Help with LM2917

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rrvroja

Joined Feb 20, 2014
5
Hi

I'm trying to use the LM2917 IC as a frequency to voltage converter circuit. However I am unable to get a change in the output voltage which is proportional to the frequency. I have used the circuit given in the TI data sheet for LM 2917. The Fin is a 1kHz square wave with an amplitude of 5Vpp. The capacitance connected to pin 2 is 0.02 microF and the resistor connected to pin 3 is 560 kilo ohm. There is no change in my output voltage if I vary the frequency. I have tried this circuit with both the 8pin and 14 pin versions of LM 2917. Please help.

Thank You
 

pwdixon

Joined Oct 11, 2012
488
If the diagram is right with 66Hz/V then 1kHz will be 15V, with a 12V rail I would guess nothing would change until your frequency was at least below 792Hz. Even then that assumes the device can swing rail to rail and it probably can't so I doubt that you would see any change before something less than 792Hz. What frequencies have you tried?
 

pwdixon

Joined Oct 11, 2012
488
You also said that the resistor value was not as per the schematic and that probably makes a difference to the Hz/V ratio. Did you calculate the resistor from the datasheet? I haven't looked at the datasheet for that calc yet.
 

pwdixon

Joined Oct 11, 2012
488
Quick look at the datasheet and there's a calculation for output voltage that if you put in 12V for the supply, 1kHz, 0.02uF and 560kOhm looks like more than 120V to me, obviously not possible and means fin might be limited to 100Hz.
 

Thread Starter

rrvroja

Joined Feb 20, 2014
5
Thanks for help. When we tried the lower values of frequency, we were able to get a proportional output voltage.
Our main aim for this circuit was to create a capacitance to voltage converter. Instead of varying the frequency, we kept a fixed frequency and varied the capacitance. The paper we referred to mentioned that if we used Fin as 1KHz, Vcc =10 V, and the resistor at pin 3 as 1MΩ, we would be able to measure the capacitance in the range from 0.1nF to 10mF. However, we need to design a circuit to measure capacitance in the range from 10pF to 100nF. Can you help to design the circuit for these specification?
 

pwdixon

Joined Oct 11, 2012
488
The problem is the number of decades of capacitance. If you have a processor you could try either switching in different resistors or you could change the frequency to get a reasonable output swing over the range of capacitance. Without a processor you will need something like a manually switched set of resistances. Obviously you will also need to read the output and refer back to the frequency of switched resistance to interpret the end capacitance value. All fairly simple stuff if you have a processor involved.
 

bertus

Joined Apr 5, 2008
20,330
Hello,

Did you notice that this is an VERY old thread?
The topic starter was last seen on Feb 26, 2014 and will likely not return.
If you have any questions start a new thread.

Bertus
 
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