Operator`s manual

13-1
SECTION 13. 21X MEASUREMENTS
13.1 FAST AND SLOW MEASUREMENT
SEQUENCE
The 21X makes voltage measurements by
integrating the input signal for a fixed time and
then holding the integrated value for the analog
to digital (A/D) conversion. The A/D conversion
is made with a 14 bit successive approximation
technique which resolves the signal voltage to
approximately one part in 15,000 of the full
scale range on a differential measurement (e.g.,
1/15,000 x 5V = 333µV). The resolution of a
single-ended measurement is one part in 7500.
Integrating the signal removes noise that could
create an error if the signal were
instantaneously sampled and held for the A/D
conversion. The slow integration time provides
a more noise-free reading than the fast
integration time. One of the most common
sources of noise is 60 Hz from AC power lines.
The slow integration time of 16.67 milliseconds
(1.67ms on the 5V range) is equal to one 60 Hz
cycle. During the integration time, the AC noise
will integrate to 0. With different software and a
hardware modification, the slow integration time
can be lengthened to 20 ms for countries that
have 50 Hz power.
There are several situations where the fast
integration time of 250 microseconds (25µs on
the 5V range) is preferred. The fast integration
time minimizes time skew between
measurements and increases the throughput
rate. The current drain on the 21X batteries is
lower when fast integration time is used. The
fast integration time should ALWAYS be used
with the AC half bridge (Instruction 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 and sensor decay. The induced
voltage in an LVDT decays with time as current
in the primary coil shifts from the inductor to the
series resistance; a long integration time would
result in most of the integration taking place
after the signal had disappeared.
FIGURE 13.1-1. Timing of Single-Ended
Measurement
Before making a series of measurements
prescribed by an Input 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 differential 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 ±4/5ths 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 differential channel
can be used for single ended
measurements. Each side must be
counted when numbering single-ended
channels; e.g., 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 differential voltage by