[sdiy] twilight zone question

[Jason Proctor]

hey mike,

thanks for the response.

i needed to boost the clock coming from the PIC to +15v because i've 
split the divider stages on the cgs36 board in order to cascade them. 
the PIC clock goes via a 4538 monostable directly to the 4017 in the 
/3/6 divider stage - the 4017 is running on +15 so it needs more than 
+5. the output of that stage goes to the "official" CLK input on the 
board. the previous version of this design had the initial divider 
stage on the protoboard running off +5, which then fed the CLK input, 
and that worked fine (as you say, +5 is enough, with the comparator 
parked there).

what's your source for the tl072 information? in the TI & ST 
datasheets i could only find "the magnitude of the input voltage must 
never exceed the magnitude of the supply voltage or 15 V, whichever 
is less" which seems to indicate that rail-to-rail operation in this 
situation is fine.

if the tl072 had problems with 0v input levels in a +15/0v 
environment, then i'd be seeing it all over this module. but i'm only 
seeing it where the signal is coming out of the +5 environment. so 
i'm thinking it might not be that. but then, what do i know... :-)

i don't want to tie up minds with this, and i have a hack that i 
think will work, so no sweat. i'm curious as to the root cause of the 
problem, but i've spent *quite* enough time on this damn module 
already.

j


At 11:16 PM -0700 10/16/06, Mike I wrote:
>Hi Jason,
>About an hour ago I looked at Ken's site and the schemo for his clock
>divider module and some other divider module - both circuits use LM358 in
>the same way at the inputs..... I forget which CGS circuit you have, but
>either one should be able to be connected directly to the 0 to 5V outputs of
>the PIC processor (caveat: I don't know anything about this Blacet module,
>and I assume it puts out TTL-compatible logic levels). There should be no
>need for an intervening 0 to +15V level translation, the LM358 in the
>existing CGS board will do a 0/+5 to 0/+15 translation directly. I would
>advise against replacing the LM358 in the CGS board with TL072 though. If
>you look at the TL072 specsheets, the inputs should not be brought any
>closer than 3 to 4 volts of the most negative supply rail  (in your case,
>ground). The LM358 inputs can be brought right down to ground and it will
>still behave properly.
>Good luck,
>Mike