LTC1657/LTC1657L
14
1657lfa
For more information www.linear.com/LTC1657
TYPICAL APPLICATIONS
This circuit shows a digitally programmable current source
from an external voltage source using an external op amp,
an LT1218 and an NPN transistor (2N3440). Any digital
word from 0 to 65535 is loaded into the LTC1657 and
its output correspondingly swings from 0V to 4.096V.
This voltage will be forced across the resistor R
A
. If R
A
is chosen to be 412Ω, the output current will
range from
0mA at zero scale to 10mA at full scale. The minimum
voltage for V
S
is determined by the load resistor R
L
and
Q1’s V
CESAT
voltage. With a load resistor of 50Ω, the volt-
age source can be 5V.
Digitally Programmable Current Source
0.1μF
5V
1657 TA04
5V < V
S
< 100V
FOR R
L
≤ 50Ω
Q1
2N3440
R
A
412Ω
1%
R
L
I
OUT
=
≈ 0mA TO 10mA
(D
IN
)(4.096)
(65536)(R
A
)
DATA (0:15)
CSLSB
CSMSB
WR
LDAC
CLR
GND
V
CC
REFOUTREFHI
REFLOX1/X2
LTC1657
µP
7
6
4
26 21 20
22 23
25 3
5:19
2
3
1
28
27
2
V
OUT
–
+
LT1218
