RFI AND TRANSIENT SUPPRESSION
INPUT BANDWIDTH LIMITING
Radio frequency interference and transients are a common
occurrence in 4-20mA loops, especially when long wiring
lengths are involved. RFI usually appears as a temporary
change in output and results from rectification of the radio
signal by one or more stages in the amplifier. For sensors
which are closely coupled to the IXR100 and are contained
in a common metal housing, the usual entry for RFI is via the
4-20mA loop wiring. Coaxial bypass capacitors may be used
with great effectiveness to bring these leads into the trans-
ducer housing while suppressing the RFI. Values of 100 to
1000pF are generally recommended. For sensors remote
from the IXR100, coaxial capacitors can also be used to
filter the excitation and signal leads. Additional low-pass
filtering at the IXR100 input helps suppress RFI. The easiest
way to do this is with the optional differential RC filter
shown in Figure 4. Typical values for R1 and R2 are
100-1000Ω, and for C1 are 100-1000pF.
Filtering at the input to the IXR100 is recommended where
possible and can be done as shown in Figure 4. C1 connected
to pins 3 and 4 will reduce the bandwidth with a f–3dB
frequency given by:
f–3dB = 0.159/(R1 + R2 + RTD + RZ) (C1 + 3pF)
This method has the disadvantage of having f–3dB vary with
R1, R2, RTD, and RZ may require large values of R1, and R2.
R1 and R2 should be matched to prevent zero errors due to
input bias current.
SIGNAL SUPPRESSION AND ELEVATION
In some applications it is desired to have suppressed zero
range (span elevation) or elevated zero range (span suppres-
sion). This is easily accomplished with the IXR100 by using
the current sources to create the suppression/elevation
voltage. The basic concept is shown in Figure 5. In this
example the sensor voltage is derived from RT (a thermistor,
RTD or other variable resistance element) excited by one of
the 0.4mA current sources. The other current source is used
to create the elevated zero range voltage. Figures 6a, 6b, 6c
and 6d show some of the possible circuit variations. These
circuits have the desirable feature of noninteractive span and
suppression/elevation adjustments.
Transient suppression for negative voltages can be provided
by the reverse-polarity protection diodes discussed later.
However, positive transients cannot be handled by these
diodes and do frequently occur in field-mounted loops. A
shunt zener diode is of some help, but most zener diodes
suffer from limited current-handling capacity and slow turn-
on. Both of these characteristics can lead to device failure
before the zener conducts. One type of zener, called the
TRANZORB and available from General Semiconductor
Industries, is especially effective in protecting against high-
energy transients such as those induced by lightning or
motor contactors. Choose a TRANZORB with a voltage
rating close to, but exceeding, the maximum VS which the
IXR100 will see. In combination, the coaxial bypass capaci-
tors and TRANZORB provide a very high level of protec-
tion against transients and RFI.
NOTE: Use of the optional offset null (pins 10, 11, and 12)
for elevation or suppression is not recommended. This trim
technique is used only to trim the IXR100’s output offset
current.
MAJOR POINTS TO CONSIDER
WHEN USING THE IXR100
1. The leads to RS and RLIN should be kept as short as
possible to reduce noise pick-up and parasitic resistance.
If the linearity correction feature is not desired, the RLIN
pins are left open.
2. +VS should be bypassed with a 0.01µF capacitor as close
to the unit as possible (pins 18 to 28).
0.4mA
0.4mA
1
R1(1)
4
3
7
3. Always keep the input voltages within their range of
linear operation, +2V to +4V (±VIN measured with
respect to pin 5).
–
20
RS
C1
IXR100
Span Adjust
6
2
15
(1)
R2
5
+
9
10
Elevated
Zero
Suppressed
8
Zero
RLIN
Range
5
0
Range
0.01µF
RZ
RTD
3.9kΩ
–
+
0
NOTE: (1) R1 and R2 should be made equal if used.
VIN
Figure 4. Optional Bandwidth-Limiting Circuitry.
Figure 5. Elevation and Suppression Graph.
®
7
IXR100
