AD587
THEORY OF OPERATION
NOISE PERFORMANCE AND REDUCTION
The AD587 consists of a proprietary buried Zener diode refer-
ence, an amplifier to buffer the output and several high stability
thin-film resistors as shown in the block diagram in Figure 1.
This design results in a high precision monolithic 10 V output
reference with initial offset of 5 mV or less. The temperature
compensation circuitry provides the device with a temperature
coefficient of under 5 ppm/°C.
+V
IN
2
R
S
A1
R
F
R
T
5 TRIM
R
I
6 V
OUT
NOISE
REDUCTION
8
The 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 band-
width is approximately 200
µV
p-p. The dominant source of
this noise is the buried Zener which contributes approximately
100 nV/√Hz. In comparison, the op amp’s contribution is negli-
gible. Figure 3 shows the 0.1 Hz to 10 Hz noise of a typical
AD587. The noise measurement is made with a bandpass 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.
AD587
4
GND
NOTE:
PINS 1,3, AND 7 ARE INTERNAL TEST POINTS.
NO CONNECTIONS TO THESE POINTS.
Figure 1. AD587 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 contribu-
tion of the Zener to the circuit.
APPLYING THE AD587
Figure 3. 0.1 Hz to 10 Hz Noise
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 oper-
ating 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 differ-
ent from 10.000 V, for example, 10.24 V for binary applications.
In either case, the optional trim circuit shown in Figure 2 can
offset the output by as much as 300 mV, if desired, with mini-
mal effect on other device characteristics.
+V
IN
2
OPTIONAL
NOISE
REDUCTION
CAPACITOR
C
N
1µF
V
IN
8
NOISE
V
O
REDUCTION
6
OUTPUT
If further noise reduction is desired, an external capacitor may
be added between the NOISE REDUCTION pin and ground as
shown in Figure 2. This capacitor, combined with the 4 kΩ R
S
and the Zener resistances, form a low-pass filter on the output
of the Zener cell. A 1
µF
capacitor will have a 3 dB point at
40 Hz, and it will reduce the high frequency (to 1 MHz) noise
to about 160
µV
p-p. Figure 4 shows the 1 MHz noise of a typi-
cal AD587 both with and without a 1
µF
capacitor.
Figure 4. Effect of 1
µ
F Noise Reduction Capacitor on
Broadband Noise
TURN-ON TIME
AD587
TRIM
GND
4
5
10kΩ
Figure 2. Optional Fine Trim Configuration
Upon application of power (cold start), the time required for the
output voltage to reach its final value within a specified error
band is defined as the turn-on settling time. Two components
normally associated with this are: the time for the active circuits
to settle, and the time for the thermal gradients on the chip to
stabilize. Figure 5 shows the turn-on characteristics of the
AD587. It shows the settling to be about 60
µs
to 0.01%. Note
the absence of any thermal tails when the horizontal scale is ex-
panded to 1 ms/cm in Figure 5b.
–4–
REV. D
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