This is only a preview of the October 2021 issue of Practical Electronics. You can view 0 of the 72 pages in the full issue. Articles in this series:
|
USB
Part 2
By Phil Prosser
Last month, we introduced our new USB
SuperCodec sound card design, which boasts superb recording
and playback performance. It isn’t only useful for recording and playback
though; with some inexpensive software, it can make a very advanced
audio signal analysis system. Now it’s time to describe the details of the
circuitry behind its phenomenal performance.
W
e covered the basic operating principles of
the USB SuperCodec in last month’s introductory
article, but we ran out of space to fit the full circuit
details. As you will see from this article, that’s mainly due
to the number and size of the circuit diagrams.
Since the circuit of the USB SuperCodec is too large to fit
across two pages, we have broken it up into five sections:
the computer interface with galvanic isolation (Fig.12), local
clock generation and asynchronous sampling rate conversion
(Fig.13), the ADC section (Fig.14), the DAC section (Fig.15)
and the power supply (Fig.16).
Galvanic isolation
The galvanic isolation is provided by IC12, a Maxim
MAX22345 (see Fig.12). This is a fast, low-power, fourchannel galvanic isolator chip. We are using the 200Mbps
version as we wanted to be able to transfer clock signals at
more than 12MHz (the bit clock [BCLK]) and 24MHz (the
master clock [MCLK]).
The version that we are using provides three ‘left to right’
and one ‘right to left’ channels. This is ideal for isolating the
I2S output from the MCHStreamer.
When we had the computer ground electrically connected
to the USB sound card ground in a real-world system, we
found it impossible to get rid of residual 50Hz-related noise
and a bunch of ‘spurs’ in the noise floor. While these were
low enough to be inaudible, putting the galvanic isolation
into the system saw these drop significantly. Indeed, even
allowing the USB earth to connect to the case of the USB
SuperCodec increased the 50Hz hum by 10-20dB!
This chip is not that expensive, but the benefit of using it
as part of a measurement system is huge.
USB3.3V
I S data OUT Ch1&2
2
J1
26
51
1
2
1
J2
10
7
2
2
USB BCLK
(VIA CON2)
(VIA CON3)
1
I S data IN Ch1&2
12
1
76
100nF
OPTICAL
INPUT
MINIDSP MCHSTREAMER MODULE
USB
TYPE B
DVDD3.3V
2
OUT1 18
MINIDSP
I2S_DAC
4 IN2
OUT2 17
MINIDSP
B CLK
5 IN3
OUT3 16
MINIDSP
LRCLK
6 OUT4
IN4 15
MINIDSP
I2S_ADC1
9
USB LRCLK
VDDB
14
DEFB
3 IN1
8
OPTICAL
OUTPUT
1
J3
IC12
MAX22345 20
VDDA
DEFA
DVDD3.3V
100nF
2
ENA
ENB
NC
NC
GNDA
GNDA
GNDB
GNDB
10
11
13
12
19
RESET_L
10k
USB GND
3
USB3.3V
BC549
VCC
DS1233
B
E
USB3.3V
1k
OPTO1 4N28
6
1
2
C
2
5
4
C
B
Q1
BC549
RESET
IC13
DS1233
GND
1
E
3 2 1
SC
SuperCodec
(USB Sound Card) MiniDSP MCH Streamer and Galvanic Isolation Circuitry
2020
Fig.12: this section of the full circuit connects the MCHStreamer to a MAX22345 high-speed isolator and a bogSUPERCODEC
(USBTheSOUND
CARD)
MiniDSP
MCH
Streamer
& Galvanic
Isolation
Circuitry
standard 4N28 optocoupler.
latter releases
the ADC and
DAC reset
lines
350ms after
plugging in
USB.
24
Practical Electronics | October | 2021
We must make it clear that
while this device provides a
high degree of isolation, we have
not designed the circuit board to
handle significant voltage differences between the two domains.
Do not, in any circumstances, rely
on this design to provide safety
isolation between the PC
and the sound interface!
It is purely intended to improve
the performance, and allow a few volts of
difference between your computer and audio
grounds, as can sometimes occur.
The data rates from the USB interface are quite
high. The MCLK signal is at 24.576MHz for the
192kHz sampling rate, and the BCLK is half this, at
12.288MHz.
Design and layout of a board for reliable operation at 25MHz requires attention to detail, careful
grounding and termination for long traces.
We have used series termination on the 25MHz
clock signal, and managed to keep high-speed traces
tidy and with a minimum of vias. They all run over a
solid ground plane for their entire length. Where we
have had to route across these signals, we made the
aperture in the ground plane as small as possible.
We came close to using a four-layer PCB for this
design, but by constraining the digital signals to
a limited area, and with careful layout, we have
avoided the cost this would incur.
In the final version of the design, we are using a
local clock oscillator for the 24.576/25MHz clock,
so while we can access the master clock from the
MiniDSP MCHStreamer, it is not used, as we can
do better with a local clock source. Hence, Fig.12
does not show any connection to the MCLK pin
of the MCHStreamer module.
In case you’re wondering how the MAX22345
works, isolators like this generally get the signal
across the isolation barrier using either magnetic
or capacitive coupling (high-speed optical isolators
exist but are usually bulkier). Maxim does not explicitly state which, but it appears to be capacitive.
We’re also using an ordinary old 4N28 optocoupler. This tells the audio side whether or not
power being received from the computer.
If there is no power, the ADC and DAC are
held in reset. Once there is 3.3V power from
the USBStreamer, the ADC and DAC are taken
out of reset after 350ms. The DS1233 provides
this delay; the signals from the USB Streamer
should have stabilised after 350ms. From a user’s
perspective, this means that when you plug the
USB SuperCodec in, it looks after its own reset
and ‘just works’.
Local clock generation and ASRC
This section has been the subject of a lot
of work. It would be possible to drive the
ADC and DAC directly from the miniDSP
MCHStreamer, as isolated by the MAX22345.
But what if the user wants to operate the
card at 44.1kHz, 48kHz, 96kHz, 192kHz or
some other rate? How do the ADC and DAC
get set up for this?
The CS4398 and CS5381 chips both have
mode pins that must be set depending on the
sampling rate at which we want to operate.
Practical Electronics | October | 2021
In the prototype, we used jumpers to set the sampling rate
for the ADC and DAC. We quickly decided that users will
want to plug the card in and have it sort this out for itself.
It would be possible to, say, use a microcontroller to sense
the sampling rate and set the chips up accordingly. But there
is a better way – using a device called an asynchronous
sampling rate converter (ASRC).
ASRCs are found in professional recording studios and
also consumer equipment which have digital-audio-todigital-audio interfaces.
Imagine you have two digital audio devices, say an amplifier and a CD player. Each is a standalone device with
its own clocks and generally looks after itself. When
We’ll get onto the construction next month, after we’ve finished with
the rather involved description. To whet your appetites, here’s the
completed PCB mounted on the input/output socket, shown life size.
25
Fig.13: the ASRC circuitry sits in between the
galvanic isolation section and the ADC and
DAC chips. Its job is to pass digital audio data
between two clock domains: that of the USB
MCHStreamer, with a nominal 24.576MHz
master clock, and the ADC and DAC, clocked
by 25MHz crystal oscillator module XO1. The
relative drift of these two clocks is taken care
of by the digital filters in IC6 and IC7.
Supercodec (USB Sound Card) Sample Rate Converter Circuitry
you plug these together, if you want to have the CD player
provide digital data to the amplifier, what happens if (as is
inevitable), the CD player’s clock is just slightly different in
frequency to the clock in the amplifier?
Eventually, the CD player will provide either too much
or too little data to the amplifier. In serious situations (eg,
professional mixing rigs), you can have a master clock distribution system. But most devices don’t have provision for
that. Alternatively, you can use an ASRC.
Instead of locking the clocks of different chips together, the
ASRC flips the problem on its head. It allows our ADC and
DAC to have their own clocks, and does a bunch of maths
to pass the correct digital values to and from the computer
at whatever sampling rate it happens to be running at.
This involves the ASRC monitoring the different sampling
rates, then implementing digital filters to deliver the exact
digital value needed at every sample interval.
The upshot of this is that we can use a local 25MHz clock
source to drive both the ADC and DAC. The clock we have
chosen is good without getting silly. Its typical RMS jitter
is less than 1ps (one million millionth of a second!). You
26
could go for a better unit, but our analysis suggests that the
difference would be essentially unmeasurable. Indeed using
a ‘better’ clock is a tweak that some serious audiophiles do.
We have used a sample rate converter in each of the ADC
and DAC lines, as we need to perform this translation for
both recording and playback.
The devices we’re using are both Cirrus Logic CS8421s.
If you are worried about what these things may do to the
sound, fear not. These are rated for 175dB dynamic range
and –140dB (0.00001%) THD+N!
So the impact of these devices is so low that it is not at all
detectable, let alone audible. (We have donned our asbestos
underwear as we await the flame throwers of the anti-ASRC
audiophile crowd!)
The actual implementation of these chips is not complex,
as shown in Fig.13. The digital audio signals go into pins
7, 8 and 9 at one particular sampling rate and emerge from
pins 12, 13 and 14 at a different rate, to match up with the
clock signal applied to pins 2.
Using an ASRC has a couple of implications on how the
ADC and DAC are set up and driven.
Practical Electronics | October | 2021
First, we must provide a low-noise clock. This is from
XO1, a 25MHz clock oscillator module.
Second, we need the local left/right clock (ie, sampling
rate) at a higher rate than the 192kHz that the MiniDSP
USBStreamer uses, to ensure no degradation of the digital signal. 25MHz divided by 32 (bits each in the L and R
samples) divided by 2 then 2 again is 195.3125kHz. So that
suits us fine.
We need to set the ASRC for the CS4398 DAC as a master
output so that it generates the 195.3125kHz left/right clock
(LRCK) and control signals for this ADC on its output – ie,
the ASRC drives the DAC at this rate at all times.
We need the ASRC for the CS5381 ADC as a master input
so that it generates the 195.3152kHz clock and control signals
for the MCHStreamer on its input.
Pin 6 ( BYPASS) allow the ASRC action to be disabled, but
since we always want it active, we have tied this to GND.
Similarly, we are not using the Time Domain Multiplexing
(multi-channel) feature, so pins 11 are tied low.
The MS_SEL pin of IC6 is pulled down via a 2kΩ resistor,
which sets the device to slave mode on its input side (clocks
are inputs), and master mode on its output side (clocks are
outputs). The 1kΩ resistor from pin 19 (SAIF) to ground
sets the inputs of both devices to 32-bit I2S mode; one of
six different digital audio protocols this chip supports. This
matches the data format from the MCHStreamer.
Similarly, the 4kΩ total resistance from pins 18 (SAOF)
to ground sets the output side to I2S mode with 24-bit data,
to suit our ADC and DAC chips. This is one of 16 possible
formats the chip supports.
Once set up as above, this forms a neat interface between
parts of a system that may have differing clocks. Is there a
downside? They are not cheap devices, but we think they’re
worth it for the flexibility they provide.
Analogue-to-digital conversion
We’re using the CS5381-KZZ chip. Cirrus Logic make two
similar devices, the CS5361 and CS5381. They are pin-compatible, but the CS5381 has better distortion performance.
We have specified the better of the two. You could drop
in the CS5361 instead, and will lose a bit of performance
on the input channels.
The circuitry surrounding this chip, shown in Fig.14, is
close to what is recommended by the Cirrus Logic application note. However, we have gone to extra lengths to ensure
very symmetrical drive of the input, and to make sure that
the sound card has a high-impedance input. Ferrite beads
FB3 and FB4, with the following 100pF capacitors to ground,
form RF filters at the inputs.
Bipolar electrolytic capacitors block DC voltages, with a
–3dB cutoff well below 1Hz. Schottky diodes D5, D10, D15
and D16 protect the op amp inputs against spikes and excess
voltage. In normal operation, these do not affect the signal.
IC2a/IC4a operate as unity-gain buffers. They provide a
low-impedance drive for the following two stages without
SOURCING THE COMPONENTS
Some of the components for this project are rather specialised
and might be difficult to track down.
To assist you in this endeavour, we have produced a spreadsheet which gives catalogue codes for each part needed,
from six different sources:
• Altronics • Jaycar • Digi-Key • Mouser • element14 • RS
You’ll find this spreadsheet at: https://bit.ly/pe-oct21-codec
Practical Electronics | October | 2021
affecting the input. IC2b/IC4b operate as inverters. We have
used 1.2k feedback resistors, as low as practical, to keep
noise down while allowing the operational amplifier to
drive the following stage without any concern of increasing
distortion by overloading the output.
We could have gone a touch lower in resistance, but feel
this is a good compromise on performance and power use.
IC3a/IC5a and IC3b/IC5b drive the differential inputs of
the ADC, and all four stages are configured in a very similar
manner. There are a couple of things going on here.
The non-inverting inputs are held at a 2.5V bias via 10kΩ
resistors from IC1’s VQ (quiescent voltage) pin, pin 22. These
resistors have 10nF local bypass capacitors to ensure the op
amps see a very low source impedance.
The inverting inputs of these op amps are driven by the
in-phase and inverted signals from the previous stage, which
are capacitively-coupled to support the DC offset. You might
be concerned that the input signal could affect the 2.5V, but
these signals are balanced, so their effects on the reference
voltage essentially cancel out.
The 470pF feedback capacitors form low-pass filters in
combination with the 680Ω and 91Ω resistors. This has a
cutoff way above the audio band, at around 500kHz, to ensure
stability and get rid of any RF noise which makes it past the
input filter.
Note that at audio frequencies, these four stages form
unity-gain buffers. The fact that the output is taken from
the junction of the resistors reduces transient loading on
the operational amplifier.
Some low-pass filtering is provided by the combination of
these resistors and the 2.7nF capacitors across the pairs of
differential ADC input pins. These capacitors are mounted
very close to the input pins. Our testing showed that these
capacitors are critical to the performance of the ADC.
Do not use any old capacitor. Do not use an ‘audiophile’
capacitor. Do use a ceramic NP0 or C0G type capacitor, surface
mounting, of known provenance. We built a prototype with a
film capacitor here, and the distortion went up by a factor of
ten. We also tried silver mica caps, and they were no better.
Clearly, it isn’t just the linearity of this capacitor that is
critical; the oversampling ADC draws pulses of current from
these caps at a high frequency, so we need caps with a low
ESR at several megahertz, as well as linearity. Only NP0/C0G
ceramics provide both.
The ADC input pins have BAT85 diodes to each rail for
protection. Reviewing the data sheet, it seems that the ADC
should survive the maximum output current of a NE5532,
but it might not survive the maximum output current of an
LM4562. We suspect that some people might try different op
amps – and since IC1 costs around £27.50 (!) – it’s worthwhile
providing protection.
The VA analogue supply to IC1 is nominally 5V, and we
have a local low-dropout linear regulator (REG5) to provide a
3.3V digital logic supply rail for IC1. We have done this locally
as it draws little current and made the layout so much easier.
Pin 15 of the ADC provides an overflow indication. This
drives the LED on the front of the unit. Should this flash
during operation, you are driving the ADC into clipping, and
need to lower the input level.
Generally, you should be running the input substantially
lower than this. The noise and distortion are optimal at a
decibel or so below clipping, and even if you run this 10dB
lower, the impact on performance will be minimal.
The ADC pins at upper right are tied either to VL or GND
to set it up in ‘hardware mode’ (ie, not being controlled by
a microcontroller), with the correct audio format selected.
The digitised audio signals appear at pin 9 of IC1 and goes
onto ASRC IC7, as shown in Fig.13. That same ASRC chip
and XO1 provide the clock signals at pins 3, 4 and 5 of IC1.
27
Digital-to-analogue conversion
The CS4398 DAC is configured in a
fairly conventional manner – see Fig.15.
Discussing the right channel, IC9’s differential outputs drive two low-pass
filters formed by IC8a and IC8b. The
filter on each pin is set up to present
the same load to the two outputs. The
impedances have been kept low to
minimise noise. This filter is the same
as used in the DSP Crossover (PE, January – March 2020) and limits the output
of supersonic signals.
We have specified C0G ceramic capacitors (or NP0; same thing) where
ceramic types are used. This is very
important as other dielectrics will introduce more distortion.
For the 1.5nF, 10nF and 22nF capacitors we used MKT capacitors. The selfresonance of low-value MKTs is typi-
cally in the 10MHz region, so the filter
behaved well and provided excellent
performance. They are easier to obtain
than NP0/C0G ceramics with those same
values, so you might as well stick with
the MKTs. But if you use very highspeed op amps in place of the NE5532s,
things could change.
IC10b forms a differential-to-singleended signal converter. The 1.2kΩ resistor values are low enough to minimise
noise while not overloading the op amp,
and leave headroom for it to drive a load.
The 470pF capacitors in this stage form
the final stage of the low-pass filter.
The DC output level of the DAC
is 2.5V. This runs through the filters
formed by IC8a and IC8b. Rather than
AC-coupling the signal to the differential to single-ended converter, we have
used the converter to remove the bulk
of the DC offset itself. The AC-coupling
capacitor at its output removes any
residual DC – though in our prototype,
this was a very low level.
The power supply
The power supply, shown in Fig.16,
may look over the top. This design
makes no apology for taking power supplies and grounding to something of an
extreme as we aim to deliver solid ADC
and DAC performance, at the parts-permillion level. In particular, any noise
on the +5VA rail is a very bad thing, and
we want the +5VL and ±9V rails to be
clean of noise and clocking artefacts.
The first version of this unit used a
toroidal transformer mounted on the
opposite side of the case from the sensitive analogue parts. It even included
a copper shorting ring to reduce radi-
SCSupercodec
SUPERCODEC(USB
(USBSound
SOUNDCard)
CARD)
28
Analogue-to-Digital Converter Circuitry
Practical Electronics | October | 2021
ated noise. Even so, we could still see
the 50Hz leaking into the plots down
around the –110 to –130dB levels.
So we changed it to run off a single
+12V DC plugpack. It uses two LM2575
buck regulators (REG1 and REG2) to
generate a +6.5V DC rail and –12V
DC rail. This choice might raise a few
eyebrows as switchmode converters are
not famous for low levels of radiation.
And you may wonder how the same
chip is used to generate both positive
and negative rails.
Let’s start with that negative rail.
In essence, we are turning REG2 on
its head; its positive output connects
to GND (after the LC filter), while its
GND pin is actually ‘floating’ on the
negative rail! It may seem strange, but
if you analyse the circuit carefully, you
will see that this will work.
But there are a few things you need to
be aware of when using a buck regulator
this way. On startup, it tends to draw
a lot of current for a short period. The
Texas Instruments data sheet warns of
this, and they were right to! The peak
startup current is about 2A, so be sure
to use the recommended plugpack, or
check that yours works OK.
There is also an LM2576, which is
a beefier version of the LM2575. This
draws closer to 4.5A on startup. It works,
but watch that startup current.
So how does this work? Here’s a brief
explanation: REG2 ‘tries’ to keep the
feedback voltage at pin 4 about 1.25V
above its ground pin, pin 3. As the –12V
rail is initially at 0V, so is pin 4, so the
output switches on hard. This means
that current can pass from the 12V input,
through inductor L3 and to ground.
www.poscope.com/epe
- USB
- Ethernet
- Web server
- Modbus
- CNC (Mach3/4)
- IO
- PWM
- Encoders
- LCD
- Analog inputs
- Compact PLC
- up to 256
- up to 32
microsteps
microsteps
- 50 V / 6 A
- 30 V / 2.5 A
- USB configuration
- Isolated
PoScope Mega1+
PoScope Mega50
Fig.14: the stereo analogue audio signals applied to RCA sockets
CON6a and CON6b are buffered and pass through a series of RF filters
before being converted to balanced (differential) signals, which are
then fed to the pairs of ADC inputs at pins 16/17 and 20/21 of IC1. The
2.7nF filter capacitors are critical to getting good results, while numerous schottky diodes protect the various ICs from signal overload.
Practical Electronics | October | 2021
- up to 50MS/s
- resolution up to 12bit
- Lowest power consumption
- Smallest and lightest
- 7 in 1: Oscilloscope, FFT, X/Y,
Recorder, Logic Analyzer, Protocol
decoder, Signal generator
29
Tweaking the SuperCodec’s performance
Phil Prosser delivered a prototype to us
with excellent performance. However, on
measuring its performance, we detected
an anomaly. The DAC THD+N figure
increased for test frequencies below
200Hz, rising from 0.00054% at 1kHz
to around 0.00085% at 20Hz.
This was not what we expected, as
performance usually improves as the
test signal frequency drops.
At first, we suspected that the 22µF
bipolar output coupling capacitors could
be the culprits, as rising distortion with
decreasing frequency is a signature of
coupling-capacitor-induced distortion.
However, replacing these with 100µF
high-quality devices (which you may
have noticed in our photographs) failed
to yield any improvement.
This led us to suspect that the lowfrequency signal was modulating a
voltage rail, so we turned our attention to
the capacitors surrounding the CS4398
DAC, IC9.
The most critical capacitors are the
electrolytic filter capacitor on pin 26,
VQ, which stabilises the half-supply
rail (quiescent output voltage, hence
VQ); the 33µF filter capacitor at pin 17
(VREF), which also helps to smooth the
VA (analogue supply voltage) 5V rail that
it’s connected to; and the electrolytic
capacitor at pin 15 (FILT+). The capacitor from pin 26 to ground was originally
3.3µF. After soldering a 47µF capacitor
across it, we re-tested the unit and found
two things.
One, it took a lot longer to reach normal
operating conditions (presumably the
larger capacitor takes longer to charge).
And two, while the THD+N figures did
drop around 25% at lower frequencies
(and a bit across the board), there was
still a rise in distortion below 200Hz.
Adding a 470µF capacitor from pin 17
(VREF) to ground did nothing, indicating
that this rail was sufficiently noise-free.
But moving that capacitor to go from pin
15 (FILT+) to ground, which originally had
a 100µF in parallel with the 100nF, totally
eliminated the rise in distortion at lower
frequencies and also slightly lowered
distortion across the board.
So we decided to compromise with the
VQ filter capacitor at 10µF; higher than
the original 3.3µF for improved overall
performance, but not so high that the unit
takes ages to stabilise when powered on.
And we definitely upgraded the 100µF
capacitor at the FILT+ pin to a high-quality
470µF unit, which just fits, as this was
the ‘cherry on top’ in terms of obtaining
the ultimate performance.
The regulator switches its output in
pulses at about 50kHz. When it switches
off, the inductor’s magnetic field causes
current to continue to flow. This can no
longer come from the LM2575, so the
voltage at pin 2 drops and the current
flows from the negative pin of the output capacitor, through D3. As a result,
the voltage across the output capacitor
increases, meaning its negative end gets
more negative.
This cycle continues, with the capacitor charging further, resulting in
the ground pin falling negative relative
to the output. As the voltage across the
feedback divider is increasing, the voltage
at feedback pin 4 relative to pin 3 also
increases. Eventually, the capacitor is
charged to 12V, and the ground pin is now
12V below the feedback pin. Pin 4 is then
at around –10.75V, ie, 1.25V above pin 3.
The regulator then operates normally,
varying its mark to space ratio to keep
this voltage as required. The regulator
is essentially driving a short-circuit at
startup, hence the fairly impressive but
brief initial current demand.
SCSupercodec
(USB
Sound
Card)
Digital-to-Analogue Converter Circuitry
SUPERCODEC
(USB
SOUND
CARD)
30
Practical Electronics | October | 2021
Still using the NE5532 – really?
We have specified NE5532 op amps for this
project. This may be a point of contention
with some readers.
We built eight of the DAC modules as
used in the DSP Active Crossover, allowing a comparison of NE5532 and LM4562
devices, and were unable to conclusively
measure one as better than the other. We
expect that we were measuring the actual
ADC and DAC performance. Given that the
LM4562 costs more than the NE5532 (and
consumes more power) there seemed to be
no good reason to use them.
We have also used LM833 op amps; they
work too, but not as well; they can’t drive as
low impedances as NE5532s, so require more
of a distortion/noise tradeoff.
If you have a favourite op amp that you
want to use, we recommend you install high
quality machined sockets, as desoldering
op amps from a double-sided PCB generally
kills the op amp, and may damage the PCB.
(Suitable sockets are the Altronics P0530
or similar). Things you would need to check
if you do this include oscillation, ringing
and leakage of high-frequency products
from the DAC to the output.
We also suspect that you will, in the
best case, get equivalent performance, and
quite possibly worse. If you want to get the
rated performance, it’s best to stick with
the devices that we tested!
To keep radiated noise from the
switchmode supplies low, we have been
rather careful with the layout, making
sure current loops are small.
We have also used low-ESR capacitors throughout, as well as oversized
toroidal inductors. This contains the
magnetic field inside the inductors and
avoids saturation, which would lead to
increased radiation.
The switchmode supplies are also
located as far from the low-level analogue electronics as we can manage. On
our test plots, there is a tiny bit of noise
visible around the 50kHz operating
frequency, but it’s so low that it doesn’t
matter. Also, that’s above the range of
our hearing, a fact that is no coincidence.
We used a large output capacitor of
2200µF to minimise noise. Then we
added a 47µH/100µF LC low-pass filter
to reduce noise at the output further.
At this point, the ripple on the supply
rail is only a few millivolts.
The +6.5V supply is provided by a
conventional implementation of a buck
regulator, using REG1. Again, we have
put in a 2200µF filter capacitor and
47µH/100µF post-regulator filter. This
also uses low-ESR capacitors.
Why 6.5V? One problem you find
with high-speed logic is that it can
draw a fair current from low-voltage
rails. We do not want to use a linear
regulator to generate a 2.5V or 3.3V rail
that might have to deliver 100-200mA.
We would need to dissipate 1.7W:
(12V – 3.3V) ×0.2A. This is possible,
but is a real nuisance to dissipate in a
small enclosure.
So instead, we are using switchmode
regulators to generate +6.5V and –12V
rails, and then feeding these into four
linear regulators to produce very clean
+5V, +3.3V, +2.5V, +9V and –9V supplies
for the ICs. The input of each linear
regulator is fed through a ferrite bead,
to minimise the chance of any RF type
signals passing through the regulator.
The +12V and –12V ‘noisy’ rails are
regulated to +9V and –9V using LM317
and LM337 adjustable regulators. These
have especially good ripple and noise
rejection. The ±9V rails power the op
amps for the ADC and DAC sections.
Note that there is a further RC filter in
the ADC and DAC domains, formed by
10Ω resistors and 47µF capacitors, to
ensure isolation between the ADC and
DAC supply rails.
A low-dropout AZ1117H regulator is
used to generate the +5V VA rail. This
is a low-noise rail, and if you analyse
the PCB, you will find that it is routed
away from the digital section. The
DVDD +3.3V and VD +2.5V rails are
for digital purposes, and use ordinary
LM317 devices.
PCB layout trick
We’ll be presenting the PCB design next
month, along with the PCB assembly,
testing and wiring instructions. But
there are a few performance-related
things to consider about the PCB, which
we’ll briefly mention before signing off.
With the power supply at the bottom,
all the digital signals and power supplies run up the left-hand side of the
board, and the low noise and analogue
signals up the right-hand side. This is
Next month . . .
Fig.15: IC9 converts the digital audio signals from the ASRC stage to
balanced analogue outputs at pin pairs 19/20 and 23/24. These are then
filtered to remove digital artefacts and converted to single-ended audio,
to be fed to RCA output sockets CON7a and CON7b.
Practical Electronics | October | 2021
In the third and final article next month
we’ll have all the construction details,
plus the test procedures after each stage
of construction, to ensure that everything
is working correctly before you proceed
to the next step.
We’ll then cover a final set of tests;
how to download, install and set up the
USB drivers, and some useful information on using the finished product.
31
SCSupercodec
(USB
Sound
Card)
Power Supply Regulators
SUPERCODEC
(USB
SOUND
CARD)
Fig.16: the power supply circuitry efficiently produces five very clean supply rails from the possibly noisy 12V DC input.
These are ±9V for the op amps, +5V for the ADC and DAC chips, +3.3V for the digital section of the DAC chip and the two
ASRC chips (IC6 and IC7) plus the isolator (IC12) and +2.5V for DC-biasing the analogue signals fed to the ADC. The ADC also
has a local regulator (REG5) to produce its 3.3V digital rail from the +5V rail, as it was easier to lay out the board that way.
intentional, to maintain isolation between these domains.
The switchmode section that generates the –12V and –6.5V rails has a
separate ground plane. At the output
of this are the final 47µH/100µF filters.
After that, there is a wire jumper from
the ‘noisy ground’ at the input to the
larger ground plane for the linear regulators. The aim here is to avoid allowing
currents in the ‘noisy ground’ injecting
noise into the remainder of the circuit.
32
There is also a vertical cut on the
left-hand side of the ground plane
which isolates the digital section
from the power supplies. This ensures
that the digital circuitry is operating
in a ground plane largely separated
from the analogue section, with the
‘connection’ being around the DVDD
+3.3V output.
The aim is to avoid the digital circuitry injecting noise onto the analogue
ground plane.
There is a ground plane across almost
the entirety of the top of the board
(bottom under the digital section), and
ground fills everywhere practical.
So here we have a range of low-noise,
carefully isolated power supplies that
are distributed in a manner to minimise
contamination of the analogue parts
with any switching or digital noise.
Reproduced by arrangement with
SILICON CHIP magazine 2021.
www.siliconchip.com.au
Practical Electronics | October | 2021
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