User manual

DBK7, pg. 12 879895 DBK Option Cards and Modules
The following equation determines the time interval needed to measure a frequency:
Minimum Measurement Period (sec) = (4096 x 0.5 µs) [F
max
/(F
max
- F
min
)]
In this equation: 4096 derives from 12-bit precision; 0.5 µs is the resolution of the DBK7’s timing circuits;
and F
max
/ (F
max
- F
min
) is the ratio the measurement time must be increased to achieve 12-bit accuracy over
the selected range.
To see how the measurement period varies, consider two examples:
• To measure frequencies from 59 to 61 Hz, the measurement period is at least
4096 x 0.5 µs x 61/2 = 62.5 ms, or about 16 measurements per second.
• To measure frequencies from 1 to 61 Hz, the measurement period is at least
4096 x 0.5 µs x 61/60 = 2.1 ms. Note that as the DBK7 only measures frequency once per
cycle, it would take from 1 to 61 measurements per second.
Thus, measuring frequencies over a narrow range takes longer than over a wide range as the ratio of
F
max
/(F
max
- F
min
). The actual measurement time is the sum of several items: the minimum measurement
period (from the equation above), the actual input period, and a variable processing time of 0 to 4 ms.
Note: If the Sequence Rep Rate is set faster than the measurement rate, multiple readings of the
same measurement will occur.
After the frequency (F) is measured to the required accuracy, it is scaled to a 12-bit number (D) for use by
the Digital to Analog Converter (DAC). This 12-bit number is determined by the formula:
D = 4096 [(F - F
min
) / (F
max
- F
min
)]; where: 0 < DAC < 4096
If the measured frequency is F
min
, then the scaled result is 0. If the measured frequency were F
max
, then the
scaled result would be 4096 but is limited to 4095. Measured frequencies below F
min
are scaled as 0;
frequencies above F
max
are scaled as 4095. The highest frequency that produces an accurate result is the
one that converts to a DAC value of 4095; that is, F
min
+ 4095/4096 (F
max
- F
min
) which is the same as
F
max
- 1/4096 (F
max
- F
min
).
D/A Conversion
The 12-bit scaled result is stored in the DAC to be read by the data acquisition system. Instead of having
four DACs with their associated circuits, the DBK7 shares a single DAC among the four channels. Using
the DAC this way makes the DBK7 more reliable, cost-efficient, and easier to calibrate. Each time the
LogBook or Daq device addresses a different DBK7 channel, the DAC supplies the corresponding voltage
(V
out
) according to the formula:
V
out
= 10.0 (D/4096) - 5.0 V
Since DAC values (D) range from 0 to 4095, DBK7 output voltages range from -5.0000 to +4.9976 V.
Calibration for the DBK7 is automatic. When the DBK7 is initialized through software, its gain and offset
errors are measured. The output circuits are then adjusted so the LogBook or Daq device measurements
correspond to the DAC setting. The DBK7’s software-adjustable gain and offset can correct for small
errors in the DBK7 or the LogBook/Daq device. This automatic calibration eliminates the periodic need
for manual calibration.