Data Translation DT9837, User Manual

Page: 83 / 150

Principles of Operation
83
The software automatically synchronizes the value of the tachometer input with the analog
input measurements, so that all measurements are correlated in time. The tachometer input is
treated like any other channel in the analog input channel list; therefore, all the triggering and
conversion modes supported for analog input channels are supported for the tachometer
input.
When you read the value of the tachometer input as part of the analog input data stream, you
might see results similar to the following (note that this assumes that the previous
measurement value is returned between new measurement values):
Using the count that is returned from the tachometer input, you can determine the following:
• Frequency of a signal pulse (the number of periods per second). You can calculate the
frequency as follows:
−
Frequency = 12 MHz/(Number of counts – 1)
where 12 MHz is the internal counter/timer clock frequency
For example, if the count is 21, the measured frequency is 600 kHz (12 MHz/20).
• Period of a signal pulse. You can calculate the period as follows:
−
Period = 1/Frequency
−
Period = (Number of counts – 1)/12 MHz
where 12 MHz is the internal counter/timer clock frequency
Table 2: An Example of Reading the Tachometer Input as Part of the Analog Input Data Stream
Time A/D Value
Tachometer
Input Value Status of Operation
10 5002 0 Operation started, but is not complete
20 5004 0 Operation not complete
30 5003 0 Operation not complete
40 5002 12373 Operation complete
50 5000 12373 Next operation started, but is not complete
60 5002 12373 Operation not complete
70 5004 12373 Operation not complete
80 5003 14503 Operation complete
90 5002 14503 Next operation started, but is not complete