How Analog-to-Digital Converter (ADC) Works

ADC On the PC

The most logical place you will find an ADC on a PC is on the sound card. Whenever you use a mike – for recording your own voice or for talking with friends using VoIP programs like Skype – or the line in input for transforming audio produced by other equipment (like your tape deck) into Wav or even MP3 files, you are in fact using your sound card’s analog-to-digital converter.

Nowadays all PCs have a sound card embedded on the motherboard – even if you won’t use it. This is because the motherboard chipset – more specifically, the south bridge chip – has an audio interface. The ADC (for recording audio) and DAC (for reproducing audio), however, isn’t integrated in the south bridge, but on a separated chip called codec (coder/decoder).

Of course if you use an add-on sound card you will be using the ADC and the DAC provided by the sound card and not by the codec chip. Also, some motherboards instead of using the audio capability of the chipset provide an extra complete audio chip, which has its own ADC and DAC. For a more detailed discussion about on-board audio options, we suggest you to read our tutorials “How Does On-Board Audio Work” and “Understand On-Board Audio Features”.

What we wish to show you is how a codec works. For this, take a look at Figure 16, where we present the block diagram of Analog Devices AD1888A codec. What can you learn by just looking at this diagram? Can you find the ADC?

Figure 16: Analog Devices AD1888A codec block diagram.

In Figure 16 you can easily see all inputs connected to the 16-bit ADC through a mixer. One question you might have is where the 16 data outputs are located – after all, this codec uses a 16-bit ADC. As we mentioned, the codec is connected to the chipset (south bridge chip). The communication of these two chips is done using a serial interface, not parallel – i.e., the codec transmits each bit one by one to the chipset. So there are just two data wires connecting them (one input, SDATA_IN, and one output, SDATA_OUT). The AC’97 interface unit is the responsible for such interface.

Analyzing Figure 16 you will also discover that AD1888A has six individuals 20-bit DACs – one for each channel, so it provides six-channel audio – and it has a digital audio (SPDIF) output, and several other features, like EAPD (External Amplifier Power Down), which can disable an external amplifier if no audio is being produced, and jack sense (JS pins) which can automatically reconfigure the jacks located on the motherboard depending on what kind of device you plugged on them.