[sdiy] Crosspoint Switch [was: Which programable chip?]

[Neil Johnson]

Hi,

Tony Rolando wrote:
> Neil Johnson wrote:
>> Hi,
>>
>> Joe Grisso wrote:
>>> Sorry to jump in like this, but using the 405x in current mode was
>>> discussed in the last thread. Check out this reference:
>>>
>>> http://www.dself.dsl.pipex.com/ampins/mixer/mixerdes.htm#7
>>>
>>> You wire the switch in between Ri on the op-amp and the inverting
>>> input, which turns the voltage presented on the other side of Ri  
>>> into
>>> a current flowing through the inverting input. It's that virtual
>>> ground, man! ;-)
>>>
>>
>> But ... only if you have the matching switch to ground when the  
>> 'through' switch is open.  Otherwise there won't be any protection  
>> and the input will see pretty much the input voltage ( assuming  
>> that Rsw(open) >> Ri ).
>
> Notice the diodes to gnd on every input pin. Notice the chip is  
> powered by +14.4V and -.6V   :)

In Ken's design - yes I agree its a neat little trick.  But neither I  
nor Joe linked to that page but to the page on mixer design.

Neil
--
http://www.njohnson.co.uk