Datasheet

REV. B
AD1895
–18–
The digital servo loop is essentially a ramp filter that provides
the initial pointer to the address in RAM and ROM for the start
of the FIR convolution. The RAM pointer is the integer output
of the ramp filter, while the ROM is the fractional part. The
digital servo loop must be able to provide excellent rejection of
jitter on the f
S_IN
and f
S_OUT
clocks as well as measure the arrival
of the f
S_OUT
clock within 4.97 ps. The digital servo loop will
also divide the fractional part of the ramp output by the ratio of
f
S_IN
/f
S_OUT
for the case when f
S_IN
> f
S_OUT
, to dynamically alter
the ROM coefficients.
The digital servo loop is implemented with a multirate filter. To
settle the digital servo loop filter quicker upon startup or a change
in the sample rate, a Fast Mode was added to the filter. When
the digital servo loop starts up or the sample rate is changed, the
digital servo loop kicks into Fast Mode to adjust and settle on the
new sample rate. Upon sensing the digital servo loop settling down
to some reasonable value, the digital servo loop will kick into
Normal or Slow Mode. During Fast Mode, the MUTE_OUT
signal of the sample rate converter is asserted to let the user
know that they should mute the sample rate converter to avoid
any clicks or pops. The frequency response of the digital servo
loop for Fast Mode and Slow Mode are shown in Figure 8.
The FIR filter is a 64-tap filter in the case of f
S_OUT
≥ f
S_IN
and is
(f
S_IN
/f
S_OUT
) × 64 taps for the case when f
S_IN
> f
S_OUT
. The FIR
filter performs its convolution by loading in the starting address
of the RAM address pointer and the ROM address pointer
from the digital servo loop at the start of the f
S_OUT
period.
The FIR filter then steps through the RAM by decrementing its
address by 1 for each tap, and the ROM pointer increments its
address by the (f
S_OUT
/f
S_IN
) × 2
20
ratio for f
S_IN
> f
S_OUT
or 2
20
for f
S_OUT
≥ f
S_IN
. Once the ROM address rolls over, the con-
volution is completed. The convolution is performed for both
the left and right channels, and the multiply accumulate circuit
used for the convolution is shared between the channels.
The f
S_IN
/f
S_OUT
sample rate ratio circuit is used to dynamically
alter the coefficients in the ROM for the case when f
S_IN
> f
S_OUT
.
The ratio is calculated by comparing the output of an f
S_OUT
counter to the output of an f
S_IN
counter. If f
S_OUT
> f
S_IN
, the
ratio is held at 1. If f
S_IN
> f
S_OUT
, the sample rate ratio is updated
if it is different by more than two f
S_OUT
periods from the previous
f
S_OUT
to f
S_IN
comparison. This is done to provide some
hysteresis to prevent the filter length from oscillating and causing
distortion.
10
0
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–220
0.01 0.1 1 10 100 1e3 1e4 1e5
FREQUENCY – Hz
SLOW MODE
FA ST MODE
Figure 8. Frequency Response of the Digital Servo Loop. f
S_IN
is the x-axis, f
S_OUT
= 192 kHz, master clock
frequency is 30 MHz.