Semtech 2006
www.semtech.com
18-11
XE8805/05A
The resistor value Rsense is generally chosen between 50 and 150 resulting in a 1V to 3V voltage drop at
maximal loop current. The resistor Rf2 is then chosen much larger depending on the current error requirement.
Allowing for an error of 0.1% gives Rf2=Rsense/0.001.
The resistor Roffset may be omitted but it will reduce the useful code range of the D/A.
Using the large bandwidth of the amplifier is recommended since this increases the rejection of supply current
variations of the other components in the loop. A bypass capacitor between VBAT and VSS will also reduce the
high frequency current variations. Values will depend on the voltage regulator used. The software in the XE8805
should keep the current supply of the circuit as stable as possible. This means that the clock frequency should kept
constant, peripherals should not be switched on and off, the current in the sensor is kept constant, the processor
should not use the halt or sleep modes, etc.
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
Rf1
Rf2
Cf
Rlim1
Rlim2
Voltage
regulator
4-20mA
Roffset
Sensor
VEXT+
VEXT-
Vf
VSS
VBAT
Figure 18-4. 2-wire 4-20mA loop with first order filter
The resistor Rf1 can then be calculated to set the full scale D/A code range (depends on the PWM modulation m,
see section 18.4.3.2) equal to the full scale signal current of 16mA:
mA
R
VSS
VBAT
R
sense
m
f
m
f
16
2
)
1
2
(
)
(
2
1
≤
The resistor Roffset can be calculated in order to obtain a 4mA current while the D/A code is 0:
mA
R
VSS
VBAT
R
sense
f
offset
4
)
(
2
≤
Not
Recommended
for
New
Designs