Campbell Scientific CR23X Micrologger - Manual (Page 201)

13-1
SECTION 13. CR23X MEASUREMENTS
NOTE: Highlighted portions of this section have not been updated for the CR23X.
13.1 FAST AND SLOW MEASUREMENT
SEQUENCE
The CR23X 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 15 bit successive approximation
technique which resolves the signal voltage to
approximately one part in 60,600 of the full
scale range on a differential measurement (e.g.,
1/60,600 x 10 V = 165 µV). The resolution of a
single-ended measurement is one part in
30,300. Table 13-9 lists possible input voltage
ranges.
Integrating the signal removes noise that could
create an error if the signal were
instantaneously sampled and held for the A/D
conversion. Slow integration times provides a
more noise-free reading than fast integration
times. One of the most common sources of
noise is 60 Hz from AC power lines. For all
input ranges, except the
±
5000 mV range, the
60 Hz integration time of 16.67 milliseconds is
equal to one 60 Hz cycle. During the integration
time, the AC noise will integrate to 0. The 50
Hz integration options lengthen the integration
time to 20 ms for countries that have 50 Hz
power. This is the preferred method of AC
rejection.
The CR23X also has options for faster AC line
rejection. Two integrations are made spaced
1/2 cycle apart (Figure 13.1-1), which results in
the AC noise integrating to 0. This is the
method used by the
±
5000 mV range for AC
noise rejection. This method is also available
on other ranges by preceding the range code
with a 4 (60 Hz) or 5 (50 Hz), i.e., 41, 42..45.
There are several situations where the fast
integration time of 250 µsec is preferred. The
fast integration time minimizes time skew
between measurements and increases the
throughput rate. The current drain on the
CR23X 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.
TABLE 13.1-1. Input Voltage Ranges and Codes
Differential**
Range Code Full Scale Range Resolution
Fast Faster Faster
250
µ
s 60 Hz 50 Hz 60 Hz 50 Hz
Integ. Reject. Reject. Reject. Reject.
10 20 30 40 50 Autorange ***
11 21 31 41 51 ± 10 mV 0.33 µV
12 22 32 42 52 ±50 mV 1.67 µV
13 23 33 43 53 ±200 mV 6.66 µV
14 24 34 44 54 ±1000 mV 33.3 µV
15 45 55 ±5000 mV 166. µV
* Same as 2X 60Hz Rejection Range. Made available for compatibility with 21X Micrologger programs.
** Differential measurement, resolution for single-ended measurement is twice value shown.
*** See autoranging precaution in Table 3.5-1.