Campbell Scientific CR5000 CR5000 Measurement and Control Module - Page 55

Section 3. CR5000 Measurement Details 3.1 Analog Voltage Measurement Sequence The CR5000 measures analog voltages with either an integrate and hold or a sample and hold analog to digital (A/D) conversion. The A/D conversion is made with a 16 bit successive approximation technique which resolves the signal voltage to approximately one part in 60,000 of the full scale range. The maximum conversion rate is 5000 per second or one measurement every 200 µs (10,000 measurements per second on a single channel). The timing of CR5000 measurements is precisely controlled. The measurement schedule is determined at compile time and loaded into memory. This schedule sets interrupts that drive the measurement task. Using two different voltage measurement instructions with the same voltage range takes the same measurement time as using one instruction with two repetitions. (This is not the case in the CR10X, 21X, CR23X and CR7 dataloggers where there is always a setup time for each instruction.) There are four parameters in the measurement instructions that may vary the sequence and timing of the measurement. These are options to measure and correct the ground offset on single-ended measurements each time measurements are made (MeasOfs), reverse the high and low differential inputs (RevDiff), to set the time to allow the signal to settle between switching to a channel and making a measurement (SettlingTime), and the length of time to integrate a measurement (Integ), and to reverse the polarity of excitation voltage (RevEx). 3.1.1 Voltage Range The CR5000 has 5 fixed voltage ranges and autorange. The 16 bit A/D has a resolution of 1 part in 216 (65,536). To allow for some overrange capabilities the A/D is applied to a range approximately 9% greater than the Full Scale Range resulting in the 1 part in 60,000 resolution over the FSR. For example, on the ±20 mV range the full scale range is 40 mV [20 - (-20)] and the resolution is two thirds of a microvolt; 0.04 / 0.000000667 = 60,000. The smaller the voltage range, the better the absolute resolution. In general, a measurement should use the smallest fixed voltage range that will accommodate the full scale output of the sensor being measured. If the voltage exceeds the range, the CR5000 indicates the overrange by returning Not-ANumber (NAN) for the measurement. For signals that do not fluctuate too rapidly, AutoRange allows the CR5000 to automatically choose the voltage range to use. AutoRange causes the CR5000 to make two measurements. The first measurement determines the range to use. It is made with no integration on the ±5000 mV range. The second measurement is made on the appropriate range using the integration specified in the instruction. Both measurements use the settling time programmed in the instruction. AutoRange optimizes resolution but takes longer than a measurement on a fixed range, because of the two measurements required. 3-1