Measurement Computing DBK Part 2 - Manual (Page 116)

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
).
Digital-to-Analog Conversion
The 12-bit scaled result of the frequency measurement is stored in a DAC to be read by the data acquisition
system. The DBK55 makes use of two DACs. The first DAC is shared by channels 0 through 3 and the
second by channels 4 through 7. Each time the LogBook or Daq device addresses a different DBK55
channel, the associated 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, DBK55 output voltages range from -5.0000 to +4.9976 V.
Calibration for the DBK55 is automatic. When the DBK55 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 settings. The DBK55’s software-adjustable gain and offset can
correct for small errors in the DBK55 or the LogBook or Daq device. This automatic calibration
eliminates the periodic need for manual calibration.
Hardware Setup
CAUTION
DBK55 modules must be configured before connecting them to inputs and outputs.
Failure to do so could result in damage to equipment.
Hardware-related steps for setting up DBK55 include:
• Configuring the DBK55 for the application
• Configuring the Daq device to which the DBK55 is being connected to
• Connecting the input cables to sensors
• Connecting the module’s output cable to a Daq device or LogBook.
Configuring the DBK55 Module
Unless the factory default settings are going to be used, several jumpers and one switch must be set on the
DBK55 module to match both the system setup and the signal-conditioning requirements. Each channel
can have its own individual hardware and software settings.
The default settings are:
• Input Circuit – set to Analog
• Attenuation – set to Enabled (less sensitive)
• Low Pass Filter – set to 100 kHz
The following table indicates the possible settings of each jumper and includes an illustration to facilitate
the location of jumpers. More detailed information concerning jumper settings immediately follows the
table.
DBK55, pg. 6 988793 DBK Option Cards and Modules