[sdiy] Digi-Mod schematic

[Seb Francis]

>> - page 8: given the critical nature of the 220p capacitor I would put 
>> the BAT54S on the left-side of C47.  Or, an alternative approach 
>> would be to put the BAT54S on the input side of R85, and put, say, 
>> 100R on the output of the 4580 inside the FB loop to protect its o/p 
>> stage when going -ve.  Also simplifies layout as you don't need to 
>> protect up to +15V rail then (one less track to worry about).
>
> I'm not sure about putting the protection on the left of C47 as the DC 
> bias levels are quite different: 0V to the left and 2.5V to the 
> right.  Can't see a neat way to do it other than how I have it.
>
> The BAT54S datasheet quotes a capacitance of 10pF max, presumably per 
> diode.  Do you really think this will make much of a difference?
>

Actually, looking at this again, if I keep the gain at unity right up 
until after U14:A, then do all the gain reduction with the R85/R86 
potential divider, it won't need protection anyway.  The gain will be 
0.1636 so a 15V swing either side of 0V becomes a 2.454V swing either 
side of 2.5V which is within the abs max.

Still interested to know why you think the BAT54S would have been a bad 
idea.  Is it only the extra capacitance or are there other parasitic 
effects that I've overlooked?

Seb