Datasheet

Data Sheet AD5231
Rev. D | Page 25 of 28
PROGRAMMABLE BIDIRECTIONAL CURRENT
SOURCE
For applications that require bidirectional current control or
higher voltage compliance, a Howland current pump can be a
solution. If the resistors are matched, the load current is
( )
WL
V
R2B
R1
R2BR2A
I ×
+
=
(8)
–15V
OP2177
V+
V–
+15V
+
–
C1
10pF
R2
15kΩ
R1
150kΩ
R
L
500Ω
R2B
50Ω
V
L
R1
150kΩ
R2A
14.95kΩ
I
L
OP2177
V+
V–
+15V
+
–
–15V
A1
AD5231
A
B W
+2.5V
–2.5V
A2
02739-052
Figure 53. Programmable Bidirectional Current Source
R2B, in theory, can be made as small as necessary to achieve the
current needed within the A2 output current-driving capability.
In this circuit, OP2177 delivers ±5 mA in both directions, and
the voltage compliance approaches 15 V. It can be shown that
the output impedance is
)(
)(
R2BR2AR1'R1R2'
R2AR1R2BR1'
Z
O
+−
+
=
(9)
Z
O
can be infinite if resistors R1 and R2 match precisely with R1
and R2A + R2B,
respectively. On the other hand, Z
O
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 from the
negative impedance.
RESISTANCE SCALING
The AD5231 offers 10 kΩ, 50 kΩ, and 100 kΩ nominal
resistance. For users who need lower resistance but want to
maintain the number of adjustment steps, they can parallel
multiple devices. For example, Figure 54 shows a simple scheme
of paralleling two AD5231s. To adjust half the resistance
linearly per step, users need to program both devices coherently
with the same settings and tie the terminals as shown.
A1
B1
W1
W2
A2
B2
LD
02739-053
Figure 54. Reduce Resistance by Half with Linear Adjustment Characteristics
In voltage diver mode, by paralleling a discrete resistor as
shown in Figure 55, a proportionately lower voltage appears at
Terminals A–B. This translates into a finer degree of precision,
because the step size at Terminal W is smaller. The voltage can
be found as follows:
DD
AB
AB
W
V
D
R2RR3
)R2R
DV ××
+
=
1024//
//(
)(
(10)
R1
R2
A
B
W
R3
02739-059
Figure 55. Lowering the Nominal Resistance
Figure 54 and Figure 55 show that the digital potentiometers
change steps linearly. On the other hand, pseudo log taper
adjustment is usually preferred in applications such as audio
control. Figure 56 shows another type of resistance scaling. In
this configuration, the smaller the R2 with respect to R1, the
more the pseudo log taper characteristic of the circuit behaves.
R1
R2
V
O
A
B
W
02739-055
Figure 56. Resistor Scaling with Pseudo Log Adjustment Characteristics