Datasheet

Data Sheet AD5263
Rev. F | Page 25 of 28
PROGRAMMABLE 4 TO 20 MA CURRENT SOURCE
A programmable 4–20 mA current source can be implemented
with the circuit shown in Figure 63. 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 Terminal B to Terminal W of the digital
potentiometer divided by R
S
:
N
S
REF
L
R
DV
I
2×
×
=
(7)
03142-062
–2.048V TO V
L
0 TO (2.048V + V
L
)
+5V
2
3
+5V
U1
4
U2
–5V
V
L
R
S
102Ω
R
L
100Ω
REF191
GND
VIN
VOUT
SLEEP
AD5263
C1
1µF
6
A
B
W
I
L
OP8510
V+
V–
Figure 63. Programmable 4–20 mA Current Source
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 is reduced.
For applications that demand higher current capabilities, a few
changes to the circuit in Figure 63 produce an adjustable current in
the range of hundreds of mA. 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 AD5263. 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 past the positive rail.
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 64). If the resistors are matched, the load
current is
WL
V
R2B
R1R2BR2A
I ×
+
=
)(
(8)
03142-063
AD5263
+5V
R1
150kΩ
R2
A
14.95kΩ
|
L
R2
B
50Ω
R2
15kΩ
C1
10pF
C2
10pF
R1
150kΩ
V
L
R
L
500Ω
A
2
+15V
+15V
A
W
V+
OP2177
OP2177
V–
V+
V–
A1
–5V
–15V
–15V
Figure 64. Programmable Bidirectional Current Source
R2B, in theory, can be made as small as needed to achieve the
current needed within the A2 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
)(
)(2
R2BR2A1R2RR1
R2AR1BR1R
Z
o
+
′
−
′
×
+×
′
=
(9)
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.