Elan Digital Systems Ltd.
8
AD125 USER’S GUIDE
2.3 POSSIBLE SOURCES OF MEASUREMENT ERROR
The following is a list of possible error sources that should be
considered when taking measurements with the AD125:
1.
The offset voltage of the A to D device and the front end
electronics will mean that an input voltage of 0V will not produce
an output code of 000000000000b. Software could be used to
correct for zero-point offset errors by using one of the analogue
inputs to the AD125 and tying it to AGND. Switch to this channel
and measure its level to obtain the code for bipolar zero. A set of
factory calibration constants is held on the card and can be
accessed by software to compensate for this error is required.
2.
Gain errors in the A to D device and the front end electronics will
cause full scale “end-point” errors. In other words the A to D
output code may reach 011111111111b before or after the input
gets to nominal full positive scale and similarly the code may
reach 100000000000b before or after the input voltage gets to
nominal negative full scale. A set of factory calibration constants
is held on the card and can be accessed by software to compensate
for this error is required.
3.
Avoid ground loops. These can be caused when the source’s -ve
side is connected via the AD125 cable to AGND and to the shield
on the AD125’s connector. This shield is connected to the PC’s
chassis and so to “earth”( via the gold ESD strips on each side of
the card). The source’s -ve side (unless floating) will be the local
GND and if this too is connected “earth” then if there is any
difference between the two “earth” potentials current will flow in
the AGND wire between the source and the AD125 causing offset
voltages (due to I x R losses) (the current will return through the
mains wiring). Avoid such loops by not connecting the AD125
connector shield to any other terminal (the AD125 already
internally links the shield to the PC’s “earth”). You may also get
problems if you simply connect the shield to “earth” at the source
end; again differences in local “earth” potential will cause currents
to flow in the shield.
4.
Avoid long connections to the AD125’s analogue inputs.
5.
Keep AGND and GND separate. Any digital switching currents
that are allowed to share the same return path as analogue signals
will result in induced voltage noise. AGND and GND are linked
