Flicker noise, also known as 1/f noise, is a low-frequency noise that affects the device output voltage which can affect precision measurements in ADCs. This noise increases proportionally with output voltage and operating temperature. The noise is measured by filtering the output from 0.1 Hz to 10 Hz. The 1/f noise is an extremely low value, therefore the frequency of interest must be amplified and band-pass filtered. This is done by using a high-pass filter to block the DC voltage. The resulting noise is then amplified by a gain of 1000. The bandpass filter is created by a series of high-pass and low-pass filter that adds additional gain to make it more visible on a oscilloscope as shown in Figure 8-7. Figure 8-8 shows the effect of flicker noise over 10 second. Flicker noise must be tested in a Faraday cage enclosure to block environmental noise.

Figure 8-7 Low-Frequency (1/f) Noise Test Setup

Figure 8-8 0.1 Hz to 10 Hz Voltage Noise

Figure 8-9 shows the typical 1/f noise (0.1 Hz to 10 Hz) distribution across various load conditions. The REF35-Q1 device also offers noise reduction functionality by adding an optional capacitor between NR (pin 5) and ground pins.

Figure 8-9 0.1 Hz to 10 Hz Noise Distribution vs Load Conditions

Figure 8-10 0.1 Hz to 10 Hz Noise Distribution vs NR Capacitance