How Do I Eliminate “Flicker Noise” in My Amplifier Design?
What is “Flicker Noise”?
While there are many different sources of noise within an operational amplifier (op amp), perhaps the most mysterious and frustrating noise source is what is known as flicker noise. This is a low-frequency phenomenon caused by irregularities in the conduction path and noise due to the bias currents within the transistors. Flicker noise increases inversely with frequency, at a rate of 3 dB per octave, hence it is often referred to as 1/f noise (with “f” referring to frequency). This 1/f noise is still present at higher frequencies, but other noise sources within the op amp begin to dominate, negating the effects of 1/f noise. For most op amps, these other noise sources form a consistent white noise (meaning it is constant across frequency) floor across most of the frequency range, but low frequency is still dominated by the 1/f noise. Figure 1 highlights a typical noise shape for a standard operational amplifier. At higher frequencies, the noise floor is constant with frequency, however, at lower frequency the 1/f noise begins to dominate, rising above the white noise.
A common metric is to locate the 1/f corner, which is the frequency at which the magnitude of the 1/f noise intersects the magnitude of the white noise. In the above example this corner occurs around 120 Hz. The frequency of this 1/f corner will vary based on the design and process technology of the amplifier, but 1/f noise will always be present. This low-frequency noise can be very problematic if the input signal is low frequency, such as the outputs from strain gauges, pressure sensors, thermocouples, or any slow-moving sensor signal.
How Can I Eliminate “Flicker Noise” in My Amplifier Design?
So how does one deal with this dominating, low frequency noise? Attempting to filter out this noise without affecting the signal of interest is virtually impossible given the small bandwidth. However, all hope is not lost. Although a system designer cannot control the internal 1/f noise of an amplifier, the designer can minimize this noise source by selecting the proper amplifier for the application. If 1/f noise is a big concern, then selecting a zero-drift amplifier is the best solution.
The industry standard term “zero-drift” refers to any amplifier that uses a continuously self-correcting architecture, regardless if it is auto-zero topology, a chopper-stabilized topology, or some hybrid of the two. The goal of zero-drift amplifiers, regardless of the specific architecture employed, is to minimize offset and offset drift. In the process, other dc characteristics such as common-mode and power supply rejection are also greatly improved. Another key benefit of these self-correcting architectures is that the 1/f noise is removed as part of the offset correction process. This noise source appears at the input and is relatively slow moving, hence it appears to be a part of the amplifiers offset and gets compensated accordingly. This results in an amplifier with a flat noise floor that extends all the way to dc.
Figure 2 illustrates a typical noise shape for a zero-drift op amp that utilizes a chopper stabilized architecture. As noted above, the 1/f noise is eliminated, resulting in a flat noise floor all the way to dc.
One downside to this chopping architecture is the noise energy is modulated to show up around the chopping clock frequency and its odd harmonics. Almost all chopper-stabilized amplifiers include internal low-pass or notch filters that will significantly attenuate this noise. Another noise-related characteristic of chopper-stabilized amplifiers is that they exhibit some noise peaking; in this example it is around 19 kHz. This is due to the multi-path topology and the need to compensate each section of the path while also meeting overall amplifier design goals, such as unity-gain stability and proper settling behavior. For low frequency applications, this higher frequency noise can more easily be reduced, either with the use of lowpass filtering or configuring the amplifier with a higher closed-loop gain, such that this part of the noise spectrum will be significantly attenuated due to the amplifier’s gain roll-off.
Flicker or 1/f noise is a physical phenomenon that affects all electronics, including operational amplifiers. However, this noise source does not have to be a limitation in low frequency data acquisition systems. In addition to providing superior dc performance such as low initial offset and low offset drift, zero-drift amplifiers also have the added advantage of eliminating 1/f noise, which is critical for low frequency applications. Please click here to learn more about Microchip’s line of operational amplifiers, including zero-drift amplifiers.