Filtered noise generator using a voltage regulator source

-February 24, 2014

Noise generators are very useful devices. They are used in test equipment, random number generators, etc. Usually, designers use special Zener noise diodes (at very low current) and transistors (with reverse-biased BE junction) as a source of noise. All of these sources of noise have low level noise voltage and need additional amplifiers.

But voltage regulators can also be used as source of noise signals. Output noise voltage of a low dropout regulator, for example, the LM2931-5.0, is 500μV (typ., 10Hz - 100kHz, COUT = 100μF). It is a few times higher than the noise level of commonly used noise diodes. A circuit which shows an LM2931-5.0 as a noise generator has been published. It needs an additional amplifier, because the noise signal is about 0.5mV.

This Design Idea shows another way of using a voltage regulator which I accidentally discovered. It is a proven circuit, but this startling effect does not show in computer simulations, for example, using Multisim. The effect is inherent to all voltage regulator ICs, and such reference ICs as the ADR425ARZ. I checked this effect with: LM2931-5.0, L78L15ABU, SPX1117M3-L, and others. The highest noise is generated by the LM2931-5.0; the lowest, by the ADR425ARZ.

The simple circuit (Figure 1) generates hundreds of millivolts of noise. The regulator supplies power to filter IC2 and works as a noise source at the same time.


Figure 1  A simple noise generator


As opposed to the LM2931 circuit linked above, the source of noise here is the input of the voltage regulator IC1. R2 is used to convert the noise current to a voltage, resulting in about 800mV P-P for R2=1.5kΩ. A 4th-order active RC lowpass filter in IC2 selects the desired noise bandwidth (about 20Hz-5kHz), and presents a small load to IC1.

R2 is calculated as R2 < (VIN – (Vs+VDV) – VPN) / (Iout+Iq); where VIN = min. input voltage (+12V in this case); Vs = max. output voltage of IC1 (5.19V); VDV = max. dropout voltage of IC1 (0.2V); Iout = max. output current of IC1 (power supply current in LP mode of IC2 is 2.5mA max.); Iq = quiescent current of IC1 (1mA max.); and VPN = estimated max. amplitude of noise signal (0.6V). So, R2 < (12 – (5.19+0.2) – 0.6) / (2.5 + 1) = 1.7kΩ. Thus a resistor 1.5kΩ was used.

The Q1 circuit enables startup of IC1. The LM2931-5.0 needs an initial current of 26mA for 40ms during startup. The Q1 circuit may not be necessary when using other types of voltage regulators (L78Lxx, SPX1117, etc). This circuit can be used with another filter, or without a filter. In this case it will be necessary to provide at least 1.5mA load on IC1. Figure 2 shows the output noise:


Figure 2  Noise of LM2931-5.0 (Vp-p=0.6V)


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