Analog LPF Included
So I want to build up a quick PCB with a TI PCM USB DAC on it.
If there is any special reason for using PCM2904? Because, there are similar parts from TI, but with I2S output, that can be used as standalone audio DAC or as interface to another (maybe better) DAC chip.
If there is any special reason for using PCM2904? Because, there are similar parts from TI, but with I2S output, that can be used as standalone audio DAC or as interface to another (maybe better) DAC chip.
I used PCM2707 as interface to AD1865 and some Philips TDA devices, long time ago.
The datasheet also states the output voltage is 0.6Vpp which is pretty much fine without an output amp, if driving a line input.
It mentions the LPF I assume it means:
-3db @ 250kHz and -0.03db at 20kHz
Does that sound fine?
The datasheet also states the output voltage is 0.6Vpp which is pretty much fine without an output amp, if driving a line input.
It mentions the LPF I assume it means:
-3db @ 250kHz and -0.03db at 20kHz
Does that sound fine?
Not familiar with that part, but my read of the datasheet agrees with yours. With the maximum sample rate of 48kHz, the on-chip filter may not be enough of a smoothing filter for your application (whatever that may be). If you wanted to keep it simple, just leave an option on your board for an additional simple RC filter.
The 2nd order filter in the app note seems easy enough to implement...not sure why you would not go for it.
Still, from the response of its internal FIR filter, the PCM2904 is a lousy chip, and is nowhere near HiFi.
For this reason, I wouldn't bother, I will just grab a reference design.
Still, from the response of its internal FIR filter, the PCM2904 is a lousy chip, and is nowhere near HiFi.
For this reason, I wouldn't bother, I will just grab a reference design.
Can you explain what you mean?
I have a PCM2707 here that sounds just as good as my sound card in my PC.
When you say "reference design" what do you mean? Can you give me an example?
I have a PCM2707 here that sounds just as good as my sound card in my PC.
Define good.When you say "reference design" what do you mean? Can you give me an example?
It gotta have some sort of dev kit or something, and the demo board usually come with a schematic. Search TI website.
I wouldn't worry about "quality". Unless you're planning on listening to it with some expensive equipment, at very high volume I doubt you will hear the difference between that and a 2707, or one with an external "boutique" DAC. Whilst there are better specs out there, some of the distortion from USB DACs comes from jitter in the PLL anyway which is just the nature of isochronous USB audio, and I very much doubt anyone can actually "hear" that.
Its a cool project especially as its something that is useful!
The PCM2912 just looks like a 2904 with more features that I don't want. It's in a TQFP package, but the schematic looks very close, except the extra stuff for mic input etc. and the optional filter tap off pins. The headphone output looks useless. It's quite low power and designed for 16Ohm headphones. I only need line out to go to a separate headphone amp.
Just copy the circuit, or design a passive 2nd order LPF with cut off at >>20kHz, <=250kHz.
One separate question. The 12Mhz resonator. When I search for these as SMD I get 3 pin devices, but the schematics only show two pins. Even the datasheet for the resonators I looked at where confusing and didn't actually include a coherent pin out.
Any 12MHz crystal will work, though you need to load it properly with load capacitors.
That being said, USB1.1 has a lot of tolerance to frequency error, and I won't be bothered to get the crystal loaded properly if it oscillates.
The PCM2904/2906 requires a 12-MHz (±500 ppm) clock for the USB and audio functions. The clock can be
generated by a built-in oscillator with a 12-MHz crystal resonator. The 12-MHz crystal resonator must be
connected to XTI (pin 21) and XTO (pin 20) with one high-value (1-M?) resistor and two small capacitors, the
capacitance of which depends on the load capacitance of the crystal resonator. An external clock can be
supplied to XTI (pin 21). If an external clock is used, XTO (pin 20) must be left open. Because there is no clock
disabling signal, use of the external clock supply is not recommended. SSPND (pin 28) is unable to use clock
disabling.
The PCM2912 just looks like a 2904 with more features that I don't want. It's in a TQFP package, but the schematic looks very close, except the extra stuff for mic input etc. and the optional filter tap off pins. The headphone output looks useless. It's quite low power and designed for 16Ohm headphones. I only need line out to go to a separate headphone amp.
Just copy the circuit, or design a passive 2nd order LPF with cut off at >>20kHz, <=250kHz.
It has the filter pins in use with the 2904 doesn't have. The only thing that looks like a LPF on the outputs is RCR with 3.3k, 100uF, 3.3k which doesn't look right.
I was going to go with the active one from that app note earlier. I means another IC, the line driver, but that might not be a bad thing. Downside is the minimum order at RS is 10 of them. So I can have 10 of them for £10 or 1 of them from ebay for £5.QuoteOne separate question. The 12Mhz resonator. When I search for these as SMD I get 3 pin devices, but the schematics only show two pins. Even the datasheet for the resonators I looked at where confusing and didn't actually include a coherent pin out.
Any 12MHz crystal will work, though you need to load it properly with load capacitors.
That being said, USB1.1 has a lot of tolerance to frequency error, and I won't be bothered to get the crystal loaded properly if it oscillates.
I'm almost more confused now. Quoting the datasheet:QuoteThe PCM2904/2906 requires a 12-MHz (±500 ppm) clock for the USB and audio functions. The clock can be
generated by a built-in oscillator with a 12-MHz crystal resonator. The 12-MHz crystal resonator must be
connected to XTI (pin 21) and XTO (pin 20) with one high-value (1-M?) resistor and two small capacitors, the
capacitance of which depends on the load capacitance of the crystal resonator. An external clock can be
supplied to XTI (pin 21). If an external clock is used, XTO (pin 20) must be left open. Because there is no clock
disabling signal, use of the external clock supply is not recommended. SSPND (pin 28) is unable to use clock
disabling.
But nobody makes "Crystal resonators"!
RS Components: We couldn't find any results for 'crystal resonator'
They make "Crystals oscilators" and they make "Ceramic Resonators"
I assume the IC is making it's own clock with a PLL and it wants a resonator to stablise it?
If I choose a crystal does the 1 meg resistor across it still apply?
When they say "generated by an external clock", does a Crystal count?
I've been pondering this. So I went looking and sure enough the 2707 also has I2S output.... but it has a normal analouge output too.
So I have to ask... why would you do this? Why would you use one DAC to interface with USB and output serial so a second DAC can read it and produce analogue?
I can only think of a few reasons:
1. You want to isolate the Analogue side from USB side and I2S is opti-isolatable?
2. You want to use an audiophool DAC which high bitrates or higher than 16bit (or you want to use it for things other than audio).
3. You want to use a high end DAC and don't want to limit yourself to one that has USB built in.
The only one that actually appeals to me would be 1.
When searching for DACs on RS Components it's not easy to find them for each purpose as the parameter search is a bit pants.
In my current use case a simple, all-in-one chip is fine.
I am seriously considering splitting the DAC and opto isolating it to get rid of USB power noise, but not this time round.
For USB audio 1.0, maybe hat's okay. If you are looking for 24/192, then you need some more innovative isolation mechanisms.
Thanks. I believe the PCM29xx has clock reconstruction to prevent jitter. Though I'm not sure I fully understand the problem of if that is a solution.
This is a first iteration. I am seriously considering splitting the DAC and opto isolating it to get rid of USB power noise, but not this time round.
QuoteThis is a first iteration. I am seriously considering splitting the DAC and opto isolating it to get rid of USB power noise, but not this time round.
That isolation isn't necessary. Careful PCB layout (you will need four layers) is important. And you will want to clean up the bus power. A buncha years ago, I did a USB audio design based on the now-obsolete TAS1020B. It had to be bus powered. I prototyped two power supplies that took in the bus +5V (which can be as low as 4.25 V, I think, and up to 5.25 V) and generated a "clean" +5 V for the op-amps and the CODEC. One of the supplies was based on an LT1613 switcher in SEPIC mode, and the other was based on a TI TPS60111 charge pump. Both worked well enough, meaning the noise from the supply wasn't affecting performance, and since the 60111 solution was cheaper, that's what was used. Oh, yeah, the 60111 has a clock input, so I took the I2S BCLK, divided it using 74HCT161 down to a frequency in the 60111's acceptable range, and used that to drive the charge pump. The micro needed 3.3V which was driven by an LDO hanging on the USB VBUS.