AD586
THEORY OF OPERATION
NOISE PERFORMANCE AND REDUCTION
The AD586 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 5 V output
reference with initial offset of 2.0 mV or less. The temperature
compensation circuitry provides the device with a temperature
coefficient of under 2 ppm/°C.
Using the bias compensation resistor between the Zener output
and the noninverting input to the amplifier, a capacitor can be
added at the NOISE REDUCTION pin (Pin 8) to form a low-
pass filter and reduce the noise contribution of the Zener to the
circuit.
The noise generated by the AD586 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
AD586. 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.
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 12 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 typical AD586
both with and without a 1
µF
capacitor.
Figure 1. AD586 Functional Block Diagram
APPLYING THE AD586
The AD586 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 5 V output. No external components
are required; the degree of desired absolute accuracy is achieved
simply by selecting the required device grade. The AD586 re-
quires less than 3 mA quiescent current from an operating sup-
ply of +12 V or +15 V.
An external fine trim may be desired to set the output level to
exactly 5.000 V (calibrated to a main system reference). System
calibration may also require a reference voltage that is slightly
different from 5.000 V, for example, 5.12 V for binary applica-
tions. In either case, the optional trim circuit shown in Figure 2
can offset the output by as much as 300 mV, if desired, with
minimal effect on other device characteristics.
Figure 3. 0.1 Hz to 10 Hz Noise
Figure 4. Effect of 1
µ
F Noise Reduction Capacitor on
Broadband Noise
TURN-ON TIME
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
AD586. It shows the settling to be about 60
µsec
to 0.01%.
Note the absence of any thermal tails when the horizontal scale
is expanded to l ms/cm in Figure 5b.
–4–
REV. C
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