AD1890

The AD1890 and AD1891 SamplePorts™ are fully digital, stereo Asynchronous Sample Rate Converters (ASRCs) that solve sample rate interfacing and compatibility problems in digital audio equipment.Conceptually, these converters interpolate the input data up to a very high internal sample rate with a time resolution of 300 ps, then decimate down to the desired output sample rate. The AD1890 is intended for 18- and 20-bit professional applications,and the AD1891 is intended for 16-bit lower cost applications where large dynamic sample-rate changes are not encountered.These devices are asynchronous because the frequency and phase relationships between the input and output sample clocks (both are inputs to the AD1890/AD1891 ASRCs) are arbitrary and need not be related by a simple integer ratio. There is no need to explicitly select or program the input and output sample clock frequencies, as the AD1890/AD1891 automatically sense the relationship betweenthe two clocks. The input and output sample clock frequencies can nominally range from 8 kHz to 56 kHz, and the ratio between them can vary from 1:2 to 2:1.

The AD1890/AD1891 use multirate digital signal processing techniques to construct an output sample stream from the input sample stream. The input word width is 4 to 20 bits for the AD1890 or 4 to 16 bits for the AD1891. Shorter input words are automatically zero-filled in the LSBs. The output word width for both devices is 24 bits. The user can receive as many of the output bits as desired. Internal arithmetic is performed with 22-bit coefficients and 27-bit accumulation. The digital samples are processed with unity gain.

The input and output control signals allow for considerable flexibility for interfacing to a variety of DSP chips, AES/EBU receivers and transmitters and for I²S compatible devices. Input and output data can be independently justified to the left/right clock edge, or delayed by one bit clock from the left/right clock edge. Input and output data can also be independently justified to the word clock rising edge or delayed by one bit clock from the word clock rising edge. The bit clocks can also be independently configured for rising edge active or falling edge active operation.

The AD1890/AD1891 SamplePort™ ASRCs have on-chip digital coefficients that correspond to a highly oversampled 0 kHz to 20 kHz low-pass filter with a flat passband, a very narrow transition band, and a high degree of stopband attenuation. A subset of these filter coefficients are dynamically chosen on the basis of the filtered instantaneous ratio between the input sample clock(LR_I) and the output sample clock (LR_O), and these coefficients are used in an FIR convolver to perform the sample rate conversion. Refer to the "Theory of Operation" section of this data sheet for a more thorough functional description. The lowpass filter has been designed so that full 20 kHz bandwidth is maintained when the input and output sample clock frequencies are as low as 44.1 kHz. If the output sample rate drops below the input sample rate, the bandwidth of the input signal is.