Campbell Scientific CR1000KD CR800 and CR850 Measurement and Control Systems - Page 281

Section 8. Operation Figure 88: Ac power line noise rejection techniques ac Noise Rejection on Large Signals If rejecting ac-line noise when measuring with the 2500 mV (mV2500) and 5000 mV (mV5000) ranges, the CR800 makes two fast measurements separated in time by one-half line cycle (see figure ac Power Line Noise Rejection Techniques (p. 281) ). A 60-Hz half cycle is 8333 µs, so the second measurement must start 8333 µs after the first measurement integration began. The A/D conversion time is approximately 170 µs, leaving a maximum input-settling time of approximately 8160 µs (8333 µs - 170 µs). If the maximum input-settling time is exceeded, 60Hz line-noise rejection will not occur. For 50-Hz rejection, the maximum input settling time is approximately 9830 µs (10,000 µs - 170 µs). The CR800 does not prevent or warn against setting the settling time beyond the half-cycle limit. Table ac Noise Rejection on Large Signals (p. 281) lists details of the half-cycle acpower line-noise rejection technique. Table 56. ac Noise Rejection on Large Signals Applies to analog input voltage ranges mV2500 and mV5000. ac Power Line Frequency Measurement Integration Time CRBasic Integration Code Default Settling Time Maximum Recommended Settling Time* 60 Hz 250 μs x 2 _60Hz 3000 μs 8330 μs 50 Hz 250 μs x 2 _50Hz 3000 μs 10000 μs *Excitation time and settling time are equal in measurements requiring excitation. The CR800 cannot excite VX / EX excitation channels during A/D conversion. The one-half-cycle technique with excitation limits the length of recommended excitation and settling time for the first measurement to one-half-cycle. The CR800 does not prevent or warn against setting a settling time beyond the one-half-cycle limit. For example, a settling time of up to 50000 µs can be programmed, but the CR800 will execute the measurement as follows: 1. CR800 turns excitation on, waits 50000 µs, and then makes the first measurement. 281