XE8805 SEMTECH [Semtech Corporation], XE8805 Datasheet - Page 131

• Manufacturer Part Number: XE8805
• Description: Data Acquisition MCU
• Manufacturer: SEMTECH [Semtech Corporation]

18.7 4-20mA loop
18.7.1
The amplifier can be used to build a 4-20mA loop externally. Figure 18-4 shows the principle of such a 2-wire loop
using a first order low pass filter.
In a 2-wire loop, the current consumption of the sensor and read-out electronics is drawn on the same wires as the
signal current. The current consumption of the sensor and read-out electronics should therefore remain below
4mA. The signal current is then added by the bipolar transistor. The resistors R
the bipolar transistor against high transient currents during power-up. R
of R
If V
up to the 4mA budget, a component with sufficiently low quiescent current has to be selected.
The resistor R
the gain and R
that the code 0 of the D/A corresponds to 4mA. The amplifier will force a current through the bipolar transistor so
that the voltage on the filter V
I
© Semtech 2006
loop
XE88xx
=(VSS-V
EXT
lim2
is larger than 5.5V, a voltage regulator has to be inserted. Since the quiescent current of the regulator adds
is chosen as a function of the external loop voltage V
R
lim
2-wire loop with first order filtering
EXT
2
=
sense
-)/R
f2
o
n
o
c
t
r
l
(
V
and C
sense
EXT
measures the total current in the loop (if R
.
−
f
V
to set the bandwidth of the filter. The resistor R
D
CEsat
f
−
and VSS on R
R
A
sense
Figure 18-3. Second order low pass filter.
⋅
20
amp
mA
)
sense
20
mA
is equal. This transforms the filter voltage into a loop current
DAS_AI_M
DAS_AI_P
DAS_OUT
18-10
DAS_AO
EXT
sense
and the transistor saturation voltage V
<<R
analog ground
f2
). The resistors R
R
nR
mC
lim1
offset
is generally set to a few kΩ. The value
adds an offset to the filter voltage so
kR
C
lim1
and R
f1
lim2
and R
XE8805/05A
are added to protect
f2
are used to set
www.semtech.com
CEsat
.