AD561
circuit will not produce both 1 to 5 volt and
4-to-20 mA outputs simultaneously.)
Figure 10. Digital 4-to-20 mA or 1-to-5 Volt Line Driver
DIGITALLY PROGRAMMABLE SETPOINT
COMPARATOR
Figure 9. Fast Precision Analog to Digital Converter
A much faster converter can be constructed by using higher
performance external components. Each individual high-order
bit settles in less than 250 ns; the low-order bits in less than
200 ns. Because of this, a staged clock, which speeds up for
lower bits will improve the speed. Also, a faster comparator and
Schottky TTL or ECL logic would be necessary. 10-bit convert-
ers in the 3
µs
to 5
µs
range could be built around the AD561
with these techniques.
DIGITAL 4-TO-20 mA OR 1-TO-5 VOLT CONVERTER
Figure 11 demonstrates a high accuracy systems-oriented
setpoint comparator. The 2.5 volt reference is buffered and
amplified by the AD741K to produce an exact 10.000 volt
reference which could be used as a primary system reference for
several such circuits. The +10 volt compliance of the AD561
then allows it to generate a zero to +10 volt output swing
through the 5 kΩ application resistor without an additional op
amp. The digital code for this system will be complementary
binary (all 1s give 0.00 volts out).
A direct digital 4-to-20 mA or 1-to-5 volt line driver can be built
with the AD561 as shown in Figure 10. The 2.5 volt reference is
divided to provide 1 volt at the op amp noninverting input – thus a
zero input code results in a 1 volt output at the Darlington emitter
(V
OUT
). The 2 k feedback resistance converts the nominal 2 mA
(± 20%) full-scale output from the AD561 to 4 volts, for a
total output of 5 volts FS. The voltage at the emitter forces a
proportional current through the 250
Ω
(which appears at the
collector as I
OUT
) The AD561 current is added to the 4–20 mA
line; thus 5 volts full scale gives 22 mA in the current loop. For
exactly 20 mA, trim the 1 k pot for 4.5 V FS. (A single op amp
Figure 11. Digitally Programmable Set-Point Comparator
OUTLINE DIMENSIONS
Dimensions shown in inches and (mm).
–8–
REV. A
PRINTED IN U.S.A.
16-Pin Ceramic Package
D-16
16-Pin Plastic Package
N-16
C394e–1–5/97
ROCHESTER [ Rochester Electronics ]
