Analog input circuitry, Analog input circuitry -2 – National Instruments BNC -2140 User Manual
Page 19
Chapter 4
Theory of Operation
4-2
ni.com
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 susceptible to
external noise. ICP-type sensor manufacturers embed a charge amplifier
within the sensor to reduce the effect of cable length, noise, and other
spurious effects. The BNC-2140 supplies the constant current required
to power the embedded charge amplifier in the ICP sensor that allows you
to use inexpensive cables such as BNC cables. Taking advantage of 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 the
section in Chapter 1,
, for a supplier
recommendation.
If you attach an ICP-type of accelerometer or microphone preamplifier to
an analog input channel, you must turn on the BNC-2140 ICP power switch
and enable the ICP circuit for that channel in order to generate the required
power. The ICP circuitry of any input channel can be enabled or disabled
independently of that of any other input channel. When you disable ICP for
a channel, the connection from the ICP circuit to that channel breaks and
has no effect on the incoming signal for that channel. If you do not require
ICP to be enabled on any of the four input channels, disable ICP on all four
channels and 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 in
either DIFF or SE mode.