Datasheet

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SBOS146A − OCTOBER 1986 − REVISED AUGUST 2004
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9
SELECTING THE R
S
R
SPAN
is chosen so that a given full-scale input span
(e
INFS
) will result in the desired full-scale output span of
∆I
OFS
:
ƪ
ǒ
0.016
amps
volt
Ǔ
)
ǒ
40
R
S
Ǔ
ƫ
De
IN
+ DI
O
+ 16mA.
Solving for R
S
:
R
S
+
40
DI
O
ńDe
IN
* 0.016
amps
volt
For example, if ∆e
INFS
= 100mV for ∆I
OFS
= 16mA,
R
S
+
40
ǒ
16mAń100mV
Ǔ
* 0.016
+
40
0.16 * 0.016
+
40
0.144
+ 278W
See the Typical Characteristics for a plot of R
S
vs ∆e
INFS
.
Note that in order not to exceed the 20mA upper range
limit, e
IN
must be less than 1V when R
S
= ∞ and
proportionately smaller as R
S
decreases.
BIASING THE INPUTS
Because the XTR operates from a single supply, both e
1
and e
2
must be biased approximately 5V above the
voltage at pin 7 to assure linear response. This is easily
done by using one or both current sources and an external
resistor, R
2
. Figure 5 shows the simplest case—a floating
voltage source eȀ
2
. The 2mA from the current sources
flows through the 2.5kΩ value of R
2
and both e
1
and e
2
are
raised by the required 5V with respect to pin 7. For linear
operation the constraint is:
+4V ≤ e
1
≤ +6V
+4V ≤ e
2
≤ +6V
The offset adjustment is used to remove the offset voltage
of the input amplifier. When the input differential voltage
(e
IN
) equals zero, adjust for 4mA output.
Figure 6 shows a similar connection for a resistive
transducer. The transducer could be excited either by one
(as shown) or both current sources. Also, the offset
adjustment has higher resolution compared to Figure 5.
+
−
8
10
11
3
5
6
4
R
S
e
IN
+
−
XTR101
R
2
2.5k
Ω
7
2mA
0.01
µ
F
0.01
µ
F
D
1
I
O
2
1
4−20mA
e
1
e
2
R
L
Offset
Adjust
1M
Ω
e’
2
Adj.
2mA
+5V
+
14
e
L
+
24V
−
−
+
I
O
+ 4mA ) ǒ0.016
amps
volt
)
40
R
S
Ǔe
IN
e
IN
+ e
2
Figure 5. Basic Connection for Floating Voltage
Source
−
+
8
10
3
5
6
4
R
S
e
IN
+
−
XTR101
R
2
2.5k
Ω
7
1mA
0.01
µ
F
D
1
2
1
e
1
e
2
Offset
Adjust
100k
Ω
e
2
2mA
+5V
+
0.01
µ
F
1mA
−
R
T
1M
Ω
11
Alternate circuitry
showninFigure8.
14
+
24V
−
−
+
R
L
e
L
I
O
+ 4mA ) ǒ0.016
amps
volt
)
40
R
S
Ǔe
IN
e
IN
+ eȀ
2
+ 1mA R
T
Figure 6. Basic Connection for Resistive Source
CMV AND CMR
The XTR101 is designed to operate with a nominal 5V
common-mode voltage at the input and will function
properly with either input operating over the range of 4V to
6V with respect to pin 7. The error caused by the 5V CMV
is already included in the accuracy specifications.
If the inputs are biased at some other CMV, then an input
offset error term is (CMV − 5)/CMRR, where CMR is in dB,
and CMRR is in V/V.
(1)