1.4.1 Ground Offsets and Single-Ended Signals

Ground offsets are especially dangerous when measuring single-ended signals. The ground return impedance can generate voltage offsets, which adds error to the measurement. The current (I_FLOW) flowing through the impedance of the ground return (Z_GROUND) generates voltage offsets (V_OS).

V_{O S} = I_{F L O W} \times Z_{G R O U N D}

From this formula, there are two ways to reduce the size of a voltage offset - one is to reduce the amount of current flowing through the ground return, and the other is to lower the impedance of the return.

Lowering the current flow through the ground return is straightforward. High current paths, such as those from a motor or a heating element, must be given their dedicated ground return isolated from the microcontroller’s ground return. In the case of a ground plane, the current will be dispersed throughout the copper plane.

Note: The microcontroller must use a dedicated via rather than a via shared with other devices when it uses a via to connect to ground or power.

The other way to lower the offset is to reduce the impedance of the ground return. Impedance has two components associated: A real resistance (R) and an imaginary reactance (X).

Z = R + j X

The actual resistance is derived from the resistivity of the material connecting point A to point B. Attaching the microcontroller directly to a ground plane is the best way to minimize the amount of resistivity. However, in many cases, it is necessary (from a layout perspective) to instead use a via to connect the device to a ground plane.

Note: Take caution whenever a ground plane is interrupted. This will increase the parasitic inductance and resistance (see Tips for Measuring Single-Ended Signals for more information on the topic). For best performance, ground returns and planes must be uninterrupted and as short as possible.

The reactance of the ground return is created primarily from the parasitic inductance (L). This causes the reactance to increase as the frequency (f) rises.

X = 2 π f L

One of the challenges with digital signals is that they are not composed of a single frequency like sine waves. Instead, square waves are composed of odd-numbered sinusoidal harmonics of their switching frequency (f). For example: 3f, 5f, 7f, etc., as shown in Figure 1-6.

One of the ways to suppress the high-frequency currents by digital logic is by placing decoupling capacitors near the microcontroller. These capacitors act as short-circuits to the high-frequency elements, which reduces the reactance and the impedance seen by these elements.

Figure 1-6. Spectrum of a Generated 1 kHz Square Wave; 0 to 100 mV, 50Ω Termination at Receiver (amplitudes are in dBV)