ad1890jpz Analog Devices, Inc., ad1890jpz Datasheet - Page 8

• Manufacturer Part Number: ad1890jpz
• Description: Sampleport Stereo Asynchronous Sample Rate Converters
• Manufacturer: Analog Devices, Inc.

AD1890/AD1891
Polyphase Filter Bank Model
Although less intuitively understandable than the interpolation/
decimation model, the polyphase filter bank model is useful to
explore because it more accurately portrays the operation of the
actual AD1890/AD1891 SamplePort hardware. In the polyphase
filter bank model, the stored FIR filter coefficients are thought
of as the impulse response of a highly oversampled 0 to 20 kHz
low-pass prototype filter, as shown in Figure 2. If this low-pass
filter is oversampled by a factor of N, then it can be conceptu-
ally decomposed into N different “subfilters,” each filter consist-
ing of a different subset of the original set of impulse response
samples. If the temporal position of each of the subfilters is
maintained, then they can be summed to recreate the original
oversampled impulse response. Since the original impulse
response is highly oversampled, the more sparsely sampled
subfilters still individually meet the Nyquist criterion (i.e., they
AMP
Figure 2. Four Polyphase Subfilters in the Time and Frequency Domains
DECOMPOSED INTO
FOUR SUBFILTERS
OVERSAMPLED
LOW PASS FILTER
IMPULSE RESPONSE
TIME
–8–
AMP
are adequately sampled). The baseband magnitude and phase
responses of the subfilters are identical. The out-of-band (i.e.,
alias) regions of the subfilters however have phase responses
which are shifted relative to one another, in a manner that
causes them to cancel when they are summed.
The subfilter coefficients are then aligned to the left, as shown
in Figure 3, so that the first coefficient of each subfilter is
aligned to the first point on a coarse time scale. (This concep-
tual step accounts for how the hardware implementation is able
to operate at the slower rate corresponding to the coarse time
scale.) Each subfilter has been shifted in time by a different
amount, and though they still share identical magnitude
responses, they now have in-band phase responses which have
fractionally different slopes (i.e., group delays).
1/4Fs
1/4Fs
1/4Fs
1/4Fs
1/4Fs
180
PHASE
1/2Fs
1/2Fs
1/2Fs
1/2Fs
1/2Fs
270
90
0 Deg
3/4Fs
3/4Fs
3/4Fs
3/4Fs
3/4Fs
Fs
Fs
Fs
Fs
Fs
FREQ
REV. 0