PLAN B analog blog

I have received a couple of letters from owners of Model 23's
regarding a rise in held voltages which begins after a few seconds of
the voltage being sampled.  Please hear me out on this:

This anomaly is known as voltage droop and is a characteristic of the
Analog Devices SMP04.  I've uploaded the SMP04 sped sheet to the misc
folder in the photos section of this yahoo group so you can see what
I'm talking about.  

When I set off to design the Model 23, I attempted to address the main
issues that ASR's are famous for f'ing up :  somewhat long refresh time between taps (mening how long it took the device to grab and hold the voltage form the previous tap), gross variations in thevoltages once fanned form tap to tap, and fluctuations caused by the
falling edges of the input trigger.  

The SMP04, in conjunction with the microproc. driving it address these
issues.  However, in order to get the accuracy of the inherent to the
SMP04 (1mv accuracy form tap to tap) , there is another spec which may
make this module problematic for people wanting to use it on slow
passages, such as processing of slow keyboard lines.

The SMP04 has a spec called voltage droop, speced form between 2mv.sec
to 25mv/sec which will cause the held voltages to rise at the same
pace after a few seconds.


Please note:  the 1mv spec refers to the accuracy (tracking if you
will) of the four sample and holds.  

In most applications where an ASR will be used, I dare say standard
ASR applications, this will not be an issue because the sampled
voltage would be updated before the rise begins.  However, if you
attempt to use it for keyboard passages or other slow applications, it
begin before you update.

- P