High Circuit Impedance on Circuit Attack Counter PCB Reverse Engineering

Since low power circuits tend to use high value resistors to conserve power, this tends to make the High Circuit Impedance on Circuit Attack Counter PCB Reverse Engineering 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:

High Circuit Impedances Increase Susceptibility to Noise Pickup

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.

As discussed in the chapter on amplifiers, current feedback amplifiers do not like to have capacitances on their inputs. To that end, ground planes should be cut back from the input pins as shown in below Figure, which is an evaluation board for the AD8001 high speed current feedback amplifier. The effect of even small capacitance on the input of a current feedback amplifier is shown in below Figure. Note the ringing on the output.