Datasheet

XTR106
12
SBOS092A
www.ti.com
NONLINEARITY vs STIMULUS
0
3
2
1
0
–1
–2
–3
Normalized Stimulus
0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1
Nonlinearity (% of Full Scale)
Negative Nonlinearity
B = –0.019
Positive Nonlinearity
B = +0.025
When using linearity correction, care should be taken to
insure that the sensor’s output common-mode voltage re-
mains within the XTR106’s allowable input range of 1.1V to
3.5V. Equation 6 in Figure 3 can be used to calculate the
XTR106’s new excitation voltage. The common-mode volt-
age of the bridge output is simply half this value if no
common-mode resistor is used (refer to the example in
Figure 3). Exceeding the common-mode range may yield
unpredicatable results.
For high precision applications (errors < 1%), a two-step
calibration process can be employed. First, the nonlinearity
of the sensor bridge is measured with the initial gain resistor
and R
LIN
= 0 (R
LIN
pin connected directly to V
REG
). Using
the resulting sensor nonlinearity, B, values for R
G
and R
LIN
are calculated using Equations 4 and 5 from Figure 3. A
second calibration measurement is then taken to adjust R
G
to
account for the offsets and mismatches in the linearization.
UNDER-SCALE CURRENT
The total current being drawn from the V
REF
and V
REG
voltage sources, as well as temperature, affect the XTR106’s
under-scale current value (see the Typical Performance
Curve, “Under-Scale Current vs I
REF
+ I
REG
). This should be
considered when choosing the bridge resistance and excita-
tion voltage, especially for transducers operating over a
wide temperature range (see the Typical Performance Curve,
“Under-Scale Current vs Temperature”).
LOW IMPEDANCE BRIDGES
The XTR106’s two available excitation voltages (2.5V and
5V) allow the use of a wide variety of bridge values. Bridge
impedances as low as 1kΩ can be used without any addi-
tional circuitry. Lower impedance bridges can be used with
the XTR106 by adding a series resistance to limit excitation
current to ≤ 2.5mA (Figure 5). Resistance should be added
FIGURE 5. 350Ω Bridge with x50 Preamplifier.
BRIDGE TRANSDUCER TRANSFER FUNCTION
WITH PARABOLIC NONLINEARITY
0
10
9
8
7
6
5
4
3
2
1
0
Normalized Stimulus
0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1
Bridge Output (mV)
Positive Nonlinearity
B = +0.025
B = –0.019
Negative Nonlinearity
Linear Response
FIGURE 4. Parabolic Nonlinearity.
1
14
13
5
3
2
XTR106
12
6
R
G
R
G
V
–
IN
V
+
IN
V+
I
O
I
RET
E
B
8
7
0.01µF
1N4148
9
10
4
11
R
LIN
Lin
Polarity
R
G
125Ω
I
O
= 4-20mA
1/2
OPA2277
1/2
OPA2277
5V
V
REF
2.5
V
REG
V
REF
5
350Ω
412Ω
10kΩ
10kΩ
1kΩ
3.4kΩ
3.4kΩ
I
REG
≈ 1.6mA
700µA at 5V
I
TOTAL
= 0.7mA + 1.6mA ≤ 2.5mA
Shown connected to correct positive
bridge nonlinearity. For negative bridge
nonlinearity, see Figure 3b.
Bridge excitation
voltage = 0.245V
Approx. x50
amplifier