[sdiy] 2164 Expo correction

[Jerry Gray-Eskue]

<< The stated output offset current of 2164 is
+/-50nA.  I must confess I'm not exactly sure how to interpret that spec (if
anyone has any insight into this spec, I'd love to hear about it)>>

This is most likely the error when the output current should be zero, if
there is no built in way to zero this offset (check you data sheet) you
probably can supply the opposite fixed current to the output to sink or
source the error current. This sort of offset zeroing mechanism has to be
calibrated for each specific 2164, the +- 50nA is the range of the
correction required. Calibrate the correction to a net 0nA when the output
is Theoretically = 0nA. Unlike a Span error offset is a constant value, it
may however drift some with temperature.  

I hope this helps and I did not misread your question and state the
obvious...


-----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, March 27, 2011 9:42 PM
To: 'Scott Gravenhorst'; synth-diy at dropmix.xs4all.nl
Subject: Re: [sdiy] 2164 Expo correction

> Small errors give instruments character...

Arrggh!

Actually, I've been thinking about it, and I see no reason why the gain
factor of 2164 should droop at VC voltages above about 1V.  The stated
dynamic range of the chip is +20 to -100dB, and it doesn't hit -100dB until
VC exceeds about 3V.

However, I realize that, at low frequencies, the current feeding the
integrator in my VCO is pretty small.  At 16Hz, given the 2.2nF cap, the
current is only about 700nA.  The stated output offset current of 2164 is
+/-50nA.  I must confess I'm not exactly sure how to interpret that spec (if
anyone has any insight into this spec, I'd love to hear about it), but if it
influences the current output at low gain settings, then increasing the size
of my integrator cap might fix the low frequencies, and it's easy enough to
try.  Stay tuned...

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