21X Measurements - Campbell 21X Operator's Manual

SECTION
13.
21X MEASUREMENTS
{D
SLOW MEASUREMENT
Setup Amplifier Iniegrotion AfD
Conversion
Settling
13.1
FAST A
SEQUENGE
The 21X
malkes
voltage measurements by
'
integrating
tfie
input signal for a fixed
time and
then holding
the integrated value for the analog
to digital (A/0) conversion. The
A,/D
conversion
is made
with a 14 bit successive approximation
technique
wflich resolves the signal voltage to
approximately one
part in 15,000
of
the full
scale range pn a differential measurement (e.9.,
1/15,000 x 5V =
333pV). The
resolution of a
single-ende( measurement is
one part in 7500.
Integrating
the signal removes noise that could
create an
erior
if
the signal were
sources
of nbise
is
60
Hz
from
AC power
lines.
The
slow
i
time
of 16.67 milliseconds
(1.67ms
on
{ne
5V range)
is
equal to one 60 Hz
cycle.
the integration time, the AC noise
will to
0.
With ditferent software and a
hardware
,
the slow integration time
can be
le
to
20 ms for countries that
have 50 Hz
There are sdveral situations where the fast
integration
tifne of 250 microseconds (25ps on
the 5V rang4) is
preferred.
The
tast integration
time minimides time skew between
measuremehts
and increases the throughput
rate. The current drain on the
21X batteries
is
lower when
fast integration time is
used. The
fast integratipn time should ALWAYS be used
with the
AC half bridge (lnstruction 5) when
measuring
AC resistance or
the output of
an
LVDT. An AC resistive sensor
will polarize
if a
DC
voltage is applied, causing erroneous
readings
anfl sensor
decay. The
induced
voltage in
aft LVDT decays with time as current
in
the primaly coil shifts from the inductor to the
series resistpnce; a long integration time would
result in mogt of
the integration taking place
after
the sigPal had disappeared.
.
450
uS
250uS
fost 550uS
Reset Inteqrotor
16.67mS
slow
FIGURE
13.1-1. Timing of Single-Ended
Measurement
Before making a
series of measurements
prescribed by an lnput Instruction, the 21X
makes a
calibration measurement. The
calibration
is
accomplished by measuring two
known voltages which are sent through
the
same amplifier circuit that
will be used for the
measurements. The calibration for a single-
ended measurement consists of measuring a
voltage which
is
4/5ths
of
full scale and then
making
a measurement with the input
grounded.
A
ditferential measurement is made
once with
the inputs as connected and a second
time with
the inputs reversed (Section
13.2);
calibration for differential
measurements uses
voltages at
r4l5ths
of full scale.
13.2 SINGLE.ENDED AND
DIFFERENTIAL VOLTAGE
MEASUREMENTS
NOTE:
The channel numbering on the 21X
panel refers to differential
channels.
Either
the high or low
side
of a ditferential channel
can be used for single
ended
measurements. Each side must
be
counted
when
numbering single-ended
channels; e.9., the high
and
low sides of
differential channel 8
are single-ended
channels
15
and 16, respectivelY.
The
timing and sequence of a single-ended
measurement is
shown in Figure 13.1-1. A
single-ended measurement is made on a single
input
which is referenced to
ground.
A single
integration is performed for each measurement.
A
differential measurement measures the
difference
in
voltage between two
inputs.
The
measurement sequence on a differential
measurement involves
two integrations: First
with
the high input referenced to the low, then
with
the inputs reversed (Figure 13.2-1). The
21X computes the
differentialvoltage by
instantaneoUsly sampled and held for the A/D
conversion. The slow integration time provides
a more noisq-free reading than the fast
integration
tiine.
One of
the most common
13-1