AD1890 datasheet(7/20 Pages) AD

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AD1890/AD1891

REV. 0

–7–

THEORY OF OPERATION

There are at least two logically equivalent methods of explaining

the concept of asynchronous sample rate conversion: the high

speed interpolation/decimation model and the polyphase filter

bank model. Using the AD1890 and AD1891 SamplePorts does

not require understanding either model. This section is included

for those who wish a deeper understanding of their operation.

Interpolation/Decimation Model

In the high speed interpolation/decimation model, illustrated in

Figure 1, the sampled data input signal (Plot A in Figure 1) is

interpolated at some ratio (IRATIO) by inserting IRATIO-1

zero valued samples between each of the original input signal

samples (Plot B in Figure 1). The frequency domain characteris-

tics of the input signal are unaltered by this operation, except

that the zero-padded sequence is considered to be sampled at a

frequency which is the product of original sampling frequency

multiplied by IRATIO.

The zero-padded values are fed into a digital FIR low-pass filter

(Plot C in Figure 1) to smooth or integrate the sequence, and

limit the bandwidth of the filter output to 20 kHz. The interpo-

lated output signal has been quantized to a much finer time

scale than the original sequence. The interpolated sequence is

then passed to a zero-order hold functional block (physically

implemented as a register, Plot D in Figure 1) and then asyn-

chronously resampled at the output sample frequency (Plot E in

Figure 1). This resampling can be thought of as a decimation

operation since only a very few samples out of the great many

interpolated samples are retained. The output values represent

the “nearest” values, in a temporal sense, produced by the inter-

polation operation. There is always some error in the output

sample amplitude due to the fact that the output sampling

switch does not close at a time that exactly corresponds to a

point on the fine time scale of the interpolated sequence. How-

ever, this error can be made arbitrarily small by using a very

large interpolation ratio. The AD1890/AD1891 SamplePort

ASRCs use an equivalent IRATIO of 65,536 to provide 16-bit

accuracy (

≈ –96 dB THD+N) across the 0 to 20 kHz audio

band.

The number of FIR filter taps and associated coefficients is

approximately 4 million. The equivalent FIR filter convolution

frequency (or “upsample” frequency) is 3.2768 GHz, and the

fine time scale has resolution of about 300 ps. Various propri-

etary efficiencies are exploited in the AD1890/AD1891 ASRCs

to reduce the complexity and throughput requirements of the

hardware implied by this interpolation/decimation model.

INPUT

SIGNAL

OUTPUT

SIGNAL

ZERO STUFF

INTERPOLATION

FIR LOW

PASS

FILTER

ZERO ORDER

HOLD

RESAMPLING

DECIMATION

TIME

AMP

Figure 1. Interpolation/Decimation Model—Time Domain View

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