Omega Engineering OMB-DBK-34A, User Manual

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Crosstalk
Crosstalk is a type of noise related to source impedance and capacitance, in which signals from one
channel leak into an adjacent channel, resulting in interference or signal distortion. The impact of source
impedance and stray capacitance can be estimated by using the following equation.
T = RC
Where T is the time constant, R is the source impedance, and C is the stray capacitance.
High source (transducer) impedance can be a problem in multiplexed A/D systems like the DBK12,
DBK13, DBK15. When using more than 1 channel, the channel input signals are multiplexed into the
circuit card. The multiplexer samples each signal for only 10 µs and then switches to the next input signal.
A high-impedance input interacts with the multiplexer’s stray capacitance and causes crosstalk and
inaccuracies in the A/D sample. In such cases, the source impedance should be less than 1 k
Ω
. If the
source impedance exceeds this value, sampling problems can be expected.
A solution to high source impedance in relation to multiplexers involves the use of buffers. The term
buffer has several meanings; but in this case,
buffer refers to an operational amplifier having high input
impedance but very low output impedance. In the example illustrated, a buffer has reduced the source
impedance from 10 k
Ω
to effectively 0 k
Ω
. Placing such a buffer on each channel (between the transducer
and the multiplexer) prevents the multiplexer’s stray capacitance from combining with the high input
impedance (10 k
Ω
in the example). This use of a buffer also stops transient signals from propagating
backwards from the multiplexer to the transducer.
Using DBK Cards and Modules for Signal Conditioning
The DBK signal-conditioning units are designed for use with Daq devices and LogBooks. Although the
DBK options can be used with ISA or PCI bus-based data acquisition boards from other vendors; they
perform best when used with an acquisition device that can dynamically select channel, gain, and range.
Dynamic channel and gain/range make it possible to have high channel-to-channel scan rates with a variety
of transducers.
DBK output signals can be bipolar, e.g., -5 to +5 V, or unipolar, e.g., 0 to 10 V. The user can select a
range of relevant values to correspond to the lowest signal (e.g., -5 or 0 V) and the highest signal (e.g., 5 or
10 V) signal. This type of range selection guarantees the highest resolution in 12-bit or 16-bit conversion.
DBK modules share the same footprint as the DaqBooks and LogBooks. This dimensional aspect provides
for convenient stacking of modules with each other and with most notebook PCs. Note that most DBK
modules have their own power supply.
1-6 Signal Management
886995
DBK Option Cards and Modules