Datasheet

AD5263
Rev. 0 | Page 22 of 28
PROGRAMMABLE 4–20 mA CURRENT SOURCE PROGRAMMABLE BIDIRECTIONAL CURRENT
SOURCE
A programmable 4–20 mA current source can be implemented
with the circuit shown in Figure 62. The 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 Terminals B-W of the digital potentiometer
divided by R
S
:
For applications that require bidirectional current control or
higher voltage compliance, a Howland current pump can be a
solution (see Figure 63). If the resistors are matched, the load
current is
N
S
REF
L
R
D
V
I
2×
×
= (8)
WL
R2
B
V
R1R2BR2A
I
×
+
=
)(
(9)
03142-0-062
–2.048V to V
L
0 to (2.048V + V
L
)
+5V
2
3
+5V
U1
C1
4
U2
–5V
R
L
V
L
R
S
REF191
GND
VIN
VOUT
SLEEP
AD5263
100Ω
102Ω
1µF
6
A
B
W
OP8510
V+
V–
I
L
03142-0-063
OP2177
V
L
A2
R1
R2B
C1
50Ω
–15V
W
R
L
+15V
–15V
+15V
OP2177
V+
V–
V+
V–
A1
R2
150kΩ
15kΩ
R2A
150kΩ
R1
10pF
10pF
C2
500Ω
14.95kΩ
AD5263
A
–5V
+5V
I
L
Figure 62. Programmable 4–20 mA Current Source
Figure 63. Programmable Bidirectional Current Source
R2B, in theory, can be made as small as needed to achieve the
current needed within A2’s output current driving capability. In
this circuit, OP2177 can deliver ±5 mA in either direction, and
the voltage compliance approaches +15 V. It can be shown that
the output impedance is
The circuit is simple, but beware of two things. First, dual-
supply op amps are ideal because the ground potential of the
REF191 can swing from –2.048 V at zero scale to V
L
at full scale
of the potentiometer setting. Although the circuit works with a
single supply, the programmable resolution of the system will be
reduced.
)(
)(2
R2BR2A1R2RR1
R2AR1BR1R
Z
o
+
′
−
′
×
+
×
′
= (10)
For applications that demand higher current capabilities, a few
changes to the circuit in Figure 62 will produce an adjustable
current in the range of hundreds of mA. First, the voltage refer-
ence 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 appro-
priate 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 past the posi-
tive rail.
This output impedance can be infinite if resistors R1′ and R2′
match precisely with R1 and R2A+R2B, respectively. On the
other hand, it can be negative if the resistors are not matched.
As a result, C1 in the range of 1 pF to 10 pF is needed to prevent
oscillation.