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SECTION
13.
21X MEASUREMENTS
averaging
the magnitude of the results from the
two integrations and using the polarity from the
first.
An exception to
this
is
the differential
measurement in Instruction
8
which makes only
one integration.
Setup Amplifier
Integrotion
A/D
Conversion
Settling
450
uS
250uS
fost
550uS
16.67mS
slow
Reset Integrotor
FIGURE
13.2-1. Differential Voltage
Measurement Sequence
Because a single-ended measurement
is
referenced to
21X ground, any difference
in
ground potential between
the sensor and the
21X
will result
in
an
error
in
the measurement.
For example,
if
the measuring lunction of
a
copper-constantan
thermocouple, being used to
measure
soiltemperature, is not insulated and
the potential of earth ground is 1mV greater at
the sensor than at
the point where the 21X
is
grounded,
the measured voltage would be 1mV
greater than the thermocouple output or
approximately
25'C high.
Another instance
where a ground potential difference creates
a
problem is in
a
case such as described
in
Section 7.2, where external signal conditioning
circuitry is powered from the same source as
the
21X.
Despite being tied to
the same
ground, differences
in
current drain and lead
resistance result in different ground potential at
the
two instruments.
For
this reason
a
differential measurement should be made on
an
analog output from the external signal
conditioner.
Ditferential measurements MUST
be used where
the inputs are known to
be
different
from ground, such as
is
the case with
the output from a
full bridge.
In
order
to make
a
differential measurement,
the inputs must
be
within the 21X common
mode range of
+5V.
The
common mode range
is
the
voltage range, relative to 21X ground,
within which both inputs of a differential
measurement must lie
in
order for the
differential measurement to be
made.
For
example,
if
the high side of a differential input
is
at
4V and
the low side
is
at
3V relative to 21X
ground,
there is no
problem;a
measurement
made on the
rSV
range would indicate a signal
of
1V.
However,
if
the high input
is
at 5.8V and
the low input is at 4.8V, the measurement
13-2
cannot be made because the high input
is
outside
of
the
common mode
range.
(The 21X
will indicate the overrange with the maximum
negative number, see Section
3.5.)
Problems with exceeding
common mode range
may
be
encountered when the
21X is used
to
read
the output of external signal conditioning
circuitry
if
a good ground connection
does
not
exist between
the externalcircuitry and the 21X
When operating where
AC power
is
available,
it
is
not
always safe to assume that a good
ground connection exists through the AC
ll
a21X
is used
to measure the output from
a
laboratory instrument (both plugged into AC
power
and referencing ground to outlet ground),
it"is
best
to run a ground wire between the 21X
and the external
circuitry.
Even with this
connection, the ground potential
of
the
two
instruments may not be at exactly
the same
level,
which
is
why
a
difterentialmeasurement
i
desired.
A
ditferential measurement has better noise
rejection than a single-ended measurement.
Integrating the signal in both directions also
reduces input offset
voltage due to thermal
effects
in
the amplifier section
of
the
21X.
lnput
offset voltage
on
a
single-ended measurement
is
less
than 5 microvolts; the input offset voltage
on a
differential measurement is less than
1
microvolt.
A single-ended measurement
is
quite
satisfactory
in
cases where noise is not
a
problem and
care
is
taken to avoid ground
potential
problems. Channels are available for
twice
as many single-ended measurements. A
single-ended measurement takes
about half the
time of a
differential measurement, which
is
valuable
in
cases
where rapid sampling
is a
requirement.
NOTE:
Sustained
voltages
in
excess of
+16 VDC
applied
to
the analog inputs will
damage the
21X
input
circuitry.
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