331 as a simple Voltage to Frequency convertor however I am having some trouble.

The Input voltage signal is only producing a output frequency for 0 -> approx. 3.5V after which the output frequency stops (i.e. 0Hz).

My Vs is 5V and components where selected according to the formula in the datasheet for my required frequency range 0-30kHz.

If anyone has any experiance with this IC please could you shed some light on this issue.

Thanks
StudentSA

hi,
Can you post a circuit, showing your component values etc.?

 

[StudentSA]

Please see attached circuit schematic.



Thanks

 

[ericgibbs]

Please see attached circuit schematic.

View attachment 43004

Thanks

hi SAA,
If you are using the components shown in that diagram, you should get 30kHz at +5Vin.????
What is the output connected too.?

 

[StudentSA]

Exactly 5V input must give me 30kHz output...

The output is currently connected to a 12bit pulse counter... so as I vary the input voltage I see that the speed of the counter is increasing. However once I hit 3.5V and above the counter stops counting completly as soon as I go below 3.5V it carrys on counting.

I will try to increase my Vsupply to 10V when I get home...because possibly Vin might be limited to Vsupply - 1.5V? This is not mentioned in the datasheet but sounds like a good place to start.

 

[ericgibbs]

Exactly 5V input must give me 30kHz output...

The output is currently connected to a 12bit pulse counter... so as I vary the input voltage I see that the speed of the counter is increasing. However once I hit 3.5V and above the counter stops counting completly as soon as I go below 3.5V it carrys on counting.

I will try to increase my Vsupply to 10V when I get home...because possibly Vin might be limited to Vsupply - 1.5V? This is not mentioned in the datasheet but sounds like a good place to start.

hi,
Does your counter have a input 'level' control' for triggering, if yes, it may be set too high.
Also does your counter have a TTL and.or analog signal input, it just could be waveform reduces slightly with frequency and the counter is not detecting the pulses.

 

[mneary]

I will try to increase my Vsupply to 10V when I get home...because possibly Vin might be limited to Vsupply - 1.5V? This is not mentioned in the datasheet but sounds like a good place to start.

That's an excellent idea. Although your schematic looks similar to ones published by National and others, the data sheet app notes never take Vin above (Vs-2V). The Simplified Schematic in the data sheet shows why this is so.

 

[StudentSA]

Yip, The Vin is limited by Vsupply.

From experimentation Vin cannot exceed Vs-1.5V

I find it interesting that this is not mentioned in the datasheet or did I miss it somewhere?

My schematic is from the datasheet with a few "Unnecessary" components erased for simplicity.

 

[mneary]

The Vin limit is given under "common mode range" under Comparator. (Vin is one of the comparator inputs.)