AD587
Rev. H | Page 6 of 12
THEORY OF OPERATION
The AD587 consists of a proprietary buried Zener diode
reference, an amplifier to buffer the output, and several high
stability thin-film resistors, as shown in
Figure 3. This design
results in a high precision monolithic 10 V output reference
with initial offset of 5 mV or less. The temperature-compensa-
tion circuitry provides the device with a temperature coefficient
of less than 5 ppm/°C.
82
6
4
5
R
T
R
I
AD587
A1
+V
IN
NOISE
REDUCTION
V
OUT
TRIM
GND
NOTE
PIN 1, PIN 3, AND PIN 7 ARE INTERNAL TEST
POINTS. NO CONNECTIONS TO THESE POINTS.
R
F
R
S
00530-003
Figure 3. Functional Block Diagram
A capacitor can be added at the NOISE REDUCTION pin
(Pin 8) to form a low-pass filter with R
S
to reduce the noise
contribution of the Zener to the circuit.
APPLYING THE AD587
The AD587 is simple to use in virtually all precision reference
applications. When power is applied to Pin 2 and Pin 4 is
grounded, Pin 6 provides a 10 V output. No external compo-
nents are required; the degree of desired absolute accuracy is
achieved simply by selecting the required device grade. The
AD587 requires less than 4 mA quiescent current from an
operating supply of 15 V.
Fine trimming may be desired to set the output level to exactly
10.000 V (calibrated to a main system reference). System cali-
bration may also require a reference voltage that is slightly
different from 10.000 V, for example, 10.24 V for binary
applications. In either case, the optional fine-trimming circuit
shown in
Figure 4 can offset the output by as much as 300 mV
with minimal effect on other device characteristics.
8
2
6
4
GND
5
AD587
NOISE
REDUCTION
TRIM
10kΩ
OUTPUT
OPTIONAL
NOISE-
REDUCTION
CAPACITOR
C
N
1µF
+
IN
V
OUT
V
IN
00530-004
Figure 4. Optional Fine-Trimming Configuration
NOISE PERFORMANCE AND REDUCTION
Noise generated by the AD587 is typically less than 4 µV p-p
over the 0.1 Hz to 10 Hz band. Noise in a 1 MHz bandwidth is
approximately 200 µV p-p. The dominant source of this noise is
the buried Zener, contributing approximately 100 nV/√Hz. By
comparison, the contribution of the op amp is negligible.
Figure 5 shows the 0.1 Hz to 10 Hz noise of a typical AD587.
The noise measurement is made with a band-pass filter made
of a 1-pole high-pass filter with a corner frequency at 0.1 Hz
and a 2-pole low-pass filter with a corner frequency at 12.6 Hz
to create a filter with a 9.922 Hz bandwidth.
1µV
5s
100
90
10
0%
1µV
0530-005
Figure 5. 0.1 Hz to 10 Hz Noise
If further noise reduction is desired, an external capacitor can
be added between the NOISE REDUCTION pin and ground,
as shown in
Figure 4. This capacitor, combined with the 4 kΩ R
S
and the Zener resistances, forms a low-pass filter on the output
of the Zener cell. A 1 µF capacitor has a 3 dB point at 40 Hz
and reduces the high frequency (up to 1 MHz) noise to about
160 µV p-p.
Figure 6 shows the 1 MHz noise of a typical AD587,
both with and without a 1 µF capacitor.
100
0%
200µV
50µs
90
10
NO C
N
C
N
– 1µF
00530-006
Figure 6. Effect of 1 μF Noise-Reduction Capacitor on Broadband Noise
