TYPICAL PERFORMANCE CURVES (CONT)
At TA = +25°C, +VCC = 24VDC, unless otherwise noted.
INPUT VOLTAGE NOISE DENSITY vs FREQUENCY
INPUT CURRENT NOISE DENSITY vs FREQUENCY
60
50
40
30
20
6
5
4
3
2
10
0
1
0
1
10
100
1k
10k
100k
1
10
100
1k
10k
100k
Frequency (Hz)
Frequency (Hz)
OUTPUT CURRENT NOISE DENSITY vs FREQUENCY
6
5
4
3
2
1
0
1
10
100
1k
10k
100k
Frequency (Hz)
THEORY OF OPERATION
A simplified schematic of the XTR101 is shown in Figure 1.
Basically the amplifiers, A1 and A2, act as a single power
supply instrumentation amplifier controlling a current source,
A3 and Q1. Operation is determined by an internal feedback
loop. e1 applied to pin 3 will also appear at pin 5 and
similarly e2 will appear at pin 6. Therefore the current in RS,
the span setting resistor, will be IS = (e2 – e1)/RS = eIN/RS.
This current combines with the current, I3, to form I1. The
circuit is configured such that I2 is 19 times I1. From this
point the derivation of the transfer function is straightfor-
ward but lengthy. The result is shown in Figure 1.
since IO is unipolar e2 must be kept larger than e1; i.e., e2 ≥
e1 or eIN ≥ 0. Also note that in order not to exceed the output
upper range limit of 20mA, eIN must be kept less than 1V
when RS = ∞ and proportionately less as RS is reduced.
INSTALLATION AND
OPERATING INSTRUCTIONS
BASIC CONNECTION
The basic connection of the XTR101 is shown in Figure 1.
A difference voltage applied between input pins 3 and 4 will
cause a current of 4-20mA to circulate in the two-wire
output loop (through RL, VPS, and D1). For applications
requiring moderate accuracy, the XTR101 operates very
cost-effectively with just its internal drive transistor. For
more demanding applications (high accuracy in high gain)
an external NPN transistor can be added in parallel with the
internal one. This keeps the heat out of the XTR101 package
Examination of the transfer function shows that IO has a
lower range-limit of 4mA when eIN = e2 – e1 = 0V. This 4mA
is composed of 2mA quiescent current exiting pin 7 plus
2mA from the current sources. The upper range limit of IO is
set to 20mA by the proper selection of RS based on the upper
range limit of eIN. Specifically RS is chosen for a 16mA
output current span for the given full scale input voltage
Ω
span; i.e., (0.016 + 40/RS)(eIN full scale) = 16mA. Note that
®
5
XTR101
BB [ BURR-BROWN CORPORATION ]
