Datasheet

21
®
DAC7644
Figure 15 shows a DAC7644 in a 4mA to 20mA current
output configuration. The output current can be determined
by Equation 3:
At full-scale, the output current is 16mA plus the 4mA for
the zero current. At zero scale the output current is the offset
current of 4mA (0.5V/125Ω).
(2)
(3)
DIGITALLY-PROGRAMMABLE
CURRENT SOURCE
The DAC7644 offers a unique set of features that allows a
wide range of flexibility in designing applications circuits
such as programmable current sources. The DAC7644 offers
both a differential reference input as well as an open-loop
configuration around the output amplifier. The open-loop
configuration around the output amplifier allows transistor
to be placed within the loop to implement a digitally-
programmable, uni-directional current source. The availabil-
ity of a differential reference also allows programmability
for both the full-scale and zero-scale currents. The output
current is calculated as:
NC
NC
NC
NC
V
OUT
A Sense
V
OUT
A
V
REF
L AB Sense
V
REF
L AB
V
REF
H AB
V
REF
H AB Sense
V
OUT
B Sense
V
OUT
B
48
47
46
45
44
43
42
41
40
39
38
37
DAC7644
I
OUT
V
PROGRAMMED
125Ω
80kΩ
+0.5V
20kΩ
+2.5V
+V
I
OUT
V
PROGRAMMED
125Ω
GND
1000pF
2200pF
+V
OPA2350
100Ω
1000pF
2200pF
100Ω
FIGURE 15. 4-to-20mA Digitally Controlled Current Source (1/2 DAC7644).
I
VHVL
R
N Value
VLR
OUT
REF REF
SENSE
REF SENSE
=


















+
()
–
•
,
/
65 536
I
V V N Value V
OUT
=
















+




25 05
125 65 536
05
125
.–.
•
,
.
ΩΩ