AD5260/AD5262
that can deliver 20 mA at 2.048 V. The load current is simply the
voltage across terminals B-to-W of the digital pot divided by RS.
IL
=
VREF ¥
RS
D
(7)
؉5V
2 U1
VIN
REF191
3 SLEEP
6
VOUT
0 TO (2.048 ؉ VL)
GND
C1
1F
AD5260
4
BW
A
+5V
–2.048V TO VL
U2 –
OP1177
+
–5V
RS
102⍀
RL
100⍀
VL
IL
Figure 19. 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 VL at full scale of the potentiometer
setting. Although the circuit works under single supply, the pro-
grammable resolution of the system will be 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 20). If the resistors are matched, the load current is:
( ) IL =
R2A + R2B
R2B
/R1
¥ VW
(8)
R1
150k⍀
R2
15k⍀
C1
10pF
+15V
+5V
A
AD5260
W
B
–5V
+15V
OP2177
A1
–15V
C2
10pF
R1
150k⍀
A2
AD8016
–15V
RL
50⍀
VL
R2A
14.95k⍀ RL
500⍀
IL
Figure 20. 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 21). The design
equations are:
VO
Vi
=
wO2
S2
+
wO
Q
S
+
wO2
(9)
1
wO = R1R2C1C2
(10)
1
1
Q = R1C1 + R2C2
(11)
Users can first select some convenient values 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.
C1
R1
R2
Vi
AB AB
W
R
W
R
+2.5V
AD8601
VO
C2
–2.5V
ADJUSTED TO
SAME SETTINGS
Figure 21. Sallen Key Low-Pass Filter
Programmable Oscillator
In a classic Wien-bridge oscillator, Figure 22, the Wien network
(R, R’, C, C’) provides positive feedback, while R1 and R2
provide negative feedback. At the resonant frequency, fo, the
overall phase shift is zero, and the positive feedback causes the
circuit to oscillate. With R = R’, C = C’, and R2 = R2A//(R2B+
RDIODE), the oscillation frequency is:
1
1
wO = RC or fO = 2pRC
(12)
where R is equal to RWA such that:
256 – D
R = 256 RAB
(13)
At resonance, setting
R2
R1
=
2
(14)
balances the bridge. In practice, R2/R1 should be set slightly larger
than 2 to ensure the oscillation can start. On the other hand, the
alternate turn-on of the diodes D1 and D2 ensures R2/R1 to be
smaller than 2 momentarily and therefore stabilizes the oscillation.
Once the frequency is set, the oscillation amplitude can be tuned
by R2B since:
2
3VO = IDR2B + VD
(15)
–16–
REV. 0