AD688SQ データシートの表示（PDF） - Analog Devices
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AD688SQ Datasheet PDF : 16 Pages
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AD688
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.
1mV
5s
100
90
1µV
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
in Figure 5. This will form a low-pass filter with the 5 kΩ RB on
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.
200µV
50µs
CN = 1µF
NO CN
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.
+VS
100
90
10V
1mV
100µs
–VS
+VOUT
10
0%
10V
Figure 8. Turn On Characteristics: Electrical Turn On
+VS
100
90
10V
1mV
2ms
–VS
+VOUT
10
0%
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.
+VS
100
90
10V
1mV
20ms
–VS
+VOUT
10
0%
10V
Figure 10. Turn On With 1 µF CN
Rev. B | Page 8 of 16
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