Your Analog Front End is a very important tool in getting quality conversion results from your ADC
There are four basic problems with connecting an ADC input directly to the "outside world":
- ADC inputs are relatively fragile (only capable of handling a few volts DC without damage), and ADC chips can be expensive/hard-to-replace (several dollars and in fine pitch SMT or BGA)
- ADC inputs have a relatively low (tens of kOhms) and variable (due to the internal sample and hold frontend stage) input impedance, which isn't compatible with the expectations of an outside world input (which needs to be at a high, fixed impedance in order to avoid losing signal voltage-divider-style)
- ADCs need an anti-aliasing filter to keep high frequencies from being undersampled and thus aliased into the desired passband, as aliases cannot be rejected in the digital domain
- Normal real-world signal levels often don't make good use of the ADC input's dynamic range -- sensor outputs and even line-level signals may only use a fraction of the input voltage range, wasting ADC bits and reference accuracy you paid dearly for.
An analog front end stage (AFE) is needed to solve all of these problems. At its simplest, an AFE can be a simple op amp integrator or low-pass filter (LPF) stage (a single low pass pole for anti-aliasing) with some fixed passband gain or attenuation to match the input signal level to the ADC's. More complex AFEs use multiple op amps to better separate gain and filter functions, reject other unwanted signals (such as DC offsets or infrasound in some audio applications), add extra LPF poles (to allow operation of the ADC closer to its Nyquist bandwith of half the sample rate), or interface to more complex inputs (such as balanced inputs, sensor bridges, etal -- think of a mic preamp or line receiver stage, for the audio world). All of this adds up to better signal quality -- you don't have your local AM talk radio station bleeding into your favorite song, loud popping noises from DC offsets interruping the program, or your volume up to 11 just to hear anything at all.
A simple (perhaps even pre-Muntzed, if you will) AFE for your PCM1804 is depicted below -- note that the OPAx134 is designed for dual supply operation, so an op-amp with better common-mode range and output swing than the OPAx134 should be favored for this job, given your supply voltage constraints. R1 and R2 are 1% resistors, while C1 is a film or C0G type and a tantalum-polymer is the best choice for DC block C3. The gain is about 6 with a single pole rolloff at 20kHz -- this brings the 0.8V p-p consumer line level up to 4.8V p-p in order to fill out the PCM1804's dynamic range fairly neatly. Vcom is the common mode voltage (2.5V) supplied by the PCM1804.
simulate this circuit – Schematic created using CircuitLab
Also, while all of this input conditioning is happening, the AFE's op amps are easier to protect from outside mayhem (such as having 48V of phantom power discharged into them all at once) than a fragile ADC input and can be made easily replaceable (very good op amps like the OPA2134 suggested by the PCM1804's datasheet are still available in standard SOIC-8/SOIC-14 and even DIP-8/DIP-14, the latter sometimes seen in sockets in industrial work where a quick op amp swap can get an analog card back up and going).
