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Re: [Discuss-gnuradio] offset at input of LF_RX

From: Matt Ettus
Subject: Re: [Discuss-gnuradio] offset at input of LF_RX
Date: Fri, 22 Jun 2007 10:12:25 -0700
User-agent: Thunderbird (X11/20070326)

address@hidden 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?


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