In my quest to get a 4050 working from VC++, I've come across a problem in
that autoranged AC RMS voltage readings through my code are not the same as
AC RMS voltage readings taken using the soft front panel set to the 20mV
range. The signals we're measuring can be down in the millivolt range. The
problem seems to be that, although autoranging goes down to the 20mV range
on DC measurements, it only goes down to the 200mV range for AC
measurements. This can give up to a factor of 10 error in the measurement.
I can get the correct reading if I manually set the range, but this isn't
ideal since it means I have to either impliment my own autorange or make
assumptions on the reading before making it- and since this code rarely
measures such small readings, the
assumptions have to be made at a higher
level than my driver which makes things messy.
I've come across a mention that autoranging is not recommended for small
signals due to the chance of noise affecting the autoranging. Noise however
does not seem to be a problem- the noise on the measured value is small.
So the point of the mail; is autoranging confirmed broken for the bottom ACV
range (even though it works for DCV), or is there something I could be doing
wrong?
--
Dr. Craig Graham, Software Engineer
Advanced Analysis and Integration Limited, UK. http://www.aail.co.uk/
