overcurrent conditions exceeding these specifications. Refer
to Figure 6. The simplest and least expensive method is a
resistor as shown in Figure 6a. The value of the resistor is
determined from the expression
R
X
= V
CC
/40mA – 75Ω
and the full scale voltage drop is
V
RX
= 20mA
x
R
X
.
For a system operating off of a 32V supply R
X
= 725Ω and
V
RX
= 14.5V. In applications that cannot tolerate such a
large voltage drop, use circuits 6b or 6c. In circuit 6b a
power JFET and source resistor are used as a current limit.
The 200Ω potentiometer, R
X
, is adjusted to provide a current
limit of approximately 30mA. This circuit introduces a
1–4V drop at full scale. If only a very small series voltage
drop at full scale can be tolerated, then a 0.032A series 217
fast-acting fuse should be used, as shown in Figure 6c.
For automatic fold-back protection, use the circuit shown in
Figure 15.
VOLTAGE REFERENCE
The RCV420 contains a precision 10V reference. Figure 8
shows the circuit for output voltage adjustment. Trimming
the output will change the voltage drift by approximately
0.007ppm/°C per mV of trimmed voltage. Any mismatch in
TCR between the two sides of the potentiometer will also
affect drift, but the effect is divided by approximately 5. The
trim range of the voltage reference using this method is
typically
±400mV.
The voltage reference trim can be used to
trim offset errors at the output of the RCV420. There is an
8:1 voltage attenuation from Ref In to Rcv Out, and thus the
trim range at the output of the receiver is typically
±50mV.
The high-frequency noise (to 1MHz) of the voltage refer-
ence is typically 1mVp-p. When the voltage reference is
used for level shifting, its noise contribution at the output of
the receiver is typically 125µVp-p due to the 8:1 attenuation
from Ref In to Rcv Out. The reference noise can be reduced
by connecting an external capacitor between the Noise
Reduction pin and ground. For example, 0.1µF capacitor
reduces the high-frequency noise to about 200µVp-p at the
output of the reference and about 25µVp-p at the output of
the receiver.
V+
V
RX
R
X
4–20mA
3
2
1
RCV420
15
14
V
O
a) R
X
= (V+)/40mA – 75Ω
V+
2N3970
200Ω
4–20mA
3
2
1
RCV420
15
14
V
O
R
X
b) R
X
set for 30mA current limit at 25°C.
V+
f
1
4–20mA
3
2
1
RCV420
15
14
V
O
c) f
1
is 0.032A, Lifflefuse Series 217 fast-acting fuse.
Request Application Bulletin AB-014 for details of a
more complete protection circuit.
FIGURE 6. Protecting the Sense Resistors.
–In
C
T
+In
1
15
2
3
10
8
20kΩ
V
REF
RCV420
11
14
V
O
±400mV adjustment at output of reference, and ±50mV
adjustment at output of receiver if reference is used for
level shifting.
FIGURE 7. Optional Voltage Reference External Trim Circuit.
®
7
RCV420
BURR-BROWN [ BURR-BROWN CORPORATION ]
