Datasheet
AD5260/AD5262
Rev. A | Page 21 of 24
–5V
OP1177
+
–
U2
+5V
R
S
102Ω
R
L
100Ω
V
L
I
L
A
B
W
AD5260
C1
1µF
GND
REF191
SLEEP
V
S
OUTPUT
+5V
U12
3
4
6
0V TO (2.048V + V
L
)
–2.048 TO V
L
02695-064
Figure 64. Programmable 4-to-20 mA Current Source
The circuit is simple, but be aware that 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.
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 65). If the resistors are matched, the load
current is
()
WL
V
BR
R1BRAR
I ×
+
=
2
22
(8)
AD8016
+15V
–15V
+5V
–5V
OP2177
AD5260
A1
W
A
B
C2
10pF
R1'
150kΩ
R1
150kΩ
R2'
15kΩ
A2
C1
10pF
R2A
14.95kΩ
R
L
500Ω
R
L
50Ω
+15V
–15V
V
L
I
L
02695-065
Figure 65. Programmable Bidirectional Current Source
PROGRAMMABLE LOW-PASS FILTER
Digital Potentiometer AD5262 can be used to construct a
second-order, Sallen-Key low-pass filter (see Figure 66). The
design equations are
2
2
2
O
O
O
i
O
S
Q
S
V
V
ω
ω
ω
++
=
(9)
R1R2C1C2
O
1
=
ω
(10)
R2C2R1C1
Q
11
+=
(11)
Users can first select any convenient value for the capacitors. To
achieve maximally flat bandwidth where Q = 0.707, let C1 be
twice the size of C2 and let R1 = R2. As a result, users can adjust
R1 and R2 to the same settings to achieve the desirable bandwidth.
A
B
V
i
AD8601
+2.5V
V
O
–2.5V
W
R
R2R1
A
B
W
R
C1
C2
ADJUSTED TO
SAME SETTINGS
02695-066
Figure 66. Sallen Key Low-Pass Filter
PROGRAMMABLE OSCILLATOR
In a classic Wien-bridge oscillator (see Figure 67), the Wien
network (R, R’, C, C’) provides positive feedback, whereas R1
and R2 provide negative feedback. At the resonant frequency, f
o
,
the overall phase shift is zero, and the positive feedback causes
the circuit to oscillate. With R = R’, C = C’, and R2 = R2A//(R2B +
R
DIODE
), the oscillation frequency is
R
C
O
1
=
ω
or
R
C
f
O
π
2
1
=
(12)
where R is equal to R
WA
such that
AB
R
D
R
256
256
−
=
(13)
At resonance, setting
2=
R1
R2
(14)
balances the bridge. In practice, R2/R1 should be set slightly
larger than 2 to ensure the oscillation can start. However, the
alternate turn-on of the diodes, D1 and D2, ensures R2/R1 to
be smaller than 2 momentarily and therefore stabilizes the
oscillation.
When the frequency is set, the oscillation amplitude can be
tuned by R2B because
DD
O
VBRIV += 2
3
2
(15)
V
O
, I
D
, and V
D
are interdependent variables. With proper
selection of R2B, an equilibrium is reached such that V
O
converges. R2B can be in series with a discrete resistor to
increase the amplitude, but the total resistance cannot be too
large to saturate the output.
In both circuits in Figure 66 and Figure 67, the frequency tuning
requires that both RDACs be adjusted to the same settings.
Because the two channels are adjusted one at a time, an intermedi-