Section 13. CR10X Measurements
13-9
Figure 13-9 Resistive Half Bridge Connected to Single-Ended CR10X
Input
The size of the peak transient is linearly related to the excitation voltage and in-
creases as the bridge resistor, R
f
, increases. Table 13-4 shows measured levels of
V
eo
for 300m lengths of three Belden wires used in Campbell Scientific sensors.
Values are given for R
f
equal to 1k
Ω
and 10k
Ω
. Table 13-4 only provides esti-
mates of the size of excitation transients encountered; the exact level depends on
the specific sensor configuration.
Equation 7 can be solved for the maximum lead length, L, permitted to maintain a
specified error limit. Combining Equations 7 and 4 and solving for L gives:
L = -(R
o
C
f
+ (t/ln(V
e
/V
eo
)))/R
o
C
w
[15]
where V
e
is the measurement error limit.
Table 13-4 Measured Peak Excitation Transients for 300m Lengths of Three
Belden Lead Wires Used by Campbell Scientific
V
eo
(mV)
V
x
(mV)
R
f
=1k
Ω
Ω
Ω
Ω
R
f
=10k
Ω
Ω
Ω
Ω
#
#
#
#
#
#
8641
8771
8723
8641
8771
8723
2000
50
100
60
100
140
80
1000
25
65
40
60
90
40
Example Lead Length Calculation for 107 Temperature Sensor
Assume a limit of 0.05
o
C over a 0
o
C to +40
o
C range is established for the tran-
sient settling error. This limit is a reasonable choice since it approximates the
linearisation error over that range. The output signal from the thermistor bridge
varies non-linearly with temperature ranging from about 100µV/
o
C at 0
o
C to
50µV/
o
C at 40
o
C. Taking the most conservative figure yields an error limit of
V
e
= 2.5µV. The other values needed to calculate the maximum lead length are
summarised in Table 13-5 and listed below:
1.
V
eo
≅
50mV, peak transient at 2V excitation
2.
V
e
≅
2.5µV, allowable measurement error
3.
t = 450µs, CR10X input settling time
4.
R
o
= 1k
Ω
, 107 probe source resistance
5.
C
f
= 3.3nF, CR10X input capacitance
6.
C
w
≅
142pF/m, lead wire capacitance
Solving Equation 15 gives a maximum lead length of:
L
≅
281m, error
≅
0.05
o
C