Square wave with zero crossing?

ericgibbs said:

I did think initially you had a -V supply on the existing circuit that could be used by the OPA, the CA3140 costs about 60pence GB.
We could use a 7660 for the -V as you suggest.

The point is adding this extra cost brings the total cost close to a MAX232 solution.
The only downside in using the MAX is that the output swing would be in the order of +/-9V.
You could reduce this to a lower voltage by using resistors and zeners.

Get the MAX232 datasheet and check to see if it would be suitable.

Click to expand...

Hi,

Cost is not an issue. We are only making a few parts. Size is a much more important consideration. So the MAX23x chip with the internal caps is very attractive. Can I presume that the low signal (0V) from the PIC gets turned into -9V by the MAX23x and the high signal gets turned into +9V? If that's true, and it works over my range of frequencies, then that would be perfect. It would fit nicely as well.

I experimented with the RC filter circuit last night. With a 10uF cap and a 10K resistor, the pic was able to fire the load device just fine - but only at frequencies over about 2.5KHz. Below that, the signal was distorted or attenuated, although it was hard to tell on the scope. So although this is somewhat workable, it would be better if it worked down to a very low frequency.

Thanks again for all of your help!

Chris C.

ChrisCam said:

Hi,

Cost is not an issue. We are only making a few parts. Size is a much more important consideration. So the MAX23x chip with the internal caps is very attractive. Can I presume that the low signal (0V) from the PIC gets turned into -9V by the MAX23x and the high signal gets turned into +9V? If that's true, and it works over my range of frequencies, then that would be perfect. It would fit nicely as well.
Thanks again for all of your help!

Chris C.

Click to expand...

hi,
The MAX is an invertor, 0Vin gives +9Vout and +5Vin gives -9Vout.

Some types require 4 external caps.