AD688

Rev. B | Page 8 of 16

NOISE PERFORMANCE AND REDUCTION

The noise generated by the AD688 is typically less than

6 µV p-p over the 0.1 Hz to 10 Hz band. Noise in a 1 MHz

bandwidth is approximately 840 µV p-p. The dominant source

of this noise is the buried Zener which contributes

approximately 140 nV/√Hz. In comparison, the op amp’s

contribution is negligible. Figure 6 shows the 0.1 Hz to 10 Hz

noise of a typical AD688.

1

µV

1mV

5s

100

90

10

0%

Figure 6. 0.1 Hz to 10 Hz Noise

If further noise reduction is desired, an optional capacitor can

be added between the noise reduction pin and ground as shown

the output of the Zener cell. A 1 µF capacitor will have a 3 dB

point at 32 Hz and will reduce the high frequency noise (to

1 MHz) to about 250 µV p-p. Figure 7 shows the 1 MHz noise of

a typical AD688 both with and without a 1 µF capacitor.

200

µV

50

µs

Figure 7. 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 is the turn on settling time. Two components normally

associated with this are: time for active circuits to settle and

time for thermal gradients on the chip to stabilize. Figure 8 and

Figure 9 show the turn on characteristics of the AD688. They

show the settling time to be about 600 µs. Note the absence of

any thermal tails when the horizontal scale is expanded to

2 ms/cm in Figure 9.

100

90

10

0%

10V

1mV

100

µs

10V

Figure 8. Turn On Characteristics: Electrical Turn On

100

90

10

0%

10V

1mV

2ms

10V

Figure 9. Turn On Characteristics: Extended Time Scale

Output turn on time is modified when an external noise

reduction capacitor is used. When present, this capacitor

presents an additional load to the internal Zener diode’s current

source, resulting in a somewhat longer turn on time. In the case

of a 1 µF capacitor, the initial turn on time is approximately

100 ms (Figure 10).

When the noise reduction feature is used, a 20 kΩ resistor

between Pin 6 and Pin 2 is required for proper startup.

100

90

10

0%

10V

1mV

20ms

10V