3424-XX-UM
ProDAQ 3424 Function Card User Manual
Page 14 of 56
Copyright,
1998-2005 Bustec Production Ltd.
CHN+
Input protection
+
24V
10M
10M
Input protection
1uF
1uF
PGA 1
Low PassFilter
24-bit
ADC
TEDS_ON
IC
P
_
O
N
POS_CPL
NEG_CPL
GAIN1SEL[1:0]
To FPGA
PGA 2
GAIN2SEL[1:0]
VREFGND_EN
VREFGND_EN
VREF-/GND
for calibration
N
E
G
_
G
N
D
TEDS reader
circuitry
-5V
VREF+/GND
for calibration
To FPGA
SENSE ICP
Current
regulator
diode
CHN-
Figure 4
– Configuration of analog front-end circuitry (single channel) of the 3424 card
3.1.2. ICP sensor conditioning
An ICP sensor requires a constant current bias. This is generated using a Current Regulator Diode
(CRD) connected to +24V. For a +24V supply the sensor output will then bias at about +10V with
10V max swing. The accuracy of the current source is not critical although it needs to be constant.
At high frequencies ICP sensors require higher current in order to operate with the same cable
length. The maximum frequency is proportional to the current & inversely proportional to the cable
capacitance (i.e. cable length). Thus, the 3424 is fitted with one 4.7mA CRD as a standard with the
second one fitted upon request.
The ICP excitation current is switched to the ICP sensor with a relay.
If ICP is selected and the ICP
excitation current is flowing then a LED lights to indicate this. Because it often happens that the
sensor do not connect directly to the VXI module, but to some external signal conditioning unit or
breakout board, it is desired that the ICP current can be indicated in this remote location. This can
be achieved with additional SENSE ICP signal routed to the front panel SCSI connector.
Comparing voltage drop across 22
sense resistor with a reference voltage level allows switching
on a LED diode in the signal conditioning unit when the ICP excitation current is really flowing.
Other so
lutions, like digital control of a remote LED, don’t allow for detection of open circuit in this
case.
The positive output of the ICP sensor connects to the positive input of the channel. During normal
operation, the signal is AC coupled in order to remove the DC bias. The negative side of the ICP
sensor needs to be connected to the same GND as the +24V supply. For the 3424 card this is
achieved with means of a relay that can switch the negative side to GND for single-ended sensors.
At low frequencies there are two important considerations. One is the Discharge Time Constant
(DTC) of the sensor. This varies from sensor to sensor and can be from milliseconds to several
hundreds of seconds. The user needs to consider this effect when measuring signals.
The second low frequency effect is the time constant of the coupling circuit when used in AC
coupling mode.
A 10MΩ resistor with a 1μF coupling capacitor requires 50 seconds to reach 5RC
(5 time constants), required for drift free stable operation.
3.1.3. TEDS reader interface