Measurement WBK21 User Manual
WBK14
Dynamic Signal Input Module
Description…… 1
Current Source …… 2
High-Pass Filter (HPF) …… 2
Programmable Gain Amplifier (PGA) …… 2
Programmable Low-Pass Filter Phase Equalizer …… 3
Programmable Low-Pass Anti-Aliasing Filter…… 2
Simultaneous Sample and Hold…… 3
Excitation Source …… 3
Calibration …… 3
Hardware Setup …… 4
Configuration…… 4
Power…… 4
Assembly…… 4
Software Setup …… 5
Using Accelerometers with WBK14 …… 6
Overview …… 6
Accelerometer Specification Parameters …… 6
Electrical Grounding…… 8
Cable Driving…… 8
WBK14 – Specifications …… 9
Description
The WBK14 is a dynamic analog signal input module for the WaveBook data acquisition system. The
WBK14 provides a complete system to interface to piezoelectric transducers that include accelerometers,
microphones, force/pressure transducers, and others.
Reference Note:
Information regarding accelerometers begins on page 6 of this document module.
Each WBK14 channel has a:
•
current source for transducer biasing
•
high-pass filter
• programmable gain amplifier
•
anti-aliasing low-pass filter
•
simultaneous sample-and-hold (SSH) amplifiers
The gain, filter cut-off frequencies and current biasing levels are software programmable.
WBK14 includes a built-in programmable excitation source. This source stimulates dynamic systems for
transfer function measurements, and serves as a reference signal for calibration.
WBK14, Dynamic Signal Input Module
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WBK14, pg. 1
Current Source
WBK14 provides constant current to bias ICP transducers. Two current levels (2 mA or 4 mA) with
voltage compliance of 27 V can be selected via software. The bias current is sourced through the center
conductor of a coaxial lead and returns to the WBK14 by the outer conductor. The output impedance is
larger than 1 MΩ and presents virtually no loading effect on the transducer’s output. For applications that
do not require bias, the current source can be removed from the BNC input by opening a relay contact.
The current sources are applied to (or removed from) the input in channel groups of two; i.e.,
channels 1-2, 3-4, 5-6, 7-8.
High-Pass Filters (HPF)
Each WBK14 channel has three High-Pass Filters (HPFs) with a 3-dB cut-off frequency (Fc). Two filters
are at 0.1 Hz and the other is 10 Hz. The 0.1-Hz HPF filters are single-pole RC filters. They are primarily
used to couple vibration signals. The 10-Hz HPF is a 2-pole Butterworth type that can be used to couple
acoustic signals or attenuate setup-induced low-frequency signals; since these can reduce the dynamic range
of the measurement (for example when using tape recorders as signal sources).
Programmable Gain Amplifier (PGA)
The HPF removes the DC voltage from the input signal. A PGA amplifies the AC voltage with flat response
up to 500 kHz. Each channel has a PGA with programmable gains (1, 2, 5, 10, 20, 50, and 100) and a
software-controlled DAC for offset nulling. The WBK14 measures only bipolar signals up to 5 V peak.
WBK14 Block Diagram
Programmable Low-Pass Anti-Aliasing Filter
The first filter stage is a programmable 2-pole continuous-time low-pass filter. The filter provides more than
65 dB alias protection to the next filter stage. In addition, it fine-tunes the phase shift of the channel to
optimize the phase-matching between channels. At calibration, the phase shift of each channel is measured
and stored in an EEPROM that is read at configuration.
WBK14, pg. 2
988396
WBK14, Dynamic Signal Input Module
Programmable Low-Pass Filter, Switch-Capacitor
Most of the signal alias rejection is performed by an 8-pole Butterworth filter. This filter is implemented
with a switch-capacitor network driven by a programmable clock (timebase control). Each channel has an
independent clock whose frequency determines the 3-dB cut-off frequency of the filter. The switchcapacitor filter provides no attenuation at the clock frequency—hence, the need for the continuous-time
low-pass filter.
Note: The Low-Pass Anti-Aliasing Filter can be bypassed to process signals with a bandwidth higher
than 100 kHz.
The External Clock input provides a path to externally control the cut-off frequency of the Low-Pass
Anti-Aliasing Filter. The input waveform can be TTL or sinusoidal, with an amplitude peak of at least
500 mV. In this mode, the cut-off frequency is set to the input frequency divided by 50.
Simultaneous Sample and Hold
All WBK14 channels are sampled simultaneously, after which the WaveBook measures each output at
1 µs/channel until all channels are digitized. The time-skew between sampling on all channels (up to 72)
is 150 ns, regardless of the number of WBK14s attached to the WaveBook.
When using WaveBook with an SSH channel enabled, the per-channel sample rates are
reduced. The rate reduction is the same as that which would occur if another channel
were added. The per-channel rate (with SSH enabled) is:
1 MHz / (n+1), where n is the number of active channels.
Excitation Source
The excitation source includes a sine/random waveform generator, a programmable gain amplifier (PGA), a
DC-level DAC, and a phase-lock loop (PLL). The PLL is used to synthesize the frequency of a fixedamplitude sine wave and control the bandwidth of the random signals. The PGA conditions the signal
amplitude to a value between 0 V to 5 V peak. The DC level of the signal is varied independently of signal
amplitude by a software-controlled DAC from -5 V to +5 V. The DC level of the excitation signal can be
used to balance static loads, while the AC signal provides the dynamic excitation.
Calibration
WBK14 is calibrated digitally, eliminating the need for all potentiometers and manual adjustments.
WaveCal, a provided Windows-based program, simplifies the calibration process.
Reference Note:
The calibration program is detailed in the WaveCal Document Module.
WBK14, Dynamic Signal Input Module
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WBK14, pg. 3
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