Mixed-Signal Design

When you design an embedded system with mixed-signal or high-precision ADCs, you must consider the path of the current flow inside of the PCB. Trace currents of just a few milliamperes can be high enough to cause significant problems for analog signals. To better understand this problem, we should know the minimum voltage level that an ADC can resolve. In other words, we need to calculate the ADC resolution. An 8-bit ADC results in 256 possible digital output values. For a range of 2V signals, the minimum value that can be detected by an 8-bit ADC (or ADC voltage resolution) is 2 V/(256-1) = 0.008V or about 8 mV. From Table 1, we can observe that the ADC voltage resolution will become smaller and smaller when we increase the bits-resolution of ADC. When the resolution is 20-bits, an ADC resolves down to 2 μV.

Table 1: ADC Resolution

Range ± 1.024 Volts
ADC Bits Resolution Units
8 8 mV
10 2 mV
12 500 μV
16 31.3 μV
20 2 μV

Offsets and noise sources below 1 mV, which were insignificant with a low-resolution ADC, are now significant when using a 12- to 20-bit ADC. These errors can easily be overlooked by designers who are not used to sensitive analog circuits.