Noise Bin (ppmp-p)

0

16

32

48

64

80

1.2

2

2.8 3.6 4.4 5.2

6

6.8 7.6 8.4 9.2 10 10.8

IL = 0mA

IL = 1mA

IL = 10mA

Figure 8-12. 0.1 Hz to 10 Hz Noise Distribution vs

Load Conditions

Noise Bin (ppmp-p)

0

8

16

24

32

40

1.2

2

2.8 3.6 4.4 5.2

6

6.8 7.6 8.4 9.2 10 10.8

CNR = Open

CNR = 1uF

CNR = 10uF

Figure 8-13. 0.1 Hz to 10 Hz Noise Distribution vs

NR Capacitance

8.5.2 Broadband Noise

Broadband noise is a noise that appears at higher frequency compared to 1/f noise. The broadband noise is

measured by high-pass filtering the output of the reference device, followed by a gain stage and measuring the

result on a spectrum analyzer as shown in Figure 8-14.

REF35xxx

CIN

GND

VIN

EN

NR

VREF

GND

Optional

CL

High-pass

Filter

Fc = 10Hz

G = 11

Amplifier

G = 11

Supply

Spectrum

Analyzer

Figure 8-14. Broadband Noise Test Setup

For noise sensitive designs, a low-pass filter can be used to reduce broadband output noise. When designing a

low-pass filter, take special care to ensure the output impedance of the filter does not degrade AC performance.

This can occur in RC low-pass filters where a large series resistance can impact the load transients due to

output current fluctuations. The REF35 device also offers noise reduction functionality by adding an optional

capacitor between NR (pin 5) and ground pins. Figure 8-15 and Figure 8-16 show the noise spectrum for

REF35250 and REF35500 devices respectively across various load capacitance.

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REF35

SNAS809A – DECEMBER 2021 – REVISED AUGUST 2022

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