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 > > > _______________________________________________ > Discuss-gnuradio mailing list > [EMAIL PROTECTED] > http://lists.gnu.org/mailman/listinfo/discuss-gnuradio > _______________________________________________ Discuss-gnuradio mailing list [EMAIL PROTECTED] http://lists.gnu.org/mailman/listinfo/discuss-gnuradio
