source: Planet Analog article
Many technical documents have been written on noise analysis. Early on in my career I became obsessed with operational amplifier (op amp) noise. Subsequently, I published a series of application notes and a book on the subject. So, why do we need another article? Many engineers don’t want to do the full detailed noise analysis and just need some quick rules of thumb to get their designs “in the ball park.” For those wanting a quick and simple method to minimize noise, this article is for you.
There are three sources of noise in op amp circuits: op amp voltage noise, op amp current noise, and resistor noise (Figure 1). The op amp current noise is translated to noise voltage when it flows through the feedback network and input source impedance.
When doing noise calculations, a common approach is to refer all the noise sources to the amplifiers input. You can calculate input-referred noise for feedback resistors by using the equivalent parallel combination of two resistors (Equation 1). You can calculate noise spectral density using Johnson’s thermal noise equation (Equation 2 and Figure 2 graph), as well as noise from the input resistor (RIN).
My last rule relates to the 1/f noise region, which often garners a lot of undue concern. In many cases the contribution of 1/f noise is small relative to the broadband noise and can be ignored. If the system’s bandwidth is ten times greater than the 1/f noise corner, ignore the contribution of the 1/f region. Generally, 1/f noise is only problematic in low-frequency systems, such as temperature-monitoring.
Rule 4: Compare the 1/f noise corner to the closed-loop op amp bandwidth. If the bandwidth is 10 times the noise corner, you can ignore 1/f (Figure 3). If not, search for amplifiers with lower 1/f noise, or use more advanced calculations to estimate the rms 1/f noise.
These four rules can help you to quickly design a low-noise system. Any calculations required are relatively simple. This method can give you a quick and intuitive way to analyze the circuit, versus getting lost in comprehensive calculations. For a more compressive analysis, TI Precision Labs and the resources provided and can help to broaden your understanding of this subject.
Join us next time when we will discuss the advantages in using current-sense amplifiers in low-side current-measurement applications.