AD5260/AD5262
5V
1
U1
VIN
AD5260 5V
VOUT 3
A
W
GND
B AD8601
VO
2 AD1582
A1
Figure 15. Programmable Voltage Reference
8-Bit Bipolar DAC
Figure 16 shows a low cost 8-bit bipolar DAC. It offers the same
number of adjustable steps but not the precision of conventional
DACs. The linearity and temperature coefficients, especially at low
values codes, are skewed by the effects of the digital potentiometer
wiper resistance. The output of this circuit is:
VO
=
Ê 2D
ËÁ 256
- 1ˆ¯˜
¥ VREF
(4)
+5V
AD5260
Vi
U2
OP2177
VO
U1
VIN
VOUT
TRIM
GND
W
B
R
+5VREF W1
A
R
+5V
A2 –5V
؊5VREF
ADR425
OP2177
A1
–5V
Figure 16. 8-Bit Bipolar DAC
Similar to the previous example, in the simpler (and much more
usual) case, where K = 1, a single digital pot AD5260, and U1
is replaced by a matched pair of resistors to apply Vi and – Vi at
the ends of the digital pot. The relationship becomes:
VO
=
ÊËÁ1 +
R2ˆ
R1¯˜
Ê 2D2
ËÁ 256
- 1ˆ¯˜
¥ Vi
(6)
If R2 is large, a few picofarad compensation capacitors may be
needed to avoid any gain peaking.
Table VIII shows the result of adjusting D, with A2 configured as a
unity gain, a gain of 2, and a gain of 10. The result is a bipolar
amplifier with linearly programmable gain and 256-step resolution.
Table VIII. Result of Bipolar Gain Amplifier
D R1 = •, R2 = 0
0
–1
64 –0.5
128 0
192 0.5
255 0.968
R1 = R2
–2
–1
0
1
1.937
R2 = 9R1
–10
–5
0
5
9.680
Programmable Voltage Source with Boosted Output
For applications that require high current adjustment such as a
laser diode driver or turnable laser, a boosted voltage source can
be considered (see Figure 18).
Vi
5V
A
W
U1
B
R1 10k⍀
A1
VO
P1
RBIAS
CC
N1
SIGNAL LO
IL
Bipolar Programmable Gain Amplifier
For applications that require bipolar gain, Figure 17 shows one
implementation. Digital potentiometer U1 sets the adjustment
range. The wiper voltage at W2 can therefore be programmed
between Vi and –KVi at a given U2 setting. Configuring A2 in
the noninverting mode allows linear gain and attenuation. The
transfer function is:
( ) VO
Vi
= ÊËÁ1 +
R2ˆ
R1¯˜
¥
Ê D2
ËÁ 256
¥
1+
K
-
K
ˆ
¯˜
(5)
where K is the ratio of RWB1/RWA1 set by U1.
VDD
U2
AD5262
W2
A2
B2
Vi
A1
B1
W1 VDD
U1
AD5262
OP2177
OP2177
A2
C1
VSS
–KVi
VO
R2
R1
A1
VSS
Figure 17. Bipolar Programmable Gain Amplifier
U1= AD5260
A1= AD8601, AD8605, AD8541
P1= FDP360P, NDS9430
N1= FDV301N, 2N7002
Figure 18. Programmable Boosted Voltage Source
In this circuit, the inverting input of the op amp forces the VO to be
equal to the wiper voltage set by the digital potentiometer. The
load current is then delivered by the supply via the P-Ch FET P1.
The N-Ch FET N1 simplifies the op amp driving requirement.
A1 needs to be the rail-to-rail input type. Resistor R1 is needed to
prevent P1 from not turning off once it is on. The choice of R1 is a
balance between the power loss of this resistor and the output turn-
off time. N1 can be any general-purpose signal FET; on the other
hand, P1 is driven in the saturation state, and therefore its power
handling must be adequate to dissipate (Vi – VO) ϫ IL power. This
circuit can source a maximum of 100 mA at 5 V supply. Higher
current can be achieved with P1 in a larger package. Note, a single
N-Ch FET can replace P1, N1, and R1 altogether. However, the out-
put swing will be limited unless separate power supplies are used.
For precision application, a voltage reference such as ADR423,
ADR292, and AD1584 can be applied at the input of the digital
potentiometer.
Programmable 4-to-20 mA Current Source
A programmable 4-to-20 mA current source can be implemented
with the circuit shown in Figure 19. REF191 is a unique low
supply headroom and high current handling precision reference
REV. 0
–15–