Hi all,
I'm playing around with a tlc04 low pass filter chip at the moment, just wonder if any one here has used one?
I have it working , the one I have sets the cut off at 1/50 of the input clock, which at the moment is a rc (it can be TTL or CMOS ), in the datasheet it says that over 250khz should be 50%duty cycle, can I assume my rc to be around 50% or did I ought drive it from a pic pin? My input frequency is 500 ish kHz
Thanks Geoff

 

According to the datasheet, the input threshold voltage has a very wide range, like 40% variation is possible. This is bad. Check with a 'scope, but the specifications are rather bad.

 

Thanks for the swift reply ,
I see what you mean, looking at it would I get better results driving it with a pwm from a pic, if the rise and fall of the pin are fast it should give a more accurate cut off point? Or should I look at a different part? It is cheap

 

I don't know squat about other filter chips, I just know how to be really picky about specifications.
The Schmidt trigger and the follower amplifier take care of, "fast" transitions but they are miserable for duty cycle.
I suggest you forget the RC method and use something else for a clock.
However, the definition of PWM is, "not a 50% duty cycle". If PWM was always a 50% duty cycle, all the widths would be the same, thus, no modulation.
Sit down and think for a minute. You need an oscillator with 50% duty cycle, and it's not width modulated.
You can probably do that with a PIC.

 

Thanks, sorry for lazy definitions , I have a 'spare' pwm module on the pic I could use to drive the filter, set it going and I can kind of forget about from a code point of view.
Cheers for looking at it , Geoff