Datasheet

AD5280/AD5282
Rev. C | Page 22 of 28
PROGRAMMABLE CURRENT SOURCE
+5V
+5V
R
S
102
V+
REF191
GND
AD5280
2
3
6
B
A
W
C1
1µF
4
U
1
0 TO (2.048 + V
L
)
V
OUT
V
IN
5V
R
L
100
V–
OP8510
U2
V
L
–2.048V TO V
L
I
L
SLEEP
OP8510
U2
02929-061
Figure 61. Programmable Current Source
A programmable current source can be implemented with the
circuit shown in Figure 61. REF191 is a unique, low supply
headroom and high current handling precision reference that
can deliver 20 mA at 2.048 V. The load current is simply the
voltage across Terminal B to Terminal W of the digital
potentiometer divided by R
S
.
N
S
REF
L
R
DV
I
2×
×
=
(8)
The circuit is simple, but attention must be paid to two things.
First, dual-supply op amps are ideal because the ground
potential of REF191 can swing from −2.048 V at zero scale to V
L
at full scale of the potentiometer setting. Although the circuit
works under single supply, the programmable resolution of the
system is reduced.
For applications that demand higher current capabilities, a
few changes to the circuit in Figure 61 produce an adjustable
current in the range of hundreds of milliamps. First, the voltage
reference needs to be replaced with a high current, low dropout
regulator, such as the ADP3333, and the op amp needs to be
swapped with a high current dual-supply model, such as the
AD8532. Depending on the desired range of current, an
appropriate value for R
S
must be calculated. Because of the high
current flowing to the load, the user must pay attention to the
load impedance so as not to drive the op amp beyond the
positive rail.
PROGRAMMABLE BIDIRECTIONAL CURRENT
SOURCE
AD5280
+5V
R1
150k
R2
A
14.95k
|
L
R2
B
50k
R2
I
15k
C1
10pF
R1
I
150k
V
L
R
L
500k
A
2
+15V
+15V
A
W
V+
OP2177
OP2177
V–
V+
V–
A
1
–5V
–15V
–15V
02929-062
Figure 62. Programmable Bidirectional Current Source
For applications that require bidirectional current control or
higher voltage compliance, a Howland current pump can be a
solution (see Figure 62). If the resistors are matched, the load
current is
( )
W
B
BA
V
R1
R2R2
L
R2
I ×
+
=
(9)
In theory, R2
B
can be made as small as needed to achieve the
current needed within the A
2
output current driving capability.
In this circuit, the OP2177 can deliver ±5 mA in either direction,
and the voltage compliance approaches 15 V. It can be shown
that the output impedance is
( )
()
B
A
A
B
O
R2R2R1'R2'R1
R2R1R2'R1
Z
+−×
×
+
= (10)
This output impedance can be infinite if Resistor R1' and
Resistor R2' match precisely with R1 and R2
A
+ R2
B
,
respectively. On the other hand, it can be negative if the
resistors are not matched. As a result, C1 must be in the range
of 1 pF to 10 pF to prevent the oscillation.