AD587
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.
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.
NOISE
REDUCTION
+V
IN
2
8
R
S
A1
V
OUT
6
5
R
F
5s
1µV
R
T
100
90
TRIM
R
I
AD587
1µV
4
GND
NOTE
PIN 1, PIN 3, AND PIN 7 ARE INTERNAL TEST
POINTS. NO CONNECTIONS TO THESE POINTS.
10
0%
Figure 3. Functional Block Diagram
A capacitor can be added at the NOISE REDUCTION pin
(Pin 8) to form a low-pass filter with RS to reduce the noise
contribution of the Zener to the circuit.
Figure 5. 0.1 Hz to 10 Hz Noise
APPLYING THE AD587
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 RS
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.
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.
50µs
200µV
100
90
C
– 1µF
N
+V
IN
NO C
10
0%
N
2
OPTIONAL
NOISE-
REDUCTION
CAPACITOR
V
IN
V
6
5
OUTPUT
OUT
NOISE
8
REDUCTION
Figure 6. Effect of 1 μF Noise-Reduction Capacitor on Broadband Noise
AD587
C
1µF
N
TRIM
10kΩ
GND
4
Figure 4. Optional Fine-Trimming Configuration
Rev. H | Page 6 of 12
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