ADR1581

Rev. 0 | Page 8 of 12

40µV/DIV

21µV rms

20µV/DIV

10µV/DIV

10ms/DIV

6.5µV rms, t = 0.2ms

(a)

(b)

(c)

2.90µV rms, t = 960ms

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

noise from a 100 kHz switching power supply is developed at

the output of the ADR1581. Figure 15 shows how a 1 μF load

capacitor connected directly across the ADR1581 reduces the

effect of power supply noise to less than 0.01%.

1k

10

0.1

1

100

FREQUENCY (Hz)

10

100

1k

10k

100k

1M

Figure 17. Total RMS Noise

TURN-ON TIME

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.

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 15. Output Impedance vs. Frequency

NOISE PERFORMANCE AND REDUCTION

The noise generated by the ADR1581 is typically less than

5 μV p-p over the 0.1 Hz to 10 Hz band.

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 μs, and there is no long thermal

tail when the horizontal scale is expanded to 2 ms/div.

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

250mV/DIV

5µs/DIV

0V

2.4V

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).

1µV/DIV

TIME (1s/DIV)

4.48µV p-p

Figure 18. Turn-On Response Time

+

–

Figure 19. Turn-On, Settling, and Transient Test Circuit

Figure 16. 0.1 Hz to 10 Hz Voltage Noise