XTR104 Burr-Brown Corporation, XTR104 Datasheet - Page 7

• Manufacturer Part Number: XTR104
• Description: 4-20mA Current Transmitter with BRIDGE EXCITATION AND LINEARIZATION
• Manufacturer: Burr-Brown Corporation

BRIDGE BALANCE
Figure 1 shows a bridge trim circuit (R
ment can be used to compensate for the initial accuracy of
the bridge and/or to trim the offset voltage of the XTR104.
The values of R
bridge, and the trim range required. This trim circuit places
an additional load on the V
trim circuit is nearly equal to R
terminal must not exceed 2mA. An approximate value for R
can be calculated:
Make R
Figure 2 shows another way to adjust zero errors using the
output current adjustment pins of the XTR104. This pro-
vides 500 A (typical) adjustment around the initial low-
scale output current. This is an output current adjustment
that is independent of the input stage gain set with R
input stage is set for high gain the output current adjustment
may not provide sufficient range.
FIGURE 2. Low-scale Output Current Adjustment.
LINEARIZATION
Differential voltage applied to the linearization inputs, V
and V
vary according to the following equation:
Where: V
Where: R
–
LIN
2
differential inputs (in V).
R
K
equal or lower in value to R
, causes the reference (excitation) voltage, V
LIN
LIN
LIN
V
B
TRIM
is the voltage applied to the V
in
is the resistance of the bridge.
16
16
V
variations in the fabrication of the XTR104).
24000 (approximately 20% depending on
1
XTR104
XTR104
is the desired voltage trim range (in V).
R
and R
10k
5k
.
R
15
15
1
5V
2
depend on the impedance of the
R
4 • V
5 V • R
14
14
output. The effective load of the
5k
V
2
. Total load on the V
±500µA typical
output current
adjustment range.
LIN
TRIM
±50µA typical
output current
adjustment range.
(a)
(b)
B
K
R
1
.
LIN
LIN
1
, R
2
+
). This adjust-
LIN
and V
R
G
. If the
output
R
+
–
, to
(3)
LIN
(4)
LIN
1
7
With V
bridge excitation voltage can be made to vary as much as
the total load on the V
maximum excitation voltage, V
Signal-dependent variation of the bridge excitation voltage
provides a second-order term to the complete transfer func-
tion (including the bridge). This can be tailored to correct for
bridge sensor nonlinearity. Either polarity of nonlinearity
(bowing up or down) can be compensated by proper connec-
tion of the V
to V
which compensates for a positive bow in the bridge re-
sponse. Reversing the connections (Figure 3) causes V
decrease with increasing bridge output, to compensate for
negative-bowing nonlinearity.
To determine the required value for R
nonlinearity of the bridge sensor with constant excitation
voltage. The linearization circuitry can only compensate for
the parabolic portion of a sensor’s nonlinearity. Parabolic
nonlinearity has a maximum deviation from linear occurring
at mid-scale (see Figure 4). Sensors with nonlinearity curves
similar to that shown in Figure 4, but not peaking exactly at
mid-scale can be substantially improved. A nonlinearity that
is perfectly “S-shaped” (equal positive and negative
nonlinearity) cannot be corrected with the XTR104. It may,
however, be possible to improve the worst-case nonlinearity
of a sensor by equalizing the positive and negative
nonlinearity.
The nonlinearity, B (in % of full scale), is positive or
negative depending on the direction of the bow. A maximum
of 2.5% nonlinearity can be corrected. An approximate
value for R
Where: K
Methods for refining this calculation involve determining
the actual value of K
later).
B is a signed number (negative for a downward-bowing
nonlinearity). This can produce a negative value for R
this case, use the resistor value indicated (ignore the sign),
but connect V
Figure 3.
This approximate calculation of R
about a 5:1 improvement in bridge nonlinearity.
Example: The bridge sensor depicted by the negative-
bowing curve in Figure 4. Its full scale output is 10mV with
constant 5V excitation. Its maximum nonlinearity, B, is
–1.9% referred to full scale (occurring at mid-scale). Using
equation 5:
0.5V in response to the bridge output voltage. Be sure that
–
IN
+
(Figure 1) causes V
LIN
V
constant 5V excitation.
B is the parabolic nonlinearity in % of full scale.
R
LIN
LIN
FS
LIN
and V
LIN
is the full-scale bridge output (in Volts) with
in
+
can be calculated by:
LIN
inputs. Connecting V
24000.
R
–
.
to V
LIN
LIN
LIN
connected to the bridge output, the
–
R
IN
for a particular device (explained
output is less than 2mA at the
R
and V
K
to increase with bridge output
LIN
0. 2 • B
XTR104
R
= 5.5V.
–
• V
LIN
LIN
LIN
+
LIN
FS
to V
generally provides
you must know the
to V
+
IN
+
as shown in
IN
and V
LIN
–
R
. In
LIN
(5)
to
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