Question about the Castle input buffers

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I am in the concept fase of designing some CMOS video modules, and I was wondering why most of the Castle modules have 0.5 volt at the input buffers (?) as shown in this picture
It looks to me that this is for crossing the threshold of the CMOS input, but I am not really sure.

Can anyone shine some light on this?

Thanks!!

The opamps are configured as comparators, the 0.5v is the threshold voltage. The resistor on the output limits the current to the diodes, which clamp the voltage to 0-5v

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thanks. I was wondering how to get to the CMOS level .

Phil, this seems like as good a place as any to ask since Martijn is touching on a related point - in a few (reputable) places I’ve read advice that it’s better to use dedicated comparators for that function, rather than op-amps (relating to continuous saturation of outputs - see the “The Voltage Comparator” section of https://www.electronics-tutorials.ws/opamp/op-amp-comparator.html). Could you share the reason why your castle designs use LM6172 instead of a high speed comparator IC?

LM6172 was the best I could come up with at the time. I had tried some other methods, lm339 (not particularly highspeed , but dedicated comp) and some other ways, transistor things etc. I tried LM6172 and it worked for the frequencies I was clocking so I just stuck with it. I’m a hacker mostly, not so much an engineer. I really started getting in to electronics when I started doing lunetta CMOS synths, which is a very hacky audio synth DIY thing, which I still adore. Anyway, it ends up giving the castles an interesting look I think.
In Fortress we used dedicated comparators, and they are really sharp, but I still like how the castles look. It’s a different tone palette.

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Thanks for that. The Castle designs do speak for themselves - they work really well and encourage a creative approach that I think is quite unique in the modular video synth space. You can’t argue with results!