XTR101 Datasheet, PDF (8/25 Pages) Burr-Brown (TI) – Precision, Low Drift 4-20mA TWO-WIRE TRANSMITTER

XTR101
SBOS146A − OCTOBER 1986 − REVISED AUGUST 2004
Although any NPN of suitable power rating will operate
with the XTR101, two readily available transistors are
recommended:
1. 2N2222 in the TO-18 package. For power-supply
voltages above 24V, a 750Ω, 1/2W resistor should be
connected in series with the collector. This will limit the
power dissipation to 377mW under the worst-case
conditions; see Figure 2. Thus, the 2N2222 will safely
operate below its 400mW rating at the upper
temperature of +85°C. Heat sinking the 2N2222 will
result in greatly reduced accuracy improvement and
is not recommended.
2. TIP29B in the TO-220 package. This transistor will
operate over the specified temperature and output
voltage range without a series collector resistor. Heat
sinking the TIP29B will result in slightly less accuracy
improvement. It can be done, however, when
mechanical constraints require it.
ACCURACY WITH AND WITHOUT AN
EXTERNAL TRANSISTOR
The XTR101 has been tested in a circuit using an external
transistor. The relative difference in accuracy with and
without an external transistor is shown in Figure 3. Notice
that a dramatic improvement in offset voltage change with
supply voltage is evident for any value of load resistor.
30
60
Span = ∆I_O = 16mA
25
50
Without External Transistor
20
40
R_L = 100Ω
15
30
R_L = 600Ω
10
R_L = 1kΩ
20
5 ^{With External Transistor}
0
R_L = 100Ω
R_L = 600Ω
10
20
30
V_CC (V)
10
R_L = 1kΩ
0
40
Figure 3. Thermal Feedback Due to Change in
Output Current
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MAJOR POINTS TO CONSIDER WHEN
USING THE XTR101
1. The leads to R_S should be kept as short as possible
to reduce noise pick-up and parasitic resistance.
2. +V_CC should be bypassed with a 0.01µF capacitor as
close to the unit as possible (pin 8 to pin 7).
3. Always keep the input voltages within their range of
linear operation, +4V to +6V (e₁ and e₂ measured with
respect to pin 7).
4. The maximum input signal level (e_INFS) is 1V with
R_S = ∞ and proportionally less as R_S decreases.
5. Always return the current references (pins 10 and 11)
to the output (pin 7) through an appropriate resistor. If
the references are not used for biasing or excitation,
connect them together to pin 7. Each reference must
have between 0V and +(V_CC − 4V) with respect to
pin 7.
6. Always choose R_L (including line resistance) so that
the voltage between pins 7 and 8 (+V_CC) remains
within the 11.6V to 40V range as the output changes
between the 4-20mA range (as shown in Figure 4).
7. It is recommended that a reverse polarity protection
diode (D₁ in Figure 1) be used. This will prevent
damage to the XTR101 caused by a momentary (such
as a transient) or long-term application of the wrong
polarity of voltage between pins 7 and 8.
8. Consider PC board layout which minimizes parasitic
capacitance, especially in high gain.
1500
1250
1000
R_L max
=
V_PS − 11.6V
20mA
750
Operating
500
Region
250
0
0
10
20
30
40
50
60
Power−Supply Voltage, V_PS (V)
Figure 4. Power-Supply Operating Range
8