Datasheet

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SBOS146A − OCTOBER 1986 − REVISED AUGUST 2004
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6
THEORY OF OPERATION
A simplified schematic of the XTR101 is shown in Figure 1.
Basically, the amplifiers A
1
and A
2
act as a single
power-supply instrumentation amplifier controlling a
current source, A
3
and Q
1
. Operation is determined by an
internal feedback loop. e
1
applied to pin 3 will also appear
at pin 5, and similarly, e
2
will appear at pin 6. Therefore, the
current in R
S
(the span setting resistor) will be
I
S
=(e
2
−e
1
)/R
S
=e
IN
/R
S
. This current combines with the
current I
3
to form I
1
. The circuit is configured such that I
2
is 19 times I
1
. From this point, the derivation of the transfer
function is straightforward but lengthy. The result is shown
in Figure 1.
Examination of the transfer function shows that I
O
has a
lower range-limit of 4mA when e
IN
= e
2
− e
1
= 0V. This 4mA
is composed of 2mA quiescent current exiting pin 7 plus
2mA from the current sources. The upper range limit of I
O
is set to 20mA by the proper selection of R
S
based on the
upper range limit of e
IN
. Specifically, R
S
is chosen for a
16mA output current span for the given full-scale input
voltage span.
For example,
ǒ
0.016
amps
volt
)
40
R
S
Ǔ
ǒ
e
IN
full−scale
Ǔ
+ 16mA.
Note that since I
O
is unipolar, e
2
must be kept larger than
e
1
(that is, e
2
≥ e
1
or e
IN
≥ 0). Also note that in order not to
exceed the output upper range limit of 20mA, e
IN
must be
kept less than 1V when R
S
= ∞ and proportionately less as
R
S
is reduced.
+V
CC
A
1
+V
CC
D
1
V
PS
I
O
7
e
L
10 11
Q
1
R
1
Voltage−Controlled
Current Source
2.5k
Ω
100
µ
A
8
56
2mA
R
2
52.6
Ω
(e
2
)
I
B1
I
B2
I
2
I
REF1
1k
Ω
e
IN
I
S
(e
2
)
(e
1
)
I
3
I
4
R
3
1.25k
Ω
−
+
I
1
+
−
A
2
A
3
(e
1
)
R
S
−
In
3
+In
4
e
IN
I
REF2
I
O
+V
CC
R
4
1.25k
Ω
+V
CC
+V
CC
R
L
I
O
+ 4mA ) ǒ0.016
amps
volt
)
40
R
S
Ǔe
IN,
e
IN
+ e
2
* e
1
Figure 1. Simplified Schematic of the XTR101