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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 13:46:12 -0700
User-agent: Thunderbird (X11/20070326)

Don Ward wrote:
> That's not what I get.  With the input open, I measure 108 mV on the
> SMA connector and a DC component from the ADC (detected using the RSSI
> register, to avoid the DC offset filter in the FPGA).  If I short the
> input to ground, I get a negative DC component from the ADC.  To get
> zero from the ADC I need to ground the input through a 50 ohm
> (approximately) resistance; in this case I measure 62 mV at the input
> to the LFRX board.  My conclusion (confirmed by inspection of the
> schematic) is that the LFRX needs to be driven by a source resistance
> of 50 ohms *at DC* to be correctly biased.

I am sorry.  You guys are both right. 

If you connect a 50 ohm resistor across the SMA, you do indeed get 62mV
at the connector, but 0V at the ADC, which is what we really care about.
If you leave the SMA open, you do get a small voltage being read at the ADC.

So yes, the LFRX does have DC bias on its input, but as long as you
drive it with a 50 ohm load, you get the right answer at the ADC.  The
LFRX wasn't designed for measuring DC voltages, although it can as long
as you have a 50 ohm source.  And if your source is not 50 ohms, you can
do the calculations necessary to scale the gain.

Also, please note that the amp is inverting, so if you put 1V through 50
ohms into the SMA, the ADC will tell you that it is -1V.


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