Available ADCs

A fast ADC is characterised by a high sampling rate, i.e. it converts voltages very often per second. A typical sampling rate is in the range of 1 MSPS (1 mega sample per second), i.e. 1,000,000 conversions per second. The unit of the sampling frequency or sampling rate is often given as Sa/s. So 1 MSPS = 1 MSa/s.

The higher the number of bits of an ADC, the more finely voltage values can be distinguished. This is called resolution. An 8-bit ADC with a 3 V reference voltage divides the 3 V into 256 numbers. So voltage changes of

\[ U_{\mathrm{Ein,ADC}} = \frac{U_{\mathrm{Ref}}}{2^n - 1} = \frac{3\,\mathrm{V}}{255} = 11.8\,\mathrm{mV} \]

lead to a change of the number at the output by 1. If, on the other hand, an ADC with the same reference voltage has a resolution of 16 bits, a change of the ADC input voltage of

\[ U_{\mathrm{Ein,ADC}} = \frac{U_{\mathrm{Ref}}}{2^n - 1} = \frac{3\,\mathrm{V}}{65535} = 45.8\,\text{µV} \]

leads to a change of the number by 1.

The quality with which the course of the input voltage at the ADC is available digitally increases with both parameters: with the resolution – i.e. the number of bits – and with the sampling rate – i.e. the number of conversions per second. The 3-bit converter is only used for explanation in this tutorial. In practice, there are mainly 8-, 10-, 12-, 16- and 24-bit converters.

In general, an ADC with a high resolution (i.e. a high number of bits) tends to need a long time for a conversion (i.e. has a low sampling rate). With more money, you can buy ADCs in which both values are higher than in cheap ADCs. Microcontrollers often have a resolution of 10 or 12 bits with sampling rates of about 1 MSPS.

You select an ADC on the basis of its data sheet. You can search for data sheets of individual ADCs online. You can also search manufacturers' websites or distributors' websites. There you can filter and compare and thus find a suitable ADC more quickly. I like to look for components at Digikey, because they have one of the largest selections of different types. I have added the link to the overview of ADCs on Digikey here. At Digikey you will find thousands of different types of ADCs from various manufacturers.