[sdiy] Outputs grounded! Re: SSI2164 datasheet updated

[Mattias Rickardsson]

On Sun, 12 Aug 2018 at 04:21, KD KD <pic24hj at gmail.com> wrote:

> 2018-08-11 21:13 GMT+02:00, Mattias Rickardsson <mr at analogue.org>:
> > I've also used a lot of 2164s in different designs,
>
> >I just watched a video shot by the driver of a car at 190 km/h on
> Autobahn.
> >He also had no problem, so I have to assume it's safe!  (-:
>
> So do Electron products leave the 2164 output open as well, maybe you
> just use them all?
>

I've always found good use for all VCA channels, there's always some
additional feature to come up with - and if not, why not use them in
parallel to win some extra SNR - so I have no experience in connecting
unused VCAs in different ways.

But *if* I would leave a VCA channel unused, I'd definitely connect the
output to ground, since that's the potential it's supposed to sit at when
used. And it *is* still used - not by me, but by the IC. If someone left it
open and didn't notice any problem, then fine - but it doesn't mean that
I'm going to do it or recommend it to anyone. It just means that someone
didn't notice any problem in the actual application. It doesn't mean that
there wasn't any problem, it doesn't mean that the person made all the
measurements needed to detect a problem, it doesn't mean that it gave
optimum results even if there was no problem. And since I don't even know
the application, I can't know that I have the suitable conditions to start
with - so I'm not interested in taking risks using methods that might both
work and fail for me when there are safer methods around. It wouldn't be a
robust design choice.

But I'm happy to hear that it seems to work well in all the existing
products out there with unconnected VCAs, it would be really sweaty to find
out it didn't! :-)

When it comes to the recommendation of grounded inputs I'm a bit more
confused though. The same applies to the "Figure 7: Ultra-Low Noise VCA"
example in the v3.0 datasheet, where all four inputs are tied together
after a common input resistor and compensation network. And also many VCF
schematics I've seen through the years, where the control currents of OTAs
are generated as one common current, and then divided by connecting it to
all OTAs with their control current inputs together.
When a (virtual ground) VCA input is grounded, how can you know it doesn't
get a substantial DC signal current due to imperfections?
When many VCA inputs (or OTA control ports) are tied together, how can you
know they get the same (current) signal?

My gut feeling tells me that the unused VCA inputs should be left
unconnected - or perhaps connected to ground via a typical input resistor
value & compensation network - in order to be "provided no signal".
Connecting it directly to ground makes me know nothing about what happens
to the input signal. But I'm sure some of you can clarify this a bit?

BTW, I did make use of paralleling VCAs, but then with separate input
resistors & compensations networks for each VCA channel.

/mr
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