High Circuit Impedances are Susceptible to Noise Pickup

High Circuit Impedances are Susceptible to Noise Pickup, Since low power circuits tend to use high value resistors to conserve power, this tends to make the circuit more susceptible to externally induced radiated noise and conducted noise. Even a small amount of parasitic capacitance can create a significant conduction path for noise to penetrate.

For example, as little as 1 pF of parasitic capacitance allows a 5 V logic transition to cause a large disturbance in a 100 kΩ circuit as illustrated in below Figure:

This serves to illustrate that high impedance circuits are full of potential parasitics which can cause a good paper design to perform poorly when actually implemented.

One needs to pay particular attention to the routing of signals. Interestingly, many high frequency layout techniques for eliminating parasitics can also be applied here for low frequency, low power circuits—for different reasons.

While circuit parasitics cause unwanted phase shifts and instabilities in high frequency circuits, the same parasitics pick up unwanted noise in low power precision circuits.