EXTERNAL TRANSISTOR
The low operating voltage (7.5V) of the XTR106 allows
operation directly from personal computer power supplies
(12V ±5%). When used with the RCV420 Current Loop
Receiver (Figure 8), load resistor voltage drop is limited to 3V.
External pass transistor, Q1, conducts the majority of the
signal-dependent 4-20mA loop current. Using an external
transistor isolates the majority of the power dissipation from
the precision input and reference circuitry of the XTR106,
maintaining excellent accuracy.
BRIDGE BALANCE
Figure 1 shows a bridge trim circuit (R1, R2). This adjust-
ment can be used to compensate for the initial accuracy of
the bridge and/or to trim the offset voltage of the XTR106.
The values of R1 and R2 depend on the impedance of the
bridge, and the trim range required. This trim circuit places
an additional load on the VREF output. Be sure the additional
load on VREF does not affect zero output. See the Typical
Performance Curve, “Under-Scale Current vs IREF + IREG.”
The effective load of the trim circuit is nearly equal to R2.
An approximate value for R1 can be calculated:
Since the external transistor is inside a feedback loop its
characteristics are not critical. Requirements are: VCEO = 45V
min, β = 40 min and PD = 800mW. Power dissipation require-
ments may be lower if the loop power supply voltage is less
than 36V. Some possible choices for Q1 are listed in Figure 1.
The XTR106 can be operated without an external pass
transistor. Accuracy, however, will be somewhat degraded
due to the internal power dissipation. Operation without Q1
is not recommended for extended temperature ranges. A
resistor (R = 3.3kΩ) connected between the IRET pin and the
E (emitter) pin may be needed for operation below 0°C
without Q1 to guarantee the full 20mA full-scale output,
especially with V+ near 7.5V.
(3)
5V • RB
R1 ≈
4 • VTRIM
where, RB is the resistance of the bridge.
VTRIM is the desired ±voltage trim range (in V).
Make R2 equal or lower in value to R1.
LINEARIZATION
10
Many bridge sensors are inherently nonlinear. With the
addition of one external resistor, it is possible to compensate
for parabolic nonlinearity resulting in up to 20:1 improve-
ment over an uncompensated bridge output.
V+
8
E
XTR106
0.01µF
Linearity correction is accomplished by varying the bridge
excitation voltage. Signal-dependent variation of the bridge
excitation voltage adds a second-order term to the overall
transfer function (including the bridge). This can be tailored
to correct for bridge sensor nonlinearity.
IO
7
IRET
6
For operation without external
transistor, connect a 3.3kΩ
resistor between pin 6 and
pin 8. See text for discussion
of performance.
RQ = 3.3kΩ
Either positive or negative bridge non-linearity errors can be
compensated by proper connection of the Lin Polarity pin.
To correct for positive bridge nonlinearity (upward bowing),
Lin Polarity (pin 12) should be connected to IRET (pin 6) as
shown in Figure 3a. This causes VREF to increase with bridge
output which compensates for a positive bow in the bridge
response. To correct negative nonlinearity (downward bow-
ing), connect Lin Polarity to VREG (pin 1) as shown in Figure
3b. This causes VREF to decrease with bridge output. The Lin
Polarity pin is a high impedance node.
FIGURE 2. Operation without External Transistor.
LOOP POWER SUPPLY
The voltage applied to the XTR106, V+, is measured with
respect to the IO connection, pin 7. V+ can range from 7.5V
to 36V. The loop supply voltage, VPS, will differ from the
voltage applied to the XTR106 according to the voltage drop
on the current sensing resistor, RL (plus any other voltage
drop in the line).
If no linearity correction is desired, both the RLIN and Lin
Polarity pins should be connected to VREG (Figure 3c). This
results in a constant reference voltage independent of input
signal. RLIN or Lin Polarity pins should not be left open
or connected to another potential.
If a low loop supply voltage is used, RL (including the loop
wiring resistance) must be made a relatively low value to
assure that V+ remains 7.5V or greater for the maximum
loop current of 20mA:
RLIN is the external linearization resistor and is connected
between pin 11 and pin 1 (VREG) as shown in Figures 3a and
3b. To determine the value of RLIN, the nonlinearity of the
bridge sensor with constant excitation voltage must be
known. The XTR106’s linearity circuitry can only compen-
sate for the parabolic-shaped portions of a sensor’s
nonlinearity. Optimum correction occurs when maximum
deviation from linear output occurs at mid-scale (see Figure
4). Sensors with nonlinearity curves similar to that shown in
(2)
(V+) – 7.5V
RL max =
– RWIRING
20mA
It is recommended to design for V+ equal or greater than
7.5V with loop currents up to 30mA to allow for out-of-
range input conditions. V+ must be at least 8V if 5V sensor
excitation is used and if correcting for bridge nonlinearity
greater than +3%.
XTR106
10
SBOS092A
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