Campbell Scientific CR10 CR10 Measurement and Control - Page 171

CR1 O HI OR LO SECTION 13. CRlO MEASUREMENTS FIGURE 13.3-2. Typical Resistive Half Bridge CR1 O HI OR LO INPUT FIGURE 13.3-3. Source Resistance Modelfor Half Bridge Connected to the CR10 DETERMINING SOURCE RESISTANCE The source resistance used to estimate the settling time constant is the resistance the CR10 input "sees" looking out at the sensor. For our purposes the source resistance can be defined as the resistance from the CR10 input through all external paths back to the CR10. Figure 13.3-2 shows a typical resistive sensor, (e.9., a thermistor) configured as a half bridge. Figure 13.3-3 shows Figure 13.3-2 re-drawn in terms of the resistive paths determining the source resistance Bo, is given by the parallel resistance of Rs and Rf, as shown in Equation 13.3-8. Ro = RsRy'(Rs+Rr) [13.3-8] lf Rl is much smaller, equalto or much greater than R., the source resistance can be approximated by Equations 13.3-9 through 1 3.3-1 1, respectively. Ro - R1, Rf..R. [13.3-e] Ro = Ry'2, Rf=R" Ro - Rs, RfttR, [13.3-10] [13.3-11] The source resistance for several Campbell Scientific sensors are given in column 3 of Table 13.3-5. DETERMINING LEAD CAPACITANCE Wire manufacturers typically provide two capacitance specifications: 1) the capacitance between the two leads with the shield floating, and 2) the capacitance between the two leads with the shield tied to one lead. Since the input lead and the shield are tied to ground (often through a bridge resistor, R) in single-ended measurements such as Figure 13.3-2, the second specification is used in determining lead capacitance. Figure 13.3-4 is a representation of this capacitance, C*, usually specified as pfd/ft. C* is actually the sum of capacitance between the two conductors and the capacitance between the top conductor and the shield. Capacitance for 3 Belden lead wires used in Campbell Scientific sensors is shown in column 6 of Table 13.3-2. 13-5