Programmable Current Source; Programmable Bidirectional Current Source; Programmable Low-Pass Filter - Analog Devices AD5235 Manual

AD5235

PROGRAMMABLE CURRENT SOURCE

A programmable current source can be implemented with the
circuit shown in Figure 52.
+5V
2 U1
V
IN
0 TO (2.048 + V
6
3
SLEEP V
OUT
REF191
C1
1 µ F
GND
4
AD5235
–2.048V TO V
L
Figure 52. Programmable Current Source
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
×
V D
=
REF
I
L ×
R 1024
S
The circuit is simple, but be aware that there are two issues.
First, dual-supply op amps are ideal, because the ground
potential of REF191 can swing from −2.048 V at zero scale to V
at full scale of the potentiometer setting. Although the circuit
works under single supply, the programmable resolution of the
system is reduced by half. Second, the voltage compliance at V
is limited to 2.5 V or equivalently a 125 Ω load. Should higher
voltage compliance be needed, users can consider digital
potentiometers AD5260, AD5280, and AD7376. Figure 53 shows
an alternate circuit for high voltage compliance.
To achieve higher current, such as when driving a high power
LED, the user can replace the U1 with an LDO, reduce R
add a resistor in series with the digital potentiometer's
A terminal. This limits the potentiometer's current and
increases the current adjustment resolution.
PROGRAMMABLE BIDIRECTIONAL CURRENT
SOURCE
For applications that require bidirectional current control or
higher voltage compliance, a Howland current pump can be a
solution (Figure 53). If the resistors are matched, the load
current is
+
R 2 A R 2 B
R 1
= ×
I V
L W
R 2 B
)
L
B
W
R
A
S
102 Ω
+5V
V+
OP1177
U2
V–
V
L
R
L
–5V
100 Ω I
L
:
S
, and
S
+2.5V
A
AD5235
W
B
–2.5V
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 either direction, and
the voltage compliance approaches 15 V. Without the additions
of C1 and C2, the output impedance (looking into V
shown as
R1'
Z =
0
R1 R2'
Z can be infinite, if resistors R1
(7) O
R1 and R2A + R2B, respectively, which is desirable. On the other
hand, Z can be negative, if the resistors are not matched and
O
cause oscillation. As a result, C1, in the range of a few pF, is
needed to prevent oscillation from the negative impedance.
L

PROGRAMMABLE LOW-PASS FILTER

In analog-to-digital conversions, it is common to include an
antialiasing filter to band limit the sampling signal. The dual-
L
channel AD5235 can, therefore, be used to construct a second-
order Sallen-Key low-pass filter, as shown in Figure 54.
R1
A
V
i
R
CONCURRENTLY
The design equations are
V
O =
V
i 2
S
(8)
Rev. B | Page 24 of 28
R1
150k Ω
+15V
–
V+
OP2177
V–
+
+15V
+
V+
–15V
OP2177
R1
V–
–
150k Ω
A1
–15V
+
R2B ( R1 R2A )
− +
R1' ( R2A R2B )
' '
and R2 match precisely with
C1
+2.5V
R2
B B
A
V+
AD8601
W W
R
V–
C2
–2.5V
ADJUSTED
Figure 54. Sallen-Key Low-Pass Filter
ω
2
f
ω
f
+ + ω
2
S
f
Q
R2
15k Ω
C1
10pF
R2B
A2
50 Ω
V
L
R2A
14.95k Ω R
L
500 Ω
I
L
) can be
L
(9)
V
O
U1
(10)