Re: [Discuss-gnuradio] USRP grounding

[Martin Dvh]

Jonathan Jacky wrote:
> 
> We are about to connect the USRP with LFRX and LFTX daughter boards to
> a laboratory data acquisition system where noise and ground loops are
> an issue.  Can anyone confirm my understanding of the grounding on the
> USRP, daughter boards, and enclosure?
I know I had to decouple the ground from an external source and the ground of 
the LFRX board with a large capacitor (several uF) to get things
working.

If I did not do this, for some reason the input of the instrumentation 
amplifier was sometimes DC-lifted out of its input range.


I didn't look into it in more detail since the capacitor helped, and I didn't 
need a DC-coupling.
(external source was a videosignal so lowest signal freq was 25 Hz)

Greetings,
Martin

> 
> As I understand it, the USRP is floating with respect to the rest of
> the laboratory.  The only ways the USRP grounds could be connected to
> grounds elsewhere in the lab are through the USB ground (depending on
> how the host computer is grounded), or through the shields of the
> cables connected to the SMA connectors on the daughterboards (see
> figure below).
> 
> Here are the details as I understand them, from the circuit diagrams
> at http://gnuradio.org/trac/browser/usrp-hw/
> 
> On the USRP motherboard:
> 
> - Power ground: The USRP is powered through a transformer.  The power
>   ground of the USRP motherboard (the ground pin of the power
>   regulator IC) is connected to one of the terminals on the output
>   side of this transformer.  Therefore the USRP power ground is not
>   connected to the rest of the lab (except for the coupling through
>   the transformer).
> 
> - Digital ground: The ground pins of the digital ICs (and the digital
>   ground pins of the AD986X) on the USRP motherboard are connected to
>   the power ground (same as the ground pin on the power regulator IC).
> 
> - USB ground: The USB connector ground is connected to the digital ground.
> 
> - Analog grounds: The analog ground pins of the two AD986X are
>   connected to two separate analog grounds, AGND_1 and AGND_2.  Each
>   analog ground is connected at one point to the power/digital
>   ground, through an inductor.
> 
> - The mounting holes are not connected to any ground.
> 
> LFRX and LFTX Daughterboards
> 
> - Power and grounds on both daugherboards are connected to the USRP
>   power and grounds through the daughterboard connector.
> 
> - Analog grounds: Each daughterboard has its own analog ground AGND
>  that is connected to the separate AGND_1 and AGND_2 on the USRP
>  motherboard.  On each daughterboard, there is no connection between
>  its analog ground and its digital ground.
> 
> - Analog inputs: The signal on the SMA connectors is connected
>   to the analog input of the instrumentation amplifier IC through a
>   resistor, and to the analog ground through resistors and capacitors.
> 
> - The shield on the SMA connectors on the LFRX and LFTX boards is
>   connected to the analog ground.
> 
> Enclosure (we have a homemade enclosure, a painted metal box):
> 
> - The USRP motherboard mounting holes are not connected to any
>   ground, so it is possible to insulate the enclosure from the
>   electronics
> 
> - If the SMA connectors on the enclosure are not insulated from the
>   enclosure, then the enclosure is connected to both analog grounds
>   (through the SMA connector shields on the daughterboards), and
>   there could be loops (see figure)
> 
> 
>                 SMA  |             USB  |             SMA  |
>                shield|           ground |            shield|
>  Enclosure ......... | ................ | ................ | .........
>                      |                  |                  |
>               LFRX   | AGND             |            AGND  | LFTX
> Daughterboards       +--+---    --+--   |   --+--    ---+--+
>                         |         |     |     |         |
>  daughterboard          |         |     |     |         |
>   connector             |         |     |     |         |
>                         |         |     |     |         |
> USRP Motherboard     ---+---))))--+-----+--+--+--))))---+---
>                     AGND_1  coil   digital |     coil  AGND_2
>                                   and power|
>                                     ground |
>                                            |     | +
>                                            )))))))
>                                Transformer )))))))
>                                            |     |
>                                           Lab power
> 
> 
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