Looking at using the ADC
ADC
Here the glue circuitry around the ADC is shown. Layout is extremely important if we are to achieve a low residual noise. High quality components and attention to leakage paths for low crosstalk. Since crossdtalk is impossible inside the DSP, this specification depends entirely upon the input circuitry.
The analogue to digital converter (ADC) details
The ADC choosen here is the Analog Devices AD1871. Further details can be obtained form the manufacturers web site. In brief this is an audio stereo sigma-delta convertor using over-sampling to sample each audio channel at 48KHz. This gives a maximum sampled frequency of 24KHz, Thus it is possible to sample any 19KHz signal present. To avoid this a finite impulse response (FIR) low pass filter (LPF) with a very sharp cut-off at 15KHz is used. The ADC incorporates a configurable high pass filter as well. This is used to remove any spurious DC or VLF level present in the input lines.
The two analogue inputs are terminated with 600Ω balanced attenuators. The attenuation required depends upon the input level to be adjusted to the ADC. The maximum ADC input is ± 2.5v. The diode bridge ensure the input is clamped to this peak level. Now in any practical signal there are transient peaks so a 6db margin is allowed (ie: double the nominal peak voltage), thus the input has to be attenuated to ± 1.25v. If we take what is common in the broadcast world a 12dm in 600Ω level we entering with ± 4.36v, so attenuate by 10.9db. The ADC incorporates a variable gain amplifier that can be used to compensate for other input levels with a single attenuator. The variable resistor in one on the audio input arms is to adjust the attenuation to be exactly equal by nulling the (L-R) signal with equal input levels on each audio channel
After conversion to digital a 32bit interleaved bit stream is sent to the DSP with 24 active bits representing the audio sample. The LRCLK indicates the left right channels with the flank being used by the DSP to achieve a known channel synchronisation and trigger the processing of the sample pair.
A Practical Note: The input circuitry has to be laid out very carefully to maximise the signal to noise ratio and minimise cross channel crosstalk, a multilayer board, separation of analogue and digital ground planes and supplies, split audio ground between the input channels. With a good design a -96dn S/N and > 90db channel seperation should be possible. This measurement has to be made by observing the samples as received by the DSP.