DIGITALLY-PROGRAMMABLE
CURRENT SOURCE
Figure 15 shows a DAC7644 in a 4mA to 20mA current
output configuration. The output current can be determined
by Equation 3:
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:
(3)
2.5V – 0.5V
N Value
65,536
0.5V
IOUT
=
•
+
125Ω
125Ω
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).
VREFH – VREF
RSENSE
L
N Value
65,536
(2)
IOUT
=
•
+ VREFL / RSENSE
(
)
IOUT
VPROGRAMMED
NC 48
NC 47
125Ω
NC 46
NC 45
+V
VOUTA Sense 44
OPA2350
VOUT
VREFL AB Sense 42
REFL AB 41
A
43
DAC7644
20kΩ
100Ω
2200pF
V
1000pF
1000pF
+0.5V
VREFH AB 40
VREFH AB Sense 39
VOUTB Sense 38
100Ω
80kΩ
+V
2200pF
V
OUTB 37
+2.5V
IOUT
VPROGRAMMED
125Ω
GND
FIGURE 15. 4-to-20mA Digitally Controlled Current Source (1/2 DAC7644).
®
21
DAC7644
BB [ BURR-BROWN CORPORATION ]
