On Fri, 22 Jun 2007, Matt Ettus wrote:
[EMAIL PROTECTED] wrote:
On Thu, 21 Jun 2007, Matt Ettus wrote:
If there is no dc-removing capacitor in the circuitry , then should I
not expect that for a board using a single (positive) supply, that the
signal is always above zero?
No, you can put a negative voltage in, as long as it doesn't go below
-3.33V. You need to look at the schematics -- the 8132 is a
differential opamp with a common mode output set at 3.3V/2.
I meant that if you operate single supply, a negative input would be
mapped to a voltage referenced to the common mode output, not to zero.
So -.1 V would become 3.3/2 -.1 = 1.55 V. That's what the data sheet
seems to indicate if I read it correctly. Am I reading it correctly?
If you put 0V in, both outputs of the amp should be at 3.3/2.
If you put in a positive voltage V, the + output should go up by V/2 and
the negative output should go down by V/2. The reverse happens for
negative voltages.
Basically, the differential amps will clip if you go outside the range
of -3.3V to +3.3V. You will also damage the differential amp if you go
below -3.3V.
The ADC will clip if you go outside the range -2V to +2V when set for
minimum gain.
Back to the subject of what your signal generator is doing, I don't
know. If you measure the voltage on the sma connector with nothing
connected, you will see that it is 0. If you connect a 1 V source
through a 50 ohm resistor, you will see that there is 0.5V at the
connector. If you connect -1 V through a 50 ohm resistor, you will see
-0.5V at the connector.
I do not. In fact, with no voltage applied to a powered up USRP, I
actually see +.109 V! So when I apply a -.1 (through a 50 ohm
resistor) I see just above zero.
I see the behavior you describe only if I unpower the USRP/LF-RX.
What gives?
Sounds broken. The LFRX has 2 inputs. Does the other one behave the
same way?
I am measuring at the output connector of the LF-RX, which is of course
the input to the 8132. I wasn't measuring at the outputs of that chip on
the circuit board. Is that what you intended me to do?
And I have 2 LFRX's and all 4 inputs behave the same way.
Matt
I'm actually giving a demo with the USRP in a couple of minutes to some
visiting students, so I'll have to wait to open up my case and make those
measurements... I'll get back to you asap though.
thanks,
eric
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