XE8805 SEMTECH [Semtech Corporation], XE8805 Datasheet - Page 133

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

The capacitor C
As an example, for VBAT-VSS=5V, m=4, R
18.7.2
A second order filter function can be implemented by replacing the resistor R
filter section as shown in Figure 18-5. The value of R
schematic.
Another possibility is shown in Figure 18-6. The advantage this solution is that it is easier to stabilize depending on
the component parasitics and board layout. But since it limits the bandwidth of the current regulation loop, it
reduces the rejection of the supply current variations.
For this schematic, all the equations of the first order schematic remain valid. The values of R
calculated from the cut-off frequency:
For the 1kHz example, we can chose R
© Semtech 2006
Sensor
R
C
R
lim1
fs
f
2-wire loop with second order filtering
C
=1kΩ, R
=
fs
2
XE88xx
f
R
π
=
can be calculated from the first order filter cut-off frequency f
f
f
1
c
2
R
π
+
1
lim2
f
f
c
R
1
R
=1kΩ, R
f
c
o
n
o
t
r
l
2
f
2
Figure 18-5. 2-wire 4-20mA loop with second order filtering.
f1
D
=560kΩ, R
VBAT
VSS
A
fs
=150kΩ and C
amp
sense
f2
=100kΩ, R
=50Ω, V
DAS_AI_M
DAS_AI_P
DAS_OUT
DAS_AO
f1a
18-12
+R
fs
EXT
C
offset
=1nF (set BW=0 in this case).
f2
VBAT
f2a
=30V and a 1kHz low pass filter, we can use:
=2.4MΩ, C
has to be chosen the same way as R
R
R
f2
f1a
f
C
=1.8nF.
f1
R
R
f1b
V
offset
f
c
f1
VSS
:
in Figure 18-4 by another first order
regulator
Voltage
R
lim1
R
lim2
XE8805/05A
R
sense
4-20mA
4-20mA
f1
fs
V
V
EXT
EXT
in the first order
and C
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+
-
fs
can be