Dave brings up an excellent point: Your scope leads can pick up noise along the way if you aren't careful. Even some of the 12.5 GHz diff probes we have from Tek pick up noise as they get draped across our PCBs and then finally to the scope. I've had to switch to 10:1 or 20:1 "shop probes" at times: tombstone a 450 or 950 ohm 0402 resistor (often, I'm in a BGA field doing this) with a nice 50 ohm coax cable leading to the scope. That coax picks up a lot less board noise along the way. Chris Padilla Cisco Systems San Jose, CA -----Original Message----- From: si-list-bounce@xxxxxxxxxxxxx [mailto:si-list-bounce@xxxxxxxxxxxxx] On Behalf Of Dave Instone Sent: Thursday, April 12, 2007 7:05 AM To: kd_kamath@xxxxxxxxx Cc: si-list@xxxxxxxxxxxxx Subject: [SI-LIST] Re: ADC Gnd Noise When you say AGND w.r.t AGND ect. where exactly are you measuring it? at the ADC pins? somewhere close to that? how long are the leads that connect scope live and ground? All these will affect what you see on the scope. If you have one use a differential probe, if you haven't tried one before for this type of measurement it will really open your eyes to how far out a single ended probing method can be. Has your scope got an FFT capability or can you feed your probe output into a spectrum analyser, this can really help in deciding where that 'noise'=20 is coming from. Regards Dave Instone +44 (0)1235 824963 OXFORD SEMICONDUCTOR LIMITED 25 MILTON PARK ABINGDON OXFORDSHIRE OX14 4SH Registered in England no 2733820 Registered Address: As above=20 deepak kamath wrote: > hi, > Thanks for the suggestion. We have measured analog ground noise with respect to digital ground and it is around 300mV(p-p) on the scope. Even if we subtract 60mV noise due to probe, the noise will be around 180mV(p-p). When there is no signal to ADC, the 8 LSB bits from ADC are toggleing. That is, ADC is seeing around 80mV(p-p) noise at its input.=20 > =20 > The clock of 7.414MHz is generated using PLL inside the FPGA and is passed through a 3.3V to 5V transceiver having driving capacity of 25mA. The clock output from the transceiver is routed to ADC without any sheilding. We are observing overshoots and undershoots of about 2V(p-p) on the ADC clock, but there is no shift in the edges of the clock.=20 > Yes, we have checked both grounds and we are observing the noise of around 300mV(p-p). That is, AGND w.r.t AGND has 300mV(p-p), DGND w.r.t DGND has 300mV(p-p) and AGND w.r.t DGND has 300mV(p-p). > =20 > Thank you and regards, > Deepak. > =20 > =20 ------------------------------------------------------------------ To unsubscribe from si-list: si-list-request@xxxxxxxxxxxxx with 'unsubscribe' in the Subject field or to administer your membership from a web page, go to: //www.freelists.org/webpage/si-list For help: si-list-request@xxxxxxxxxxxxx with 'help' in the Subject field List technical documents are available at: http://www.si-list.net List archives are viewable at: =20 //www.freelists.org/archives/si-list or at our remote archives: http://groups.yahoo.com/group/si-list/messages Old (prior to June 6, 2001) list archives are viewable at: http://www.qsl.net/wb6tpu =20 ------------------------------------------------------------------ To unsubscribe from si-list: si-list-request@xxxxxxxxxxxxx with 'unsubscribe' in the Subject field or to administer your membership from a web page, go to: //www.freelists.org/webpage/si-list For help: si-list-request@xxxxxxxxxxxxx with 'help' in the Subject field List technical documents are available at: http://www.si-list.net List archives are viewable at: //www.freelists.org/archives/si-list or at our remote archives: http://groups.yahoo.com/group/si-list/messages Old (prior to June 6, 2001) list archives are viewable at: http://www.qsl.net/wb6tpu