ADR512W
Rev. 0 | Page 8 of 12
APPLICATIONS INFORMATION
The ADR512W is a 1.2 V precision shunt voltage reference. It is
designed to operate without an external output capacitor
between the positive and negative terminals for stability. An
external capacitor can be used for additional filtering of the
supply.
As with all shunt voltage references, an external bias resistor
(R
BIAS
) is required between the supply voltage and the
ADR512W (see Figure 2). R
BIAS
sets the current that is required
to pass through the load (I
L
) and the ADR512W (I
Q
). The load
and the supply voltage can vary, thus R
BIAS
is chosen as follows:
• R
BIAS
must be small enough to supply the minimum I
Q
current to the ADR512W even when the supply voltage is
at its minimum and the load current is at its maximum
value.
• R
BIAS
also must be large enough so that I
Q
does not exceed
10 mA when the supply voltage is at its maximum and the
load current is at its minimum.
Given these conditions, R
BIAS
is determined by the supply
voltage (V
S
), the load and operating current (I
L
and I
Q
) of the
ADR512W, and the ADR512W’s output voltage.
R
BIAS
= (V
S
− V
OUT
)/(I
L
+ I
Q
) (3)
ADJUSTABLE PRECISION VOLTAGE SOURCE
The ADR512W, combined with a precision low input bias op
amp such as the AD8610, can be used to output a precise
adjustable voltage. Figure 11 illustrates the implementation of this
application using the ADR512W.
The output of the op amp, V
OUT
, is determined by the gain of the
circuit, which is completely dependent on resistors R2 and R1.
(4)
An additional capacitor in parallel with R2 can be added to
filter out high frequency noise. The value of C2 is dependent on
the value of R2.
ADR512W
09938-011
AD8610
R1
R2
C2 (OPTIONAL)
V
CC
R
BIAS
1.2V
V
OUT
= 1.2V
(1 + R2/R1)
Figure 11. Adjustable Precision Voltage Source
OUTPUT VOLTAGE TRIM
Using a mechanical or digital potentiometer, the output voltage
of the ADR512W can be trimmed ±0.5%. The circuit in Figure 12
illustrates how the output voltage can be trimmed using a 10 k Ω
potentiometer.
09938-012
ADR512W
R1
100kΩ
V
CC
R
BIAS
V
OUT
POT
50kΩ
Figure 12. Output Voltage Trim
USING THE ADR512W WITH PRECISION DATA
CONVERTERS
The compact ADR512W package and the device’s low
minimum operating current requirement make it ideal for use
in battery powered portable instruments, such as the AD7533
CMOS multiplying DAC, that use precision data converters.
Figure 13 shows the ADR512W serving as an external reference
to the AD7533, a CMOS multiplying DAC. Such a DAC
requires a negative voltage input to provide a positive output
range. In this application, the ADR512W supplies a −1.2 V
reference to the REF input of the AD7533.
09938-013
ADR512W
AD7533
V
OUT
= 0V TO 1.2V
V
DD
–V
DD
R2
G
N
9
LSB
0
MSB
+
–
+
–
1
1
3
2 1 15
Figure 13. The ADR512W as a Reference for a 10-Bit CMOS DAC (AD7533)
PRECISE NEGATIVE VOLTAGE REFERENCE
The ADR512W is suitable for use in applications where a
precise negative voltage reference is desired, including the
application detailed in Figure 13.
Figure 14 shows the ADR512W configured to provide a −1.2 V
output.
