Silicon Chip – May 2019

36 Silicon chip Australia’s electronics magazine siliconchip.com.au

on pin 13 of IC6; instead it is merely pulled up to VD (3.3V)
via a 10kresistor and connected to pin 2 of CON3. So the
micro must forcibly pull this pin low to reset IC6.
The digital audio data is fed straight to pins 3-6 of IC6.
While this chip does have an SPI control interface on pins
9-12, it can also be operated without it. This ‘hardware
mode’ is selected by keeping pin 9 (control data input) at a
DC level for a certain period after reset.
In this case, pins 9-12 become control inputs. That is
how it is being used here. Pin 12 (M0) is pulled high via
a 10kresistor to the VLC (logic supply) pin while the
other three pins (M1-M3) are connected to ground via
10kresistors. This selects single-speed (32-50kHz sampling
rate) I^2 S mode without digital de-emphasis.
Like the ADC, DAC chip IC6 needs external filter capaci-
tors for two internal reference rails, and these are connect-
ed between pin 15 (FILT+) and ground, and pin 17 (VREF)
and ground.
Analog audio appears at pins 19, 20, 23 & 24. As with the
ADC, these are differential signals. They are AC-coupled us-
ing 100μF capacitors with 100kbiasing resistors to remove
the DC component of the output signals. They are then fed
to third-order (-18dB/octave) active low-pass filters built
around low-distortion LM4562 dual op amps IC7 and IC8.
These filters are different from the recommended filter in
the CS4398 data sheet, but they have the same purpose: to
remove the high-frequency delta-sigma switching artefacts
from the analog audio signals.
These filters have a -3dB point of 30kHz and are down to
-90dB by 1MHz. But the response is down by only around
0.3dB at 20kHz, with a very flat passband, so has minimal
effect on audio frequency signals.
The differential output from the two pairs of identical fil-
ters is fed into a differential amplifier which provides fur-
ther filtering, based around either IC9a or IC9b. This also
converts them to single-ended signals.
These stages provide some gain, to boost the ~1V RMS
from the DAC up to around 2.3V RMS, a similar level to
that produced from many other audio sources like CD/DVD/
Blu-ray players
The signals are then AC-coupled by 22μF capacitors and
DC-biased to ground using 10kresistors, to remove any
remaining DC bias on the signals. They are then fed to the
inputs of IC10, a PGA2320 volume control chip.
There are two things to note about this chip. One is that
we’re feeding the left channel signal to its right channel in-
put and the right channel signal to its left channel input.
But that doesn’t matter since its channels are independent.

The other is that the CS4398 already has a built-in digi-

tal volume control. IC10 is included on the board because

it adds little noise to the signal and since the signal swing

is higher at the outputs, we thought that this would intro-

duce less distortion. And that is true, but the effect is quite

small, so we didn’t even bother wiring up the control sig-

nals from IC10 to the microcontroller.

So you can leave it off the board and instead, solder

0 resistors from its pin 9 pad to pin 11, and another from

pin 16 to pin 14, so that the signals from IC9 go straight to

the output RCA connectors, CON4.

While it may seem odd that there’s a footprint for IC10

when it isn’t connected to the microcontroller, it could be

useful if the board was used in a different project, and there

was space on the board, so we’ve left the option open.

Power supplies

As with the ADC board, the op amps run off the ±9V sup-

plies fed in from the power supply board via CON3. How-

ever, rather than passing through ferrite beads, on this board

each op amp has a 10/100μF RC low-pass filter for each

supply rail, as well as 100nF bypass capacitors for each

op amp supply pin.

Another difference from the ADC board is while that

board derived a local 5V supply from +9V using an on-

board regulator, on this board, DAC IC6 and (if fitted) vol-

ume control IC10 run from a 5V supply that’s fed from the

power supply board, via pin 22 of CON3.

The two chips have separate ferrite beads on this supply

line for isolation, plus small and large bypass capacitors.

DAC IC6 also requires three 3.3V supply rails – one for

I/O (VLC, pin 14), one for its digital circuitry (VD, pin 7)

and one for its internal PLL (VLS, pin 27).

These are all powered from the same 3.3V supply rail

via pin 20 of CON3, but again they have separate ferrite

beads for EMI suppression and isolation, plus individual

100nF bypass capacitors.

There are also 100nF and 10μF capacitors on the incom-

ing 3.3V supply rail.

Volume control

As mentioned earlier, volume control chip IC10 is not

required, but if it is fitted, it is powered from the ±9V rails

(at the VA+ and VA- pins) and also from the 5V rail via

ferrite bead FB11. The ZCEN input (pin 1) is pulled up to

+5V with a 10kresistor, while Mute (pin 8) is similarly

pulled up by a 10kresistor.

Pin 1 is the Zero Crossing Enable control and when pulled

The completed project

(June and July issues)

will include a 128

x 64 graphical LCD

which lets you set up

the unit and see how

it is configured. It is

controlled using a

rotary encoder and

two pushbuttons to

drive the menu-based

interface.