LMC567: LMC567 OPERATION WITH PULSE TRAIN AT INPUT

Ivan

Jumping ahead,  while searching the web for any app specific information, it was a great pleasure to come across old old --Signetics part # & notes on this & that. Funny, how long this parts has been around....

I conclude from the writings in various documents, that  the input has a bias level. This surely cannot be impacted by the input source.

So I cannot hook up my previous stage which is an open collector opto output with a pull-up to 3.3V direct to the input pin of LMC567

Now, TI has that info but there are other circuits that would make one believe you could do that. Fro example, in one oscillator, input  pin is grounded.

From observations above, one surely cannot put ground potential at input pin,  wont you agree?

I did have a dc coupling cap initially.

So I have to select  a value consistent with the pull up value & the input impedance.

To see 400 Hz thru, RC should give a HPF cut off at most 100 Hz

So ReqCc> 10ms

In my case, Req= 7.5K

So,  Cc>1.33 uF........so we can use 2.2uF ( Tantalum will do; not X7R at these frequencies)

Now I realize it was a small value(0.1uF)

Would you agree that dc coupling of this sort is required?

thx

robin

Ivan

So this morning, I changed the coupling cap 10x a large value consistent with  freq input I have.

I applied a slow fm form an Agilent source.

50 Ohms, my Spectrum analyzer is also 50 ohm

That aside, at the chip input pin, I see NADA, zilch. DC level there is almost 0( as it should be, from my latest review of LM567 documents)

So there IS something wrong.

Searching the web, I came across some site giving calculations  spread sheet for LM567

And this results in VERY DIFFERENT VALUES.

See attached file.LM567_NEW_VALUES.pptx

What is the real stuff here I can use to get the chip working as I indicated in the very first email text?

thnx

Thnx Ivan

Embarrassing to note that  a simple 1.1/(RC) causes so many arithmetic errors..

The web calculator someone posted does not match with TI's revised equation f=1.1/(R1C1)

If I select (as in the original)  R1=12400 Ohm

C1=.221uF

You get  f=403.22 Hz

which is close enough

The web calculator cannot be trusted

In any event, the circuit does not seem to work with my original values and the original circuit with 1uF decoupling cap.

Pl  check what is not quite right with the d/s equation... I will sweep the input & see where it locks & behaves as a "dual tone decoder"

robin

Ivan

Before I apply at the circuit real input signal, I need to clarify some confusing items in the d/s.

My input is a fixed amplitude 3v pulses with PPM/PWM nature.. This will be capacitively coupled to the input of course.

See attached item from d/s

How do I interpret this input  absolute MIN  p-p to be "2V"?

Yet, elsewhere, we have

which means one can operate with as low as 10s of mV at input(rms)

Then , further down

Here, we see input can go all the up to Vcc( in my case it is 3.3v)

There is a confusing statement about input frequency vs Fosc

this should be typed as

Fosc=2Finput

Do you see this reversal of meaning in (7)?

I need clarification about the max input voltage as soon as you can! I cannot damage any more assemblies....

appreciate

But we are almost there with a novel application of this chip.

robin

wow : the screen shots went blank

see attached pptdata_sheet_excerpts.pptx