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

[ematlis]

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?

> 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?

> Matt

thanks,
eric