AD5171(RevPrC) View Datasheet(PDF) - Analog Devices
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AD5171 Datasheet PDF : 20 Pages
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Preliminary Technical Data
AD5171
APPLICATIONS
PROGRAMMABLE VOLTAGE REFERENCE (DAC)
It is common to buffer the output of the digital potentiometer
as a DAC unless the load is much larger than RWB. The buffer
serves the purpose of impedance conversion as well as
delivering higher current, which may be needed.
5V
1 U1
VIN
ADR03
VOUT
3
AD5171
A
W
5V
U2
AD8601
V0
AD1582
GND
2
B
A1
Figure 40. Programmable Voltage Reference (DAC)
GAIN CONTROL COMPENSATION
The digital potentiometers are commonly used in gain controls
(Figure 41) or sensor transimpedance amplifier signal
conditioning applications. To avoid gain peaking or in worst-
case oscillation due to step response, a compensation capacitor
is needed. In general, C2 in the range of a few picofarads to no
more than a few tenths of a picofarad is adequate for the
compensation.
C2
4.7pF
R2 100kΩ
B
A
W
R1
47kΩ
U1
VO
VI
Figure 41. Typical Noninverting Gain Amplifier
PROGRAMMABLE VOLTAGE SOURCE WITH
BOOSTED OUTPUT
For applications that require high current adjustment, such as a
laser diode driver or tunable laser, a boosted voltage source can
be considered (Figure 42).
U3 2N7002
VIN
VOUT
U1 A
+V
AD5171 W
U2
AD8601
B
–V
CC RBIAS
IL
LD
SIGNAL
Figure 42. Programmable Booster Voltage Source
In this circuit, the inverting input of the op amp forces the VOUT
to be equal to the wiper voltage set by the digital potentiometer.
The load current is then delivered by the supply via the N-Ch
FET N1. N1 power handling must be adequate to dissipate
(VI − VO) × IL power. This circuit can source a maximum of
100 mA with a 5 V supply. For precision applications, a voltage
reference such as ADR421, ADR03, or ADR370 can be applied
at the A terminal of the digital potentiometer.
LEVEL SHIFTING FOR DIFFERENT VOLTAGE
OPERATION
When users need to interface a 2.5 V controller with AD5171, a
proper voltage level shift must be employed so that the digital
potentiometer can be read from or written to the controller;
Figure 43 shows one of the implementations. M1 and M2
should be low threshold N-Ch power MOSFETs, such as
FDV301N.
VDD1 = 2.5V
Rp
Rp
VDD2 = 5V
Rp
Rp
SDA1
SCL1
2.5V
CONTROLLER
G
S
DG
M1
S
D
M2
SDA2
SCL2
2.7V–5.5V
AD5171
Figure 43. Level Shifting for Different Voltage Operation
RESISTANCE SCALING
The AD5171 offers 5 kΩ, 10 kΩ, 50 kΩ, and 100 kΩ nominal
resistances. For users who need to optimize the resolution with
an arbitrary full-range resistance, the following techniques can
be used. By paralleling a discrete resistor (Figure 44) a
proportion tely lower voltage appears at terminal A to B, which
is applicable to only the voltage divider mode.
This translates into a finer degree of precision because the step
size at terminal W will be smaller. The voltage can be found as
VW
(D)
=
(RAB || R2)
R3 + RAB || R2
×
D
64
× VDD
(5)
VDD
R3
A
R2 R1
W
B
Figure 44. Lowering the Nominal Resistance
Rev. PrC | Page 17 of 20
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