National Instruments BNC -2140 - Manual (Page 20)

Chapter 4 Theory of Operation
© National Instruments Corporation 4-3 BNC-2140 User Manual
Analog Input Circuitry
The BNC-2140 has four identical analog input channels.
A principal function of the BNC-2140 is to supply a constant current for
ICP type accelerometers and microphone preamplifiers. Many
accelerometers use piezoelectric materials to generate a charge that is
proportional to the acceleration applied. Although these types of
accelerometers have certain advantages, they are very suseptable to
external noise. ICP-type sensor manufacturers embed a charge amplifier
within the transducer to reduce the effect of cable length, noise, and other
spurious effects. The BNC-2140 supplies the constant current required to
power that embedded charge amplyfier ICP that allows you to use
inexpensive cables such as BNC cables. Leveraging off this technology,
some manufacturers use ICP signal conditioning to power their
prepolarized microphones. If your application requires a microphone
preamplifier for use with a prepolarized microphone see
Optional
Equipment , in Chapter 1, for a list of recommended suppliers.
You must enable ICP to generate the required power for each channel that
uses these types of accelerometers and microphone preamplifiers. When
you disable ICP, the connection from the ICP circuit to the analog input
signal breaks and has no impact on the incoming signal. When you disable
ICP on an input channel, the circuitry of two channels is still energized. If
you do not require ICP to be enabled on any of the four input channels, turn
off the ICP power to de-energize the circuitry. Turning off the ICP power
removes any noise the circuitry can induce on the incoming signal.
You can also use the BNC-2140 to select between DIFF and SE input
modes. The BNC-2140 works with any DSA device that has a differential
input stage for each input channel.
In DIFF mode, one line connects to the positive input of the channel, and
the other connects to the negative input of that same channel. You can
connect the differential input to either floating or ground-referenced
signals.
You can use ICP signal conditioning when the BNC-2140 inputs are either
in DIFF or SE mode.