Fluke 2680A, Service Manual

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268XA
Service Manual
2-44
Thermocouple readings also require an isothermal block reference junction reading to be
taken. If any thermocouple channels are measured in a scan, a reference junction
measurement is taken first, before any channel measurements.
Reference Junction
The reference junction reading is similar to a V dc reading. However, no channel
selection or function relay switching is required; however, the Stallion must be
configured. The reference junction reading is converted to a floating-point value and
returned to the outguard.
Frequency
There are actually two parts to a frequency measurement. First, a normal V ac
measurement is taken using the highest range. You must do this to determine the
amplitude of the input signal, and thus the most appropriate gain setting to use for the
actual frequency measurement. The frequency measurement circuitry works best with a
large amplitude input signal.
Therefore, the gain setting used is one higher than would be used for a normal V ac
measurement. For example, if the autosensitivity reading indicates that the input
amplitude is 3 V, you take the frequency measurement is taken with the AC buffer
amplifier set to the 300 mV range. See Table 2-17, Frequency Sensitivity. For input
signals whose measured amplitude is very low, a frequency reading is still attempted,
since the frequency response of the V ac measurement circuitry rolls off more quickly at
higher frequencies than that of the frequency measurement circuitry.
Table 2-17. Frequency Sensitivity
Measured Amplitude Range Used for Frequency Measurement
Less than 3 V ACR1 (300 mV)
Between 3 V and 30 V ACR2 (3 V)
Greater than 30 V (PAI only) ACR3 (30 V)
The frequency measurement itself does not use the A/D. Frequency measurements are
taken using the Stallion chip. To reduce noise in frequency measurements, the instrument
takes eight frequency readings and averages them together to obtain a single frequency
measurement.
There is only one frequency range (this is different from sensitivity), and therefore the
range field of the channel configuration is ignored for a frequency channel.
The only difference between the three measurement rates for a frequency reading is the
length of time allowed for settling. See Table 2-14, Signal Conditioning Settling Time.
If the status bits returned by Stallion indicate a low frequency (PEROVER bit set), a
value of 0 Hz is returned. If they indicate a high frequency (PEROVER bit clear and
FREQOVER bit set), a value of PLUS_OVLD_VAL (defined in Perform Scan) is
returned.
If the FRDY interrupt is not received within 500 ms of starting a frequency measurement,
the input signal is assumed to be too low in amplitude or frequency to measure, and a
value of 0 Hz is returned.