ADR445 Analog Devices, ADR445 Datasheet - Page 15

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

APPLICATIONS
OUTPUT ADJUSTMENT
The ADR44x family features a TRIM pin that allows the user to
adjust the output voltage of the part over a limited range. This
allows errors from the reference and overall system errors to be
trimmed out by connecting a potentiometer between the output
and the ground, with the wiper connected to the TRIM pin.
Figure 35 shows the optimal trim configuration. R1 allows fine
adjustment of the output and is not always required. R
be sufficiently large so that the maximum output current from
the ADR44x is not exceeded.
Using the trim function has a negligible effect on the temperature
performance of the ADR44x. However, all resistors need to be
low temperature coefficient resistors, or errors can occur.
BIPOLAR OUTPUTS
By connecting the output of the ADR44x to the inverting
terminal of an operational amplifier, it is possible to obtain both
positive and negative reference voltages. Care must be taken
when choosing Resistor R1 and Resistor R2 (see Figure 36).
They must be matched as closely as possible to ensure minimal
differences between the negative and positive outputs. In
addition, care must be taken to ensure performance over
temperature. Use low temperature coefficient resistors if the
circuit is used over temperature; otherwise, differences exist
between the two outputs.
0.1µF
ADR440/
ADR441/
ADR443/
ADR444/
ADR445
GND
V
4
2
IN
TRIM
V
OUT
Figure 35. ADR44x Trim Function
6
5
100kΩ
R1
0.1µF
R
10kΩ
R2
1kΩ
P
V
O
= ±0.5%
P
should
Rev. A | Page 15 of 20
ADR440/ADR441/ADR443/ADR444/ADR445
NEGATIVE REFERENCE
Figure 37 shows how to connect the ADR44x and a standard
operational amplifier, such as the OP1177, to provide negative
voltage. This configuration provides two main advantages. First,
it only requires two devices; therefore, it does not require
excessive board space. Second, and more importantly, it does
not require any external resistors. This means the performance
of this circuit does not rely on choosing low temperature
coefficient resistors to ensure accuracy.
V
directly from the output of the operational amplifier. If the
negative supply voltage is close to the reference output, the
operational amplifier must be dual supply and have low offset
and rail-to-rail capability.
OUT
is at virtual ground, and the negative reference is taken
0.1µF
Figure 37. ADR44x Negative Reference
–V
ADR440/
ADR441/
ADR443/
ADR444/
ADR445
Figure 36. ADR44x Bipolar Outputs
DD
+V
GND
V
4
2
DD
IN
V
OUT
6
6
ADR440/
ADR441/
ADR443/
ADR444/
ADR445
V
OUT
+V
GND
0.1µF
V
5kΩ
2
4
DD
IN
R3
R1
10kΩ
+10V
–10V
10kΩ
–V
R2
REF
+5V
–5V