![](http://datasheet.mmic.net.cn/Semtech/XE8805AMI028LF_datasheet_100002/XE8805AMI028LF_133.png)
Semtech 2006
www.semtech.com
18-12
XE8805/05A
The capacitor Cf can be calculated from the first order filter cut-off frequency fc :
2
1
2
1
2
f
c
f
R
f
R
C
π
+
=
As an example, for VBAT-VSS=5V, m=4, Rsense=50, VEXT=30V and a 1kHz low pass filter, we can use:
Rlim1=1k, Rlim2=1k, Rf1=560k, Rf2=100k, Roffset=2.4M, Cf=1.8nF.
18.7.2
2-wire loop with second order filtering
A second order filter function can be implemented by replacing the resistor Rf1 in Figure 18-4 by another first order
filter section as shown in Figure 18-5. The value of Rf1a+Rf2a has to be chosen the same way as Rf1 in the first order
schematic.
XE88xx
VBAT
DAS_OUT
DAS_AI_P
DAS_AO
DAS_AI_M
VSS
D
A
c
o
n
t
r
o
l
amp
4-20mA
Rsense
Rf1a
Rf2
Cf2
Rlim1
Rlim2
Voltage
regulator
4-20mA
Roffset
Sensor
VEXT+
VEXT-
Vf
VSS
VBAT
Rf1b
Cf1
Figure 18-5. 2-wire 4-20mA loop with second order filtering.
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 Rfs and Cfs can be
calculated from the cut-off frequency:
c
fs
f
C
R
π
2
1
=
For the 1kHz example, we can chose Rfs=150k and Cfs=1nF (set BW=0 in this case).
Not
Recommended
for
New
Designs