REV. F

–4–

AD587

THEORY OF OPERATION

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.

8

2

6

4

5

AD587

A1

NOISE

REDUCTION

TRIM

GND

NOTE

PINS 1, 3 AND 7 ARE INTERNAL TEST POINTS.

NO CONNECTIONS TO THESE POINTS.

Figure 1. Functional Block Diagram

A capacitor can be added at the NOISE REDUCTION pin

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 components

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 calibra-

tion 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 trim circuit shown in Figure 2 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

10k

OUTPUT

1 F

OPTIONAL

NOISE

REDUCTION

CAPACITOR

Figure 2. Optional Fine Trim Configuration

NOISE PERFORMANCE AND REDUCTION

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 bandwidth 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 negligible. Figure 3

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.

Figure 3. 0.1 Hz to 10 Hz Noise

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

4k

Ω R

output of the Zener cell. A 1

µF capacitor will have a 3 dB point

at 40 Hz, and 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

µF Noise Reduction Capacitor

on Broadband Noise

TURN-ON TIME

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 expanded to

1ms/cm in Figure 5b.