ADR445 Analog Devices, ADR445 Datasheet - Page 16

• Manufacturer Part Number: ADR445
• Description: (ADR440 - ADR445) LDO XFET Voltage References
• Manufacturer: Analog Devices

ADR440/ADR441/ADR443/ADR444/ADR445
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 V
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.
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 I
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.
AD5251
AD5207
AD5242
AD5262
AD5282
AD5252
AD5232
AD5235
ADN2850
1
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.
Can also use a negative supply.
ADR440/
ADR441/
ADR443/
ADR444/
ADR445
+V
GND
V
2
4
IN
DD
V
2.00
2.00
2.00
2.00
2.00
2.00
2.00
2.00
No. of
Channels
2.00
OUT
Figure 38. Programmable Voltage Source
6
OUT
R1
10kΩ
No. of
Positions
64.00
256.00
256.00
256.00
and GND, with its wiper connected to
256.00
256.00
256.00
1024.00
1024.00
10kΩ
R2
ITF
I
SPI
I
SPI
I
I
SPI
SPI
SPI
2
2
2
2
C
C
C
C
R (kΩ)
1, 10, 50, 100
10, 50, 100
10, 100, 1M
20, 50, 200
20, 50, 100
1, 10, 50, 100
10, 50, 100
25, 250
25, 250
ADJ V
REF
2
C®
V
5.5
5.5
5.5
15
15
5.5
5.5
5.5
5.5
Rev. A | Page 16 of 20
DD
1
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.
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.
V
0.1µF
CC
ADR440/
ADR441/
ADR443/
ADR444/
ADR445
GND
Figure 39. Programmable Current Source
ADR440/
ADR441/
ADR443/
ADR444/
ADR445
V
2
4
IN
V
GND
Figure 40. Floating Current Source
OUT
V
2
4
IN
V
OUT
6
6
0.1µF
OP90
AD5259
+V
–V
2N3904
S
S
VISHAY
SST111
I
LOAD
R
SENSE