We commonly use analog to digital converters in electronic devices. For instance, we connect the output of a sensor to an ADC input and use the digital readings for our purpose. Digital signals offer good noise rejection, there is firm switching between levels, and the built-in margin available is also good. However, the analog side can be susceptible to noise.
If the analog input is noisy, it affects the digital output. Most noise levels on the analog side come from a single source—a lack of attention to the ground. To have better results with ADCs, understanding basic principles about grounding is important.
Grounding is simple for low- and mid-speed digital design. When testing the design on a breadboard, power and ground lines are well-defined. These are the two rails running along the two longer edges of the breadboard. The designer designates one of the lines as power, while the other is the ground. They connect the power and ground points in the circuit to the respective rails using short wires.
The importance of grounding increases as digital circuits start to operate faster, and the resolution of the analog side increases. In reality, the ground is not simply a zero-voltage level, it is also a return path for the current flowing in the circuit.
In ideal conditions, whatever may be the circuit behavior, it would not affect the ground. However, in the real world, the ground is rather imperfect. The return path from a narrow trace may have a tiny bit of resistance, such as that from a bad solder joint, or from a few ground pins on a chip. It is possible to see ground bounces in the form of voltage spikes. Add to this resistance some stray inductance, such as from leads of a chip package. Now, power supply noise increases as the operating frequencies go up.
When the ADC resolution is high, the step width in the digital output is in the millivolt range. This makes spikes and noise on the analog input a major problem. The input noise causes bits of error that add to the error sources within the ADC. Designers can take reference from good off-the-shelf board designs for improving the ground quality.
Boards with SoCs or microcontrollers often have a ground plane. Here, the ground is a thick copper layer that may occupy more than one layer on a multi-layered board. IC pins that need to reach the ground can do so with very short paths. Connecting resistance reduces drastically. Capacitors bypassing the power lines reduce stray inductance to a large extent. This helps to smoothen the noise from power supply lines.
Nowadays, there are smart sensors in the market. They contain built-in microcontrollers and ADCs. The input analog signals have a very small distance to travel. They have much less tendency to pick up noise. In addition, some sensors also present data in serial modes, such as on SPI or i2C interfaces, where the output is already in digital form.
Designers must pay special attention to boards with islands of unconnected ground, as these can cause the maximum level of noise at the input.