tiny inductor with every bypass cap?

This DSP has 3.3V and 1.8V supply voltages. Can the same plane have both voltages?

It would be quite hard to find material that would be lossy at such low frequencies. For instance materials listed at

start at 1 MHz (material 31), while the common material 73 have significant losses only above 10 MHz.

Paul

Talal,

You really need some consulting help if you're asking questions like this. The edge rates of your signals are likely to be very fast, and this requires some real know-how when it comes to designing the circuitry, clock distribution and trace lengths, and the general pcb layout. The previous comment regarding supply sequencing should be heeded, too.

A short answer for you is this:

If you can arrange it, the controlled-impedance traces should couple to (be adjacent to) solid (unsplit) GND plane layers. Then, you can split your supply planes as you need. Otherwise, you have to be very careful in routing your traces so as to avoid coupling gaps which will result in large high-inductance coupling loops, or add extra decoupling to couple these split planes near their splits.

You should seek some professional help for your design and/or take a class in high-speed signal integrity.

Bob

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Seems a bit overkill. If there is a supply running a PLL circuit in the DSP, you'll need the bead on the PLL supply. Look at the manufacturer's data sheet. Often, they will recommend bypass capacitor issues.

--
Mark

z.

u

do not

The Texas Instruments application note for this part doesn't inspire confidence. It might be worth looking at what other - more trustworth

- manufacturers recommend for comparable parts. I've had a couple of bad experiences with Texas Instruments and really don't trust their customer support, but even the best of companies - Analog Devices come to mind - aren't infallible.

-- Bill Sloman, Nijmegen

Maybe they are showing parasitic inductance of the leads (part plus cap)? Look at the accompanying literature.

Hello,

I designed a board with an 'F2808 about 2 years ago. Based on TI app notes I put 100nH in series with the two ADC power pins only and 2uF to ground on each (2 * 0603 caps - different values).

Was it necessary - I don't know - never tried it without.

Did the ADC work - yes - I was impressed at how well it worked.

The board was 4 layers with careful design of gorund planes/fills around the processor and decoupling caps on both sides.

If you email me privately I'll send you a picture of the board from the CAD system.

Michael Kellett

Unless my design is cost sensitive, I generally put ferrite beads on the wires coming from the power supply. I have never had this actually change how well anything works, but I can see a bit less noise with a scope, and doing it that way is fairly cheap. I have seen all sorts of schemes for power/ground, but I have never seen any thing that beats a solid ground plane and good physical placement of sensitive / noise-producing sections.

--
Guy Macon

I'd agree. Best to get some help now, not after there are problems with the design, the boss is becoming impatient and it's almost too late. Talal, get a consultant that is somewhat local or pay him/her to come to your work place. Then you can learn the main tricks via a coaching process.

Yes, it'll cost money but it is money well spent.

--
Regards, Joerg

http://www.analogconsultants.com/

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Use another domain or send PM.

Well, suffice it to mention one episode: Just after receiving my degree I met a guy at an airport. Recognized him right away because we happened to pick up our degrees around the same time. "Hey, what are you doing now?" ... "Writing application notes for XYZ Corporation".

--
Regards, Joerg

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Use another domain or send PM.

At the very minimum, get this book, and pay for the fastest shipping so you can hurry and get started reading it: High Speed Digital Design: A Handbook of Black Magic

Much of this book is about board layout, and exactly this sort of thing.

If you are going to layout the board yourself, then you need to learn good board design techniques. Even if you let someone else layout the board, you should be the one telling them what to do, not the other way around. To do otherwise can result in a board that does not work because of power decoupling and SI issues.

The inductors on this design are pretty bizarre and are likely not even used on the real board. A 50 uH inductor is rather large and using 15 of them would stick out like a sore thumb. I expect they were replaced in production with 0 ohm jumpers. In general, inductors are not needed for power supplies. Good ground planes do an excellent job of minimizing high frequency noise. Low frequency noise is another issue however. I used a CP Clare part once that had 0 dB PSRR. They didn't put that in the data sheet, they let you figure it out on your own. My analog power rail was derived from the digital power rail and had 10 mV of 300 Hz noise from a control loop in the DSP. The 10 mV of noise coupled directly onto the phone line and showed up as a low level, but very distinct hum or buzz.

0 dB of PSRR is unusual in a chip of any sort (I have to take my hat off to CP Clare). But the point is that you need to include the right capacitance to handle noise at all frequencies. By preventing the noise in the first place, you don't have to worry about it coupling into other circuits.

Rick

Rick

I am lossy at low frequencies. Met my wife at an ultrasound company and she was quited stunned that my body would absorb even 3.5MHz to the point where she couldn't get a decent ultrasound image. Never seen it this bad before. She married me anyway.

--
Regards, Joerg

http://www.analogconsultants.com/

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Use another domain or send PM.

I've found that the HoJo book is great for a reference, but find it too jumbled to be a coherent teaching tool. It's certainly a good place to start.

When I've seen Dr. Johnson in action (e.g. on some Xilinx webcasts) he's a very good teacher.

Bob

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All of us old geezers know that there is a tiny inductor in series with every bypass cap. Impossible to get away without it. The trick is to minimize it.

Jim

--
"It is the mark of an educated mind to be able to entertain a thought 
without accepting it."
        --Aristotle

Don't you know? Those who can't be the real engineers take the position of the application engineers.

Of course they are. Application engoneer is a sales position.

Vladimir Vassilevsky DSP and Mixed Signal Design Consultant

The internal ADC of TI 28xx is crap. It has tremendously inacurate internal reference and high zero offset. With 12 bits available, the true accuracy is about 8 bits unless you have the ADC calibrated.

There is no need for 4 layer board with 28xx. The layout can be done on two layers just fine. Been there, done that.

Vladimir Vassilevsky DSP and Mixed Signal Design Consultant

If you are not sensible about the layout, then God help you.

Vladimir Vassilevsky DSP and Mixed Signal Design Consultant

It is not a big deal to lay out the board for TMS28xx. Two layers work fine. With 28xx, the very important thing is the power sequensing on startup and shutdown.

Indeed. Especially as the power pins of the chip are supposed to be connected directly.

Vladimir Vassilevsky DSP and Mixed Signal Design Consultant