Campbell Scientific CR6 CR6 Measurement and Control System - Page 308
Range, InChan, SettlingTime, RevEx, RevDiff, Offset
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Section 8. Operation Measurements are made using a delta-sigma analog-to-digital (A-to-D) converter with oversampling and digital signal processing (DSP (p. 494) ) to achieve an effective resolution of 24 bits. Prior to the A-to-D converter, a high impedance programmable-gain input-instrumentation amplifier (PGIA) provides signal amplification. See figure Programmable Gain Input Amplifier (PGIA) (p. 308). The CRBasic program controls amplifier gain and input configuration (single-ended or differential). Internal multiplexers route individual terminals to the PGIA. Timing of measurement tasks is precisely controlled. The measurement schedule is determined at compile time and loaded into memory. About 2 ms are required to synchronize and calibrate the A-to-D converter, so the first measurement takes 2 to 3 ms longer than subsequent measurement of the same type in a scan. If the Reps parameter of a measurement instruction is set to an argument >1, these measurements are slightly faster because the gain is not adjusted between repetitions. Several parameters in CRBasic voltage measurement instructions vary the sequence and timing of measurements. Table CRBasic Parameters Varying Measurement Sequence and Timing (p. 309) lists these parameters. Figure 82. Programmable Gain Input Amplifier (PGIA) The voltage measurement proceeds as follows: 1. Set PGIA gain for the voltage range selected with the CRBasic measurement instruction parameter Range. Set the A-to-D converter for the first notch frequency selected with FN1. See section Digital Filter (p. 309). 2. Turn on excitation to the level selected with ExmV or ExuA. 3. Multiplex selected terminals (InChan) to the PGIA and delay for the entered settling time (SettlingTime). 4. Perform the A-to-D conversion. 5. Repeat for excitation reversal and input reversal as determined by parameters RevEx and RevDiff. 6. Apply multitplier (Mult) and offset (Offset) to measured result. The CR6 can make and store measurements from six differential or 12 singleended inputs at the minimum scan rate of 20 ms (50 Hz) using fast-measurementprogramming techniques as discussed in the section Measurements: Faster 308
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