Campbell Scientific CR10 CR10 Measurement and Control - Page 168

SECTION 13. CRlO MEASUREMENTS Setup Amplifier In tegrotion Settlin g A/D Conversion 450 uS 25OuS fost 70OuS 2.72mS slow Reset Integrotor F|GURE 13.2-1. Timing of Singld:Ended Measurement 13.2 SINGLE.ENDED AND DIFFERENTIAL VOLTAGE MEASUREMENTS NOTE: The channel numbering on the old silver CR10 wiring panel refers to ditferential channels. Either the high or low side of a differentialchannel can be used for single-ended measurements. Each side must be counted when numbering singleended channels; e.9., the high and low sides of difterentialchannel4 are singleended channels 7 and 8, respectively. The timing and sequence of a single-ended measurement is shown in Figure 13.2-1. A single-ended measurement is made on a single input which is referenced to ground. A single integration is pedormed 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 13.2-2). The CR10 computes the differentialvoltage by averaging the magnitude of the results from the two integrations and using the polarity from the first. An exception to this is the differential measurement in Instruction 8 which makes only one integration. Because a single-ended measurement is referenced to CR10 ground, any difference in ground potential between the sensor and the CR10 will result in an error in the measurement. For example, if the measuring junction of a copper-constantan thermocouple, used to measure soiltemperature, is not insulated and the potential of earth ground is 1 mV greater at the sensor than at the point where the CR10 is grounded, the measured voltage would be 1 greater than the thermocouple output or approximately 25'C high. Another instance where a ground potential difference creates a problem is in a case such as described in Section 7.2, where external signalconditioning circuitry is powered from the same source as the CR10. Despite being tied to the same ground, differences in current drain and lead resistance result in different ground potential at the two instruments. For this a differential measurement should be made on an analog output from the external signal conditioner. Differential measurements MUST be used where the inputs are known to be different from ground, such as is the case with the output from a full bridge. Setup & Integ. A/D Conv. Switch lnpt. lnteg. A/D Conv. Settle & Settle 250us fost 700us 600us 250us fost 700us 2.72ms slow 2.72ms slow FIGURE 13.2-2. Ditferential Voltage Measurement Sequence 13-2