Resolution of USRP N210 and N200

Hi,

Both of USRP N210 and N200 have 14 bit ADC, which means a range of
+/-2e13=+/-8129.

However, as I tested a N210 with LFRX, I got values from -32704 ~ 30073,
close to +/-2e15=32768.

Last year, I also tested a N200 with LFRX, I got values from -16357 ~
16241,
close to +/-2e14=16384.

The sampling rate was set to 4 MHz.

So why does the ADC has a real resolution higher than 14 bit, and are
N210
and N200 the same?

I’m really new to the world of ADC, so some basics would be very
helpful.

Thanks!

Wu

Ting -

This question really isn’t GNURadio related. If you resend it over the
USRP-Users list, though, we will help explain things quickly!

http://lists.ettus.com/mailman/listinfo/usrp-users_lists.ettus.com

Cheers,
Ben

On Sat, Nov 2, 2013 at 12:22 AM, Ting Wu <

Hi Ting -

Sorry, there really wasn’t a good reason for me to not answer your
question, regardless =)

So the ADCs are indeed 14 bits, but those samples go through a lot of
processing on a number of different platforms, and get sent over a few
different buses. At any of these stages, depending on your system
parameters, there might be conversions and scaling factors applied to
the
data, which changes the representation. Remember that since the ADC is a
14-bit ADC, you are never really getting more precision than those
14-bits;
they are just different ways of representing that data.

Does that help, at all?

Cheers,
Ben

To add to what Ben has wrote, if decimation is involved in the
processing
stages it is possible to get a higher resolution than that of the
initial
samples. For example, a decimation by a factor of two can increase the
bit
precision by one. So in this case, it could depend on where the
interface
is being defined. Perhaps the ADC chips being used are indeed 14 bits,
as
per the spec sheet, but the implementations of the different LFRX
versions
may have different post-ADC processing stages. If this is the case,
while
the precision of the ADC chips is 14 bits, the output of the LFRX could
have a higher resolution.

For those wanting to learn more, this is the principle behind
sigma-delta
ADCs http://en.wikipedia.org/wiki/Delta-sigma_modulation.

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