Frequency to Voltage Converter LM2917N(14pin) HELP!

Apr 06, 2006 4 Replies
E

im trying to explore the operation of the chip in zener regulated F>V circuit with 250mVpp @ 1khz,12V Vcc as prescirbed in the datasheet.my threshold levels are rated +-50mVpp so im quite sure that proper output Voh(8.3) and Vol(2.3) shoudl be produced at pin 2 or 3... my R1 is 100k C1 is 20nF and C2 is 1uF and a 10k transistor output(pin5)...



my problem is:


1.my reading at pin 2 is 580 mVdc max
2.my reading at pin 3 is 520mVdc max -> shouldnt it be equal to ic x R1 = 185uA x 100k = 18.5V where my ic or I2=I3 is rated to be about 185uA @ 25degree celsius as based from the datasheet graph
3.my reading at pin 5 is 420mVdc max -> the output voltage which says Vout = 66Hz/volt is nowhere to be attained. changing the input frequency from my function gen. doesnt alter the Vout in anyway...why?
4.my application is in metal detedector. i have a lc tuned oscillator and i want to use the frequency change as my indicator of metal presence as opposed to "mixing"...or dou you have any other way to do this? meaning a simple switch in which when a certain threshold frequency is attained, it outputs a certain voltge level?

thanks! help please!


datasheet is here:

formatting link



Vote
Twitter Facebook LinkedIn
E

im sorry..i forgot to connect pin 12 to ground..the new values are still problematic for me

pin5=pin4 : 80mV max pin3 = 80mV max pin2= 80mV max

Vote
Twitter Facebook LinkedIn
A

[snip]

Are you referring to the zener regulated circuit on datasheet page 9? Pin 5 is connected to Vcc and output is taken across the 10k resistor from pin 4 to ground. Note: pin 4 is also connected to pin 7.

BTW Multi-posting is much better than cross-posting because answers posted in one group automatically appear in the other(s); and you don't have to type your question twice.

Vote
Twitter Facebook LinkedIn
E

yes.im referring to the 14pin lm2917 chip. pin 5 and 10 is connected and then series with a 10k resistor.

Vote
Twitter Facebook LinkedIn
E

thank you for your kind thoughts... yes im referring to the zener F>V converter circuit

initially, i made a circuit that operates in mixing mode.i have a fixed

oscillator of about 480khz and then another one with almost the same frequency and is made variable by the loop inductance..my problem however was that i cant get my signal mix or extract the difference signal. i tried single ended bjt mixer as well as diode but cant get a good output.. also, i would like to indicate metal presence not by an audio tone but by a pic mcu.my initial setup was to rectify and extract the input frequency and input it to my pic mcu which is then compared to a reference voltage which will be pre-calibrated. but having found of chips that can do frequecny to voltage conversion, i opted for this chip now... my application is very simple.i will just calibrate my pic reference voltage and then from there any change in frequency will correspond to a certain voltage... these are my problems:

  1. the maximum input frequency based on datasheet is fmax = I2/(C1xVcc)...from datasheet, for a supply voltage of 12V,my I2=I3 is about 185uA..also, C1 should be at least 500pf due to timing considerations...so that will give me about 30.8khz maximum..my plan is

to further increase my loop inductance to b e in the range of

20-30khz.now my question is how sensitive will this be with the presence of metal? 2.what waveform will i expect from pin 2 and 3? I2 is mirrored to I3 right? isnt it i should be seing a waveform in which its voltage changes from about 8.3 then discharges to about 2.3 which indicated the Voh and Vol for the pin3? 3.my Vout should be varying when i vary my input frequency right? it seems im not gettng the sensible outputs..... please help... _i connected pin3 and 4

lastly,

any idea on a simpler way to realize my application?

Vote
Twitter Facebook LinkedIn

Join the Discussion

Have something to add? Share your thoughts — no account required.

Didn't find your answer?

Ask the community — no account required