Measurement Computing WBK17 - Manual (Page 5)

WBK17, Counter/Encoder Module 987996 WBK17, pg. 5
CMOS output driver with high current outputs used with voltages of up to 15 V. Balanced line drivers that
are used at higher voltages (e.g., +15 V and +24 V) and used in differential mode are particularly immune
to external noise sources.
Some encoder outputs will be open-collector type. These require a pullup resistor in order to operate. In
many cases, the pullup resistor is already incorporated inside the encoder, other times it must be supplied
externally. Since the “open-collector with pullup resistor” output is inherently unbalanced (drives strongly
in the low direction, weakly in the high direction) the input differential amplifier will distort the input
waveform. This distortion can be easily viewed when the input channel is scanned by the WaveBook. This
distortion may have to be taken into account when setting the comparator threshold. Many times the best
way to accommodate unbalanced inputs is to AC couple and set the threshold at 0.00V. This forces the
comparator to trigger on the center of the waveform, where the input has high slew and little noise. An
option is to DC couple and set the comparator threshold at the mid-point of the transition.
If external pullup resistors are required, they can be connected at the WBK17’s input terminal blocks.
A pullup resistor can be placed between any input channel and one of the two power supplies offered on the
output power connector (+5V and +15V.) Choose a pullup resistor value based on the encoder’s output
drive capability and the input impedance of the WBK17. Lower values of pullup resistors will cause less
distortion but also cause the encoder’s output driver to pull down with more current. Although the WBK17
has a wide input dynamic range and good common-mode rejection, you should connect the encoder GND to
the COM input, when possible.
Analog Filtering
Analog Filter
Each channel has a single-pole, low-pass filter with three programmable cut-off frequencies.
These are: 100 kHz, 20 kHz, and 30 Hz.
Use analog filtering to reject low-level noise that may otherwise interfere with the comparator. The analog
filters are most beneficial when the unwanted noise is far outside the desired bandwidth. For example, if the
desired input bandwidth is only 0 to 10 kHz, then the 20 kHz low pass filter will reduce unwanted noise
components of 20 kHz by 3dB. Unwanted noise of 200 kHz will be reduced by 23dB; and noise
components of 2 MHz will be reduced by 43dB.
Since the analog filters come before the comparator circuit and the expansion analog output path, their
effects will be observed at the comparator and in the analog waveform scanned by the WaveBook.
The analog expansion path going back to the WaveBook has a 450 kHz single pole filter.