XTR111 Datasheet, PDF (14/22 Pages) Burr-Brown (TI) – Precision Voltage-to-Current Converter/Transmitter

XTR111
SBOS375 − NOVEMBER 2006
OUTPUT ERROR FLAG AND DISABLE INPUT
The XTR111 has additional internal circuitry to detect an
error in the output current. In case the controlled output
current cannot flow due to a wire break, high load
resistance or the output voltage level approaching the
positive supply, the error flag (EF), an open drain logic
output, pulls low. When used, this digital output requires
external pull-up to logic high (the internal pull-up current is
2µA).
The output disable (OD) is a logic input with approximately
4µA of internal pull-up to 5V. The XTR111 comes up with
the output disabled until the OD pin is pulled low. Logic
high disables the output to zero output current. It can be
used for calibration, power-on and power-off glitch
reduction and for output multiplexing with other outputs
connected to the same terminal pin.
Power-on while the output is disabled (OD = high) cannot
fully suppress output glitching. While the supply voltage
passes through the range of 3V to 4V, internal circuits turn
on. Additional capacitance between pins VG and IS can
suppress the glitch. The smallest glitch energy appears
with the OD pin left open; for practical use, however, this
pin can be driven high through a 10kΩ resistor before the
24V supply is applied, if logic voltage is available earlier.
Alternatively, an open drain driver can control this pin using
the internal pull-up current. Pull-up to the internal regulator
tends to increase the energy because of the delay of the
regulator voltage increase, again depending on the supply
voltage rise time for the first few volts.
INPUT VOLTAGE
The input voltage range for a given output current span is
set by R_SET according to the transfer function. Select a
precise and low drift resistor for best performance,
because resistor drift directly converts into drift of the
output current. Careful layout must also minimize any
series resistance with R_SET and the VIN reference point.
The input voltage is referred to the grounding point of R_SET.
Therefore, this point should not be distorted from other
currents. Assuming a 5V full-scale input signal for a 20mA
output current, R_SET is 2.5kΩ. A resistance uncertainty of
just 2.5Ω already degrades the accuracy to below 0.1%.
The linear input voltage range extends from 0V to 12V, or
2.3V below the positive supply voltage (whichever is
smaller). The lowest rated supply voltage accomodates
an input voltage range of up to 5V. Potential clipping is not
detected by an error signal; therefore, safe design guard
banding is recommended.
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Do not drive the input negative (referred to GND) more
than 300mV. Higher negative voltages turn on the internal
protection diodes. Insert a resistor in series with the input
if negative signals can occur eventually during power-on
or -off or during other transient conditions. Select a resistor
value limiting the possible current to 0.3mA. Higher
currents are non-destructive (see Absolute Maximum
Ratings), but they can produce output current glitches
unless in disable mode.
More protection against negative input signals is provided
using a standard diode and a 2.2kΩ resistor, as shown in
Figure 6.
V−Signal
2.2kΩ
VIN
1N4148
Figure 6. Enhanced Protection Against Negative
Overload of V_IN
4mA−20mA OUTPUT
The XTR111 does not provide internal circuits to generate
4mA with 0V input signal. The most common way to shift
the input signal is a two resistor network connected to a
voltage reference and the signal source, as shown in
Figure 7. This arrangement allows easy adjustment for
over-and under-range. The example assumes a 5V
reference (V_REF) that equals the full-scale signal voltage
and a signal span of 0V to 5V for 4mA to 20mA (I_MIN to I_MAX)
output.
Reference
Voltage
5V
Input Voltage
0V to 5V
R₁
40kΩ
R₂
10kΩ
VIN
1V to 5V
Figure 7. Resistive Divider for I_MIN to I_MAX Output
(4mA to 20mA) with 0 to V_FS Signal Source
The voltage regulator output or a more precise reference
can be used as V_REF. Observe the potential drift added by
the drift of the resistors and the voltage reference.
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