Message: 6 From: Pete Rawson <peterawson@earthlink.net> Subject: Re: [time-nuts] Advice on 10 MHz isolation/distribution >Clay, > >Analog devices AD4899-1 voltage noise = 2nV/rtHz @ 10Hz; GBW = 300MHz. > >Pete Rawson That is an interesting part. So, how does one think about reverse isolation in a feedback amp?  Is the open-loop gain of the amp applied to the spurious signal to cancel it?  The spurious, reduced by the ratio of the 50 ohm source impedance, to the output impedance of 0.7 ohms of the amp (-18 dB) appears at the inverting input . . . And what would appear at the non-inverting input? Off to the simulator. Clay

life speed wrote: > Message: 6 > From: Pete Rawson<peterawson@earthlink.net> > Subject: Re: [time-nuts] Advice on 10 MHz isolation/distribution > > >> Clay, >> >> Analog devices AD4899-1 voltage noise = 2nV/rtHz @ 10Hz; GBW = 300MHz. >> >> Pete Rawson >> > That is an interesting part. > > So, how does one think about reverse isolation in a feedback amp? Is the open-loop gain of the amp applied to the spurious signal to cancel it? The spurious, reduced by the ratio of the 50 ohm source impedance, to the output impedance of 0.7 ohms of the amp (-18 dB) appears at the inverting input . . . > > And what would appear at the non-inverting input? > > Off to the simulator. > > Clay > > Reverse isolation is highest if one uses a non inverting amplifier rather than an inverting one. The signal at the inverting input is coupled to the non inverting input via the differential input impedance (predominately capacitive) of the opamp. ie finite input stage current gain and emitter base (along with package strays) capacitances are the principal contributors at least for frequencies where the input stage output voltage swing is small. Bruce

Hi ..... and thus if you drive it from a low impedance node you can get pretty good isolation. Bob On Feb 19, 2010, at 5:59 PM, Bruce Griffiths wrote: > life speed wrote: >> Message: 6 >> From: Pete Rawson<peterawson@earthlink.net> >> Subject: Re: [time-nuts] Advice on 10 MHz isolation/distribution >> >> >>> Clay, >>> >>> Analog devices AD4899-1 voltage noise = 2nV/rtHz @ 10Hz; GBW = 300MHz. >>> >>> Pete Rawson >>> >> That is an interesting part. >> >> So, how does one think about reverse isolation in a feedback amp? Is the open-loop gain of the amp applied to the spurious signal to cancel it? The spurious, reduced by the ratio of the 50 ohm source impedance, to the output impedance of 0.7 ohms of the amp (-18 dB) appears at the inverting input . . . >> >> And what would appear at the non-inverting input? >> >> Off to the simulator. >> >> Clay >> >> > Reverse isolation is highest if one uses a non inverting amplifier rather than an inverting one. > The signal at the inverting input is coupled to the non inverting input via the differential input impedance (predominately capacitive) of the opamp. > ie finite input stage current gain and emitter base (along with package strays) capacitances are the principal contributors at least for frequencies where the input stage output voltage swing is small. > > Bruce > > > _______________________________________________ > time-nuts mailing list -- time-nuts@febo.com > To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts > and follow the instructions there. >