Datasheet ADR1581 (Analog Devices) - 8

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ADR1581 40µV/DIV 21µV rms OUTPUT IMPEDANCE VS. FREQUENCY
Understanding the effect of the reverse dynamic output impedance
(a)
in a practical application is important to successfully applying the ADR1581. A voltage divider is formed by the ADR1581 output
20µV/DIV 6.5µV rms, t = 0.2ms
impedance and the external source impedance. When an external source resistor of about 30 kΩ (IR = 100 μA) is used, 1% of the
(b)
noise from a 100 kHz switching power supply is developed at the output of the ADR1581. Figure 15 shows how a 1 μF load
10µV/DIV 2.90µV rms, t = 960ms
capacitor connected directly across the ADR1581 reduces the effect of power supply noise to less than 0.01%.
(c)
7 01 2-
1k 10ms/DIV
67 06 Figure 17. Total RMS Noise
) 100 TURN-ON TIME Ω C ( L = 0
Many low power instrument manufacturers are becoming
DANCE
increasingly concerned with the turn-on characteristics of the
E 10 P
components in their systems. Fast turn-on components often
M I UT
enable the end user to keep power off when not needed, and yet
ΔIR = 0.1IR P T IR = 100µA U C
those components respond quickly when the power is turned
L = 1µF O 1
on for operation. Figure 18 displays the turn-on characteristics
IR = 1mA
of the ADR1581.
0.1
5 Upon application of power (cold start), the time required for the
10 100 1k 10k 100k 1M
-01 72 output voltage to reach its final value within a specified error is
FREQUENCY (Hz)
066 the turn-on settling time. Two components normally associated Figure 15. Output Impedance vs. Frequency with this are time for active circuits to settle and time for thermal
NOISE PERFORMANCE AND REDUCTION
gradients on the chip to stabilize. This characteristic is generated from cold start operation and represents the true turn-on wave- The noise generated by the ADR1581 is typically less than form after power-up. 5 μV p-p over the 0.1 Hz to 10 Hz band. Figure 20 shows both the coarse and fine Figure 16 shows the turn-on settling characteristics of the device; the total settling 0.1 Hz to 10 Hz noise of a typical ADR1581. Noise in a 10 Hz to time to within 1.0 mV is about 6 μs, and there is no long thermal 10 kHz bandwidth is approximately 20 μV rms (see Figure 17a). tail when the horizontal scale is expanded to 2 ms/div. If further noise reduction is desired, a one-pole low-pass filter can be added between the output pin and ground. A time constant
2.4V
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
V 0V IN
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).
CL = 200pF 4.48µV p-p
8 01
1µV/DIV 250mV/DIV 5µs/DIV
2- 67 06 Figure 18. Turn-On Response Time
RS = 11.5kΩ RL + VIN VR CL VOUT
10 6 -0 1 0
–
72 2- 66
TIME (1s/DIV)
67 00 06 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 Document Outline FEATURES APPLICATIONS GENERAL DESCRIPTION PIN CONFIGURATION TABLE OF CONTENTS REVISION HISTORY SPECIFICATIONS ABSOLUTE MAXIMUM RATINGS ESD CAUTION TYPICAL PERFORMANCE CHARACTERISTICS THEORY OF OPERATION APPLYING THE ADR1581 TEMPERATURE PERFORMANCE VOLTAGE OUTPUT NONLINEARITY VS. TEMPERATURE REVERSE VOLTAGE HYSTERESIS OUTPUT IMPEDANCE VS. FREQUENCY NOISE PERFORMANCE AND REDUCTION TURN-ON TIME TRANSIENT RESPONSE PRECISION MICROPOWER LOW DROPOUT REFERENCE USING THE ADR1581 WITH 3 V DATA CONVERTERS OUTLINE DIMENSIONS ORDERING GUIDE