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ADR440 Datasheet(PDF) 16 Page - Analog Devices

Part No. ADR440
Description  Ultralow Noise, LDO XFET Voltage References with Current Sink and Source
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Maker  AD [Analog Devices]
Homepage  http://www.analog.com
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ADR440 Datasheet(HTML) 16 Page - Analog Devices

 
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ADR440/ADR441/ADR443/ADR444/ADR445
Rev. A | Page 16 of 20
PROGRAMMABLE VOLTAGE SOURCE
To obtain different voltages than those offered by the ADR44x,
some extra components are needed. In Figure 38, two
potentiometers are used to set the desired voltage, while the
buffering amplifier provides current drive. The potentiometer
connected between VOUT and GND, with its wiper connected to
the noninverting input of the operational amplifier, takes care of
coarse trim. The second potentiometer, with its wiper
connected to the trim terminal of the ADR44x, is used for fine
adjustment. Resolution depends on the end-to-end resistance
value and the resolution of the selected potentiometer.
ADR440/
ADR441/
ADR443/
ADR444/
ADR445
6
2
4
VIN
VOUT
GND
R2
10kΩ
ADJ VREF
+VDD
R1
10kΩ
Figure 38. Programmable Voltage Source
For a completely programmable solution, replace the two
potentiometers in Figure 38 with one Analog Devices dual
digital potentiometer, offered with either an SPI® or an I2
interface. These interfaces set the position of the wiper on both
potentiometers and allow the output voltage to be set. Table 9
lists compatible Analog Devices digital potentiometers.
Table 9. Digital Potentiometer Parts
Part No.
No. of
Channels
No. of
Positions
ITF
R (kΩ)
VDD1
AD5251
2.00
64.00
I2C
1, 10, 50, 100
5.5
AD5207
2.00
256.00
SPI
10, 50, 100
5.5
AD5242
2.00
256.00
I2C
10, 100, 1M
5.5
AD5262
2.00
256.00
SPI
20, 50, 200
15
AD5282
2.00
256.00
I2C
20, 50, 100
15
AD5252
2.00
256.00
I2C
1, 10, 50, 100
5.5
AD5232
2.00
256.00
SPI
10, 50, 100
5.5
AD5235
2.00
1024.00
SPI
25, 250
5.5
ADN2850
2.00
1024.00
SPI
25, 250
5.5
1 Can also use a negative supply.
Adding a negative supply to the operational amplifier allows
the user also to produce a negative programmable reference,
by connecting the reference output to the inverting terminal
of the operational amplifier. Choose feedback resistors to
minimize errors over temperature.
PROGRAMMABLE CURRENT SOURCE
It is possible to build a programmable current source using a
setup similar to the programmable voltage source, as shown in
Figure 39. The constant voltage on the gate of the transistor sets
the current through the load. Varying the voltage on the gate
changes the current. This circuit does not require a dual digital
potentiometer.
ADR440/
ADR441/
ADR443/
ADR444/
ADR445
VIN
VOUT
GND
ILOAD
VCC
RSENSE
AD5259
2
6
4
0.1µF
0.1µF
Figure 39. Programmable Current Source
HIGH VOLTAGE FLOATING CURRENT SOURCE
Use the circuit in Figure 40 to generate a floating current source
with minimal self heating. This particular configuration can
operate on high supply voltages, determined by the breakdown
voltage of the N-channel JFET.
ADR440/
ADR441/
ADR443/
ADR444/
ADR445
VIN
VOUT
GND
OP90
+VS
SST111
VISHAY
2N3904
–VS
2
6
4
Figure 40. Floating Current Source


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