Campbell CDM-A100 Series, Instruction Manual

Page: 56 / 61

Appendix D. Calculating Network Restrictions
C-3
f
N1
= �
measurement time
− 31
µs
repetitions
− 5
µs
2
−
T
settle
− 180
µs
�
−1
• 10
6
µs/s
With a measurement time of 5 ms (5000 µs), settling time of 100 µs, and 7
repetitions, f
N1
must be greater than 13807 Hz.
f
N1
= �
5000
µ
s − 31
µs
7
− 5
µs
2
− 100 µs − 180
µs
�
−1
• 10
6
µs/s
= 13807 Hz
Thus, to improve measurement resolution, f
N1
should be set to 15000.
According to TABLE D-1 , Analog Voltage Measurement Range and
Resolution
(p. D-1)
, this would improve the resolution to 9.012 µV
RMS
for the
largest voltage range, ±5000 mV.
C.3 Example 3
How fast can a CDM-
A116 measure ten 3 kΩ full
-bridge sensors using the
default settling time (500 µs) and both excitation and input reversal, and
achieve a resolution of less than 1 µV
RMS
? For easier wiring, limit the
sensors excited per excitation terminal to four.
The full-bridge measurement, BrFull(), makes a differential measurement. It is
given that the measurement will use input reversal, so the resolution is given in
the third column of TABLE
D-1 , Analog Voltage Measurement Range and
Resolution
(p. D-1)
. To achieve a resolution of less than 1 µV
RMS
within the
±5000 mV range, f
N1
must be 100 Hz or better.
The measurement time is figured separately for each excitation channel used,
where the repetitions are the number of sensors excited on that channel. In this
setup, three excitation terminals are used, X1 – X3. The measurement times are
given by the following equations:
X1 measurement time = repetitions on X1
• � 4 • �
T
settle
+
10
6
µs/s
f
N1
+ 180
µs
� + 8
µs
� + 46
µs
+ 31
µs
X2 measurement time = repetitions on X2
• � 4 • �
T
settle
+
10
6
µs/s
f
N1
+ 180
µs
� + 8
µs
� + 46
µs
+ 31
µs
X3 measurement time = repetitions on X3
• � 4 • �
T
settle
+
10
6
µs/s
f
N1
+ 180
µs
� + 8
µs
� + 46
µs
+ 31
µs
Because there are 10 measurements total, repetitions on X1 will be 4,
repetitions on X2 will be 4, and repetitions on X3 will be 2. Using the default
settling time (500 µs) and the first notch frequency figured earlier (100 Hz), the
sum of the above equations gives a total measurement time of 427511 µs
(427.511 ms). Therefore, a practical scan rate for this setup would be 500 ms.