Datasheet

DAC7642, DAC7643
18
SBAS233
www.ti.com
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DIGITALLY-PROGRAMMABLE
CURRENT SOURCE
The DAC7642 and DAC7643 offer a unique set of features
that allows a wide range of flexibility in designing applications
circuits, such as programmable current sources. The
DAC7642 and DAC7643 offer 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 a transistor to be placed within the
loop to implement a digitally-programmable, unidirectional
current source. The availability of a differential reference also
allows programmability for both the full-scale and zero-scale
currents. The output current is calculated as:
I
VHVL
R
N Value
VLR
OUT
REF REF
SENSE
REF
SENSE
=


















+
( )
–
•
,
/
65 536
FIGURE 15. 4-20mA Digitally Controlled Current Source.
V
OUT
A
V
OUT
A Sense
V
REF
L Sense
V
REF
L
V
REF
H
V
REF
H Sense
V
OUT
B Sense
V
OUT
B
32
31
30
29
28
27
26
25
DAC7642
DAC7643
1000pF
I
OUT
100Ω
+2.5V
20kΩ
80kΩ
+V
+V
OPA2350
100Ω
2200pF
1000pF
2200pF
V
PROGRAMMED
125Ω
I
OUT
V
PROGRAMMED
125Ω
SYMBOL DESCRIPTION MIN TYP MAX UNITS
t
RCS
CS LOW for Read 150 ns
t
RDS
R/W HIGH to CS LOW 10 ns
t
RDH
R/W HIGH after CS HIGH 10 ns
t
DZ
CS HIGH to Data Bus in High Impedance 10 100 ns
t
CSD
CS LOW to Data Bus Valid 100 150 ns
t
WCS
CS LOW for Write 40 ns
t
WS
R/W LOW to CS LOW 0 ns
t
WH
R/W LOW after CS HIGH 10 ns
t
AS
DACSEL Valid to CS LOW 0 ns
t
AH
DACSEL Valid after CS HIGH 10 ns
t
LS
CS LOW to LOADDACS HIGH 30 ns
t
LH
CS LOW after LOADDACS HIGH 100 ns
t
LX
LOADDACS HIGH 100 ns
t
DS
Data Valid to CS LOW 0 ns
t
DH
Data Valid after CS HIGH 10 ns
t
LWD
LOADDACS LOW 100 ns
t
RSS
RESET LOW 10 ns
t
RSH
RESET HIGH 10 ns
t
S
Settling Time 10 µs
TABLE II. Timing Specifications (T
A
= –40°C to +85°C).
Figure 15 shows a DAC7642 and DAC7643 in a 4-20mA
current output configuration. The output current can be
determined by Equation 3:
I
V V N Value V
OUT
=


















+






25 05
125 65 536
05
125
. – .
•
,
.
ΩΩ
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).
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