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> Signal
Generation Circuits > APP 4527

Dec 06, 2010

Keywords: white noise, pink noise, flicker noise,
1/f noise, low-noise op amps
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parts

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APPLICATION NOTE 4527

White-Noise Generator Has No 1/f Component

Abstract: In
this noise-generator circuit, the amplifier (MAX4238) has no
1/f component in its input voltage noise. It amplifies its own
input-voltage noise with a feedback network made with
low-value resistors, to avoid adding noticeable 1/f noise
either from the resistors or from the amplifier’s input noise
current.

Attend
this brief webcast by Maxim on
TechOnline

A
similar version of this article appeared in the March 20, 2008
issue of EDN magazine.

White-noise generators
(those for which the graph of output-power density vs.
frequency is flat) are useful for testing circuits that have
an extended low-frequency or dc response. For frequency ranges
that extend down to a few hertz
or below, however, the design of white-noise generators is
complicated by the presence of pink noise (also known as
flicker noise, or 1/f noise).

The noise generated by a
semiconductor
device always has the characteristic signature of pink noise:
its output power-density amplitude increases as frequency
decreases, starting with a corner frequency located at tens of
hertz up to a few kilohertz. Noise generated by a high-value
resistor has its own 1/f component, whose value and
characteristics vary with the technology used to manufacture
the resistor. If, on the other hand, the resistor value is low
and the device is built with low-noise technology, the noise
is almost completely white (power density constant with
frequency). Unfortunately, a low-value resistor also yields a
low value of noise power-density amplitude, and any device
introduced to amplify that level adds pink noise of its
own.

You can find amplifiers whose input voltage noise
includes no pink noise component, but their input current
noise will have a 1/f component (which appears at the
amplifier output) if the resistance
seen from any amplifier input has a significant
value.

In the noise-generator circuit of Figure
1, the amplifier (MAX4238) has no 1/f
component in its input voltage noise. It amplifies its own
input-voltage noise with a feedback network made with
low-value resistors, to avoid adding noticeable 1/f noise
either from the resistors or from the amplifier's input noise
current.

Figure
1. Built with an amplifier whose input voltage noise has no
1/f component, this white-noise generator produces an output
with no 1/f component.

A plot of the circuit's
output voltage as a function of frequency (Figure 2) is
almost flat from 0.01Hz to 3kHz. The voltage-density amplitude
is in the range 4-5µV/√Hz. The noise density amplitude also
depends on temperature,
so you should keep the circuit at constant temperature while
making measurements.

Figure
2. Output voltage noise for the Figure 1 circuit.

Related parts

MAX4238

Ultra-Low Offset/Drift,
Low-Noise, Precision SOT23 Amplifiers

Free
samples

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Download,
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APP 4527: Dec 06, 2010

AN4527, AN 4527, APP4527,
Appnote4527, Appnote 4527

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