A quiet, well regulated supply is important for optimum performance in a number of circuit applications. Voltage controlled oscillators (VCOs) and precision voltage controlled crystal oscillators (VCXOs) respond to small changes in their supply very quickly. Phase-locked loops (PLLs) require a stable supply, as signal on the supply translates directly to phase noise in the output. RF amplifiers require quiet supplies, as they lack the ability to reject supply variations, and regulator variation will appear as unwanted side bands and lower the signal-to-noise ratio. Low noise amplifiers and analogue-to-digital converters (ADCs) do not have infinite supply rejection and the cleaner the regulator output is, the higher their performance.
Once fully built, one can determine if the supply has low enough noise for the application. Oscillator phase noise is measured and compared against results achieved with a known good supply, ADCs are checked to ensure they are getting the maximum number of bits. These are tricky, time consuming measurements and it would be better to make sure the noise levels are low enough for your needs without expensive trials.
In addition to noise, one must also consider the supply rejection capabilities of the linear regulator. Poor rejection from a linear regulator will bring switching regulator residue or other unwanted signals through, corrupting the hard work done to ensure a clean supply. Extremely low noise from the regulator is worthless if poor supply rejection brings enough signal through to swamp noise levels.
Measuring output voltage noise: being Quiet is Nothing New
The subject of noise has been broached before. Linear Technology Application Note 83, “Performance Verification of Low Noise, Low Dropout Regulators,” published in March 2000, describes in detail a method for measuring output voltage noise of regulators down as low as 4 µVRMS with confidence. The amplifier circuit and filters in the Application Note gave 60 dB of gain across a 10 Hz to 100 kHz bandwidth. This is a good starting point to determine confidence in measurement of noise levels.
New linear regulators such as the LT3042 are now in production with much lower output voltage noise levels. While the family of regulators released around the publication of Application Note 83 operate with approximately 20 µVRMS noise in the 10 Hz to 100 kHz band, the LT3042 is now available with noise levels as low as 0.8 µVRMS across the same frequency band. Reviewing the circuit from Application Note 83 shows an input referred noise floor of 0.5 µVRMS, which provides less than 1% error when measuring noise levels as low as 4 µVRMS. With output noise levels of 0.8 µVRMS, this noise floor is now unacceptable; the regulator itself operates at noise levels only slightly above the measurement circuit. This translates to almost 20% error, making the measurement circuit too significant a factor to be able to measure signals with confidence.
Measuring less than 1 µVRMS noise is not a trivial task. Working backward from a 10 Hz to 100 kHz measurement band, this equates to a noise spectral density of 3.16 nV/√Hz (assuming white noise). This is equivalent to the Johnson noise of a 625Ω resistor! Measuring noise at these levels within 5% requires that instrumentation have an input referred noise of 1 nV/√Hz; measuring within 1% requires input referred noise of 450 pV/√Hz.
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