Despite the long-predicted demise
of the 4- to 20-mA current loop, this analog interface is still the most common
method of connecting current-loop sources to a sensing circuit. This interface
requires the conversion of a voltage signal—typically, 1 to 5V—to a 4- to 20-mA
output. Stringent accuracy requirements dictate the use of either expensive
precision resistors or a trimming potentiometer to calibrate out the initial
error of less precise devices to meet the design goals.
Neither technique is optimal in
today’s surface-mounted, automatic-test-equipment-driven production
environment. It’s difficult to get precise resistors in surface-mount packages,
and trim
ming potentiometers require human intervention, a requirement that is
incompatible with production goals.
The Linear Technology LT5400 quad
matched resistor network helps to solve these issues in a simple circuit that
requires no trim adjustments but achieves a total error of less than 0.2%
(Figure 1). The circuit uses two amplifier stages to exploit the unique
matching characteristics of the LT5400. The first stage applies a 1 to 5V
output—typically, from a DAC—to the non - inverting input of op amp IC1A. This
voltage sets the current through R1 to exactly VIN/R1 through FET Q2. The same
current is pulled down through R2, so the voltage at the bottom of R2 is the
24V loop supply minus the input voltage.
This portion of the circuit has
three main error sources: the matching of R1 and R2, IC1A’s offset voltage, and
Q2’s leakage. The exact values of R1 and R2 are not critical, but they must
exactly match each other. The LT5400A grade achieves this goal with ±0.01%
error. The LT1490A has less-than-700-μV offset voltage over 0 to 70°C. This
voltage contributes 0.07% error at an input voltage of 1V. The NDS7002A has a
leakage current of 10 NA, although it is usually much less. This leakage
current represents an error of 0.001%.
The second stage holds the
voltage on R3 equal to the voltage on R2 by pulling current through Q1. Because
the voltage across R2 equals the input voltage, the current through Q1 is
exactly the input voltage divided by R3. By using a precision 250Ω current
shunt for R3, the current accurately tracks the input voltage.
The error sources for the second
stage are R3’s value, IC1B’s offset voltage, and Q1’s leakage current. Resistor
R3 directly sets the output current, so its value is crucial to the precision
of the circuit. This circuit takes advantage of the commonly used 250Ω
current-loop-completion shunt resistor. The Riedon SF-2 part in the figure has
0.1% initial accuracy and low temperature drift. As in the first stage, offset
voltage contributes no more than 0.07% error. Q1 has less than 100-nA leakage,
yielding a maximum error of 0.0025%.
Total output error is better than
0.2% without any trimming. Current-sensing resistor R3 is the dominant source
of error. If you use a higher-quality device, such as the Vishay PLT series,
you can achieve an accuracy of 0.1%. Current-loop outputs are subject to considerable
stresses in use. Diodes D1 and D2 from the output to the 24V loop supply and
ground help protect Q1; R6 provides some isolation. You can achieve more
isolation by increasing the value of R6, with the trade-off of some compliance
voltage at the output.
If the maximum output-voltage
requirement is less than 10V, you can increase R6’s value to 100Ω, affording
even more isolation from output stress. If your design requires increased
protection, you can fit a transient-voltage suppressor to the output with some
loss of accuracy due to leakage current.
This design uses only two of the
four matched resistors in the LT5400 package. You can use the other two for
other circuit functions, such as a precision inverter, or another 4- to 20-mA
converter. Alternatively, you can place the other resistors in parallel with R1
and R2. This approach lowers the resistor’s statistical error contribution by
the square root of two.