[sdiy] Negative voltage references

Jay Schwichtenberg jays at aracnet.com
Sun May 20 21:51:06 CEST 2012


David,

" I would say that this is not worth losing sleep over."

No offence here, but I'd argue on this one. I'd go with the spec. I've been
on several projects where the EEs didn't pay attention to things like this
and its bitten them. It doesn't look good on their resume when there has
been 10000 units shipped and then most the units in manufacturing start
failing and the product line is shut down. Turns out it's the original stock
of parts has been exhausted and they've ordered the next batch of parts from
a different, cheaper vendor. It suprises me that they can get a dozen or so
units from China to the states overnight and on the engineers desk when this
happens. More than once I've seen this happen.

Even though the parts that you have work, somewhere someone will use a
different brand or batch that will not exceed the specs and they're hosed.
Also on common parts that are multi-sourced it's not a bad idea to get a
number of data sheets and go with the lowest speced one because even though
they have the same part number they may have slightly different specs.

Jay S.

-----Original Message-----
From: synth-diy-bounces at dropmix.xs4all.nl
[mailto:synth-diy-bounces at dropmix.xs4all.nl] On Behalf Of David G Dixon
Sent: Sunday, May 20, 2012 11:48 AM
To: 'Tom Wiltshire'; 'Neil Johnson'
Cc: Synth-diy at synth-diy.org
Subject: Re: [sdiy] Negative voltage references

> How do you deal with the minimum current requirement in your 
> 2164 VCAs, David?

Honestly, I wasn't aware of this requirement.  It isn't explicitly stated in
the datasheet, other than by the fact that 1 mA appears as a lower limit in
the voltage spec table entries.  This could mean simply that they only
measured it over that range.  There is a plot in the datasheet which shows
dropout voltages down to 0 mA output current, and which aren't that
different from those at 10 mA.

In my VCA circuit, I've got six 100k resistors to ground from the 79L05, so
I'm only pulling about 0.3 mA.  I just measured the output of the 79L05, and
it was -5.03V.  When I put a 4.99k resistor from the output to ground, thus
bringing the output current to 1.3 mA, the output voltage went to -5.03V.
No change within the accuracy of my DVM.

I would say that this is not worth losing sleep over.

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