ADR1581
40µV/DIV
21µV rms
OUTPUT IMPEDANCE VS. FREQUENCY
Understanding the effect of the reverse dynamic output impedance
in a practical application is important to successfully applying the
ADR1581. A voltage divider is formed by the ADR1581 output
impedance and the external source impedance. When an external
source resistor of about 30 kΩ (IR = 100 μA) is used, 1ꢁ of the
noise from a 100 kHz switching power supply is developed at
the output of the ADR1581. Figure 15 shows how a 1 ꢀF load
capacitor connected directly across the ADR1581 reduces the
effect of power supply noise to less than 0.01ꢁ.
(a)
(b)
(c)
20µV/DIV
6.5µV rms, t = 0.2ms
2.90µV rms, t = 960ms
10µV/DIV
10ms/DIV
1k
Figure 17. Total RMS Noise
TURN-ON TIME
100
C
= 0
L
Many low power instrument manufacturers are becoming
increasingly concerned with the turn-on characteristics of the
components in their systems. Fast turn-on components often
enable the end user to keep power off when not needed, and yet
those components respond quickly when the power is turned
on for operation. Figure 18 displays the turn-on characteristics
of the ADR1581.
10
1
ΔI = 0.1I
R
R
I
= 100µA
R
C
= 1µF
L
I
= 1mA
R
0.1
10
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. This characteristic is generated
from cold start operation and represents the true turn-on wave-
form after power-up. Figure 20 shows both the coarse and fine
turn-on settling characteristics of the device; the total settling
time to within 1.0 mV is about 6 ꢀs, and there is no long thermal
tail when the horizontal scale is expanded to 2 ms/div.
100
1k
10k
100k
1M
FREQUENCY (Hz)
Figure 15. Output Impedance vs. Frequency
NOISE PERFORMANCE AND REDUCTION
The noise generated by the ADR1581 is typically less than
5 ꢀV p-p over the 0.1 Hz to 10 Hz band. Figure 16 shows the
0.1 Hz to 10 Hz noise of a typical ADR1581. Noise in a 10 Hz to
10 kHz bandwidth is approximately 20 μV rms (see Figure 17a).
If further noise reduction is desired, a one-pole low-pass filter
can be added between the output pin and ground. A time constant
of 0.2 ms has a −3 dB point at about 800 Hz and reduces the high
frequency noise to about 6.5 μV rms (see Figure 17b). A time
constant of 960 ms has a −3 dB point at 165 Hz and reduces the
high frequency noise to about 2.9 μV rms (see Figure 17c).
2.4V
V
0V
IN
C
= 200pF
L
4.48µV p-p
250mV/DIV
5µs/DIV
1µV/DIV
Figure 18. Turn-On Response Time
R
R
= 11.5kΩ
L
S
+
V
IN
V
R
C
V
OUT
L
–
TIME (1s/DIV)
Figure 19. Turn-On, Settling, and Transient Test Circuit
Figure 16. 0.1 Hz to 10 Hz Voltage Noise
Rev. 0 | Page 8 of 12
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