Discussion in 'Electronic Design' started by john, Nov 17, 2008.

1. ### johnGuest

Hello All,

I have following confusions and questions about ADC based system: I am
trying to find the two ADCs for the following specifications

1. The specifications of input analog signal are as follows
a. Frequency range # 1 : 500Hz to 5MHz
b. Frequency range # 2 : 30 KHz
c. Both signals are biphasic square wave
d. Amplitude : +/- 5 volts

2. Digital interface will be USB2.0 (480 Mbps). Planning to interface
the ADC digital bus with USB2.0.

I need to choose an ADC that can sample the input analog signal
efficiently and generate the desired data bit rate (which I do not
know exactly now).

Sampling Calculations:

I have worked with an ADC from Maxim (MAX 187). This chip can do
75Ksps. So, it might be ok with the input signal of frequency of
40KHz. The Nyquist theorem says that I can not read the frequency
higher that (fs / 2). So, the maximum output analog frequency that I
can read if 37.5 KHz. Am I correct?
Can anybody suggest the part for the Frequency range # 1?
My other question about sampling is that the square wave consists of
many frequency harmonics, what harmonic should be chosen for sampling
the fundamental, second or third?

Data Rate Calculations:

The Data rate of the USB2.0 IS 480Mbps and the MAX 187 is 12 bit /
sample ADC at 75Ksps. So, the digital output rate of the MAX187 will
be 900,000 bits/sec. Is this correct?
Now, if I divide 480 Mbps with 12 bits / sample, then USB2.0 can
support 40 Mbps. Is this correct?

I will appreciate of someone suggest some parts and clear few
designing principles regarding ADC for me.

Thanks
John

2. ### Guest

Hi John, I'm not going to be much help to you, but I was wondeirng
why you need an ADC if you have square waves as an input signal. What
do you want to know besides the frequency?

George

3. ### johnGuest

Hi,

Yes, you are right! The Input signal could be discountinuous, square
wave or any other type of signal. But I still need verification of my
calculations.

Thanks
John

4. ### MooseFETGuest

Actually the Nyquist is the first frequency you can't measure not the
last one you can. You can have a sine wave a Nyquist crossing zero
exactly at the sample times and thus read zero.
This also doesn't leave you any room for an anti-alias filter.
The LTC2203 or LTC2286 will go fast enough to do 5MHz. How many bits
do you need?

That is the peak rate don't expect to maintain that rate for the whole
time.
It is sort of correct. You will have some overhead.

Can you actually obtain the MAX 187?

5. ### Guest

If the OP wants to measure a square wave why does he need any more
than one bit? I'm confused.

George

6. ### Guest

That is the peak rate don't expect to maintain that rate for the whole

I have managed to get about half the 480Mbps over usb 2.0 using the
cypress ez-usb fx2. The gnu software radio project has a lot of info
on how to do it. I found that the custom drivers were not actually
necessary and the user mode wrapper from cypress will do just fine. In
my experience multiple end points don't get you any more bandwidth and
isochronous transfers are also a waste of time. However, IO completion
ports are an absolute must. The real gotcha is that some usb host
controllers will do there best to reorder every single packet you
send, while others won't.

Good luck, Jon Pry

7. ### johnGuest

Hello,

Sampling Calculations:

I have worked with an ADC from Maxim (MAX 187).  This chip can do
75Ksps. So, it might be ok with the input signal of frequency of
40KHz. The Nyquist theorem says that I can not read the frequency
higher that (fs / 2).  So, the maximum output analog frequency that I
can read if 37.5 KHz. Am I correct?

Tim wrote: Nyquist didn't say that.http://www.wescottdesign.com/
articles/Sampling/sampling.html.

So, I am wrong about sampling the 40KHz signal with 75Ksps ADC. And
the reason is ???

Can anybody suggest the part for the Frequency range # 1?
My other question about sampling is that the square wave consists of
many frequency harmonics, what harmonic should be chosen for sampling
the fundamental, second or third?

Tim wrote:If you are saying that you have a 5MHz square wave, then
you'll have
harmonics going well above 5MHz -- the sharper the edges, the higher
the
harmonics.

So , what harmonic should I sample, The highest frequency range one,
middle frequency range ??

Data Rate Calculations:

The Data rate of  the USB2.0 IS 480Mbps and the MAX 187 is 12 bit /
sample ADC at 75Ksps. So, the digital output rate of the MAX187 will
be 900,000 bits/sec. Is this correct?
Now, if I divide 480 Mbps with 12 bits / sample, then USB2.0 can
support 40 Mbps. Is this correct?

I will appreciate of someone suggest some parts and clear few
designing principles regarding ADC for me.

Tim wrote: 480Mbps is the absolute maximum raw data transfer rate, not
the rate
that can be sustained by any one channel, and possibly not (I haven't
read the fine print of the USB specification) something that you
could
ever actually achieve in practice.
A big part of this is that you can't get all the bus bandwidth in one
channel -- it's simply not in the standard.  You want to do a bit of
digging and find out the maximum data rate that you can sustain using
isochronous transfers with your high-speed USB.

Lets suppose the USB 2.0 has stable 480Mbps. Then are my calculations
true??

Tim wrote: You never mentioned your precision requirement -- are you
just assuming
that 12 bits is both necessary and sufficient, or might you get away
with less (or absolutely need more)?

Yes, I am looking for 12 bit precision.

Regards.