On Mon, 23 Sep 2013 11:39:18 -0500, Tim Wescott <tim@seemywebsite.really> wrote:>On Mon, 23 Sep 2013 06:05:47 -0400, Spehro Pefhany wrote: > >> What's a good approach for use at 1MHz, give or take 2:1? >> >> Things like the AD630 are slow, and my usualy go-to approach (Gilbert >> Cell balanced demodulator with LO >> Vt) has input referred drift and >> offset typically in the ~1uV/K and a couple mV. I'd like both to be >> better by at least an order of magnitude, and preferably with some gain >> like the Gilbert cell things, which have 15-20dB of gain. >> >> Analog switches have negligible offset- how much trouble will all that >> charge injection (~4pC for a good one) cause at 1-2MHz? > >You've probably already considered this, but the best low-offset >multipliers I've found all live in digital hardware, and work on a data >stream that's been run through an ADC. > >Take an ADC that has good linearity, sample your signal at the absolute >fastest that the ADC will go, decimate it if necessary in digital-land, >then do the demodulation as a multiplication-and-sum in an FPGA, DSP, or >ordinary processor. > >At your speeds, you're probably at the dividing line between a really >hard-working DSP chip and an FPGA that's loafing along. I'd go with the >FPGA if I could find the talent to do the implementation; if I couldn't >then I'd flip a coin between trying to cram it into a DSP, or using my >own inexpert FPGA skills to make it work in that realm.Thanks, Tim:- Yup, have that concept working elsewhere in more than one design (using FPGAs). This one is a quickie design- analog + some supervisory digital.
Precision synchronous demodulator
Started by ●September 23, 2013
Reply by ●September 23, 20132013-09-23
Reply by ●September 23, 20132013-09-23
Den mandag den 23. september 2013 12.05.47 UTC+2 skrev Spehro Pefhany:> What's a good approach for use at 1MHz, give or take 2:1? > > > > Things like the AD630 are slow, and my usualy go-to approach (Gilbert > > Cell balanced demodulator with LO >> Vt) has input referred drift and > > offset typically in the ~1uV/K and a couple mV. I'd like both to be > > better by at least an order of magnitude, and preferably with some > > gain like the Gilbert cell things, which have 15-20dB of gain. > > > > Analog switches have negligible offset- how much trouble will all that > > charge injection (~4pC for a good one) cause at 1-2MHz? >I wonder how well a multiplying dac would work? -Lasse
Reply by ●September 23, 20132013-09-23
On 09/23/2013 03:05 PM, Lasse Langwadt Christensen wrote:> Den mandag den 23. september 2013 12.05.47 UTC+2 skrev Spehro Pefhany: >> What's a good approach for use at 1MHz, give or take 2:1? >> >> >> >> Things like the AD630 are slow, and my usualy go-to approach (Gilbert >> >> Cell balanced demodulator with LO >> Vt) has input referred drift and >> >> offset typically in the ~1uV/K and a couple mV. I'd like both to be >> >> better by at least an order of magnitude, and preferably with some >> >> gain like the Gilbert cell things, which have 15-20dB of gain. >> >> >> >> Analog switches have negligible offset- how much trouble will all that >> >> charge injection (~4pC for a good one) cause at 1-2MHz? >> > > I wonder how well a multiplying dac would work? > > -Lasse >Multiplying performance up in the megahertz is generally very disappointing. Cheers Phil Hobbs -- Dr Philip C D Hobbs Principal Consultant ElectroOptical Innovations LLC Optics, Electro-optics, Photonics, Analog Electronics 160 North State Road #203 Briarcliff Manor NY 10510 hobbs at electrooptical dot net http://electrooptical.net
Reply by ●September 23, 20132013-09-23
> I was thinking of doing something with CMOS where the signal is balanced > and any charge injection on one leg is precisely balanced by charge > injected into the other leg. The shorter transition time of CMOS > switching should give lower noise than a diode ring modulator and there > is no chance of unbalaced D.C. injection from the control waveform into > the signal. > > My particular application could be in modulating audio with a non > sinusiodal waveform by pulse-width modulation, but the charge-balancing > principle should work equally well at R.F. > >There are papers on such schemes using CMOS switches for cancellation. Also schemes regarding controlling the DV/DT of the clocks. You would probably want to go fully differential on the clock as well if you want to be anal about this.
Reply by ●September 23, 20132013-09-23
Den mandag den 23. september 2013 21.07.52 UTC+2 skrev Phil Hobbs:> On 09/23/2013 03:05 PM, Lasse Langwadt Christensen wrote: > > > Den mandag den 23. september 2013 12.05.47 UTC+2 skrev Spehro Pefhany: > > >> What's a good approach for use at 1MHz, give or take 2:1? > > >> > > >> > > >> > > >> Things like the AD630 are slow, and my usualy go-to approach (Gilbert > > >> > > >> Cell balanced demodulator with LO >> Vt) has input referred drift and > > >> > > >> offset typically in the ~1uV/K and a couple mV. I'd like both to be > > >> > > >> better by at least an order of magnitude, and preferably with some > > >> > > >> gain like the Gilbert cell things, which have 15-20dB of gain. > > >> > > >> > > >> > > >> Analog switches have negligible offset- how much trouble will all that > > >> > > >> charge injection (~4pC for a good one) cause at 1-2MHz? > > >> > > > > > > I wonder how well a multiplying dac would work? > > > > > > -Lasse > > > > > > > Multiplying performance up in the megahertz is generally very disappointing. >I just saw analog have several rated for ~10MHz multiplying BW and parallel in out so they should be real easy to drive. But I never tried them so I'm not sure what that means in the real world -Lasse
Reply by ●September 23, 20132013-09-23
On Mon, 23 Sep 2013 09:01:19 -0700, John Larkin <jjlarkin@highNOTlandTHIStechnologyPART.com> wrote:>On Mon, 23 Sep 2013 11:31:12 -0400, Spehro Pefhany ><speffSNIP@interlogDOTyou.knowwhat> wrote: > >>On Mon, 23 Sep 2013 07:49:02 -0700, John Larkin >><jjlarkin@highNOTlandTHIStechnologyPART.com> wrote: >> >>>On Mon, 23 Sep 2013 06:05:47 -0400, Spehro Pefhany >>><speffSNIP@interlogDOTyou.knowwhat> wrote: >>> >>>>What's a good approach for use at 1MHz, give or take 2:1? >>>> >>>>Things like the AD630 are slow, and my usualy go-to approach (Gilbert >>>>Cell balanced demodulator with LO >> Vt) has input referred drift and >>>>offset typically in the ~1uV/K and a couple mV. I'd like both to be >>>>better by at least an order of magnitude, and preferably with some >>>>gain like the Gilbert cell things, which have 15-20dB of gain. >>>> >>>>Analog switches have negligible offset- how much trouble will all that >>>>charge injection (~4pC for a good one) cause at 1-2MHz? >>>> >>>> >>>>Best regards, >>>>Spehro Pefhany >>> >>>Can you bandpass filter some first? That would directly take burden off the >>>synchronous detector. >> >>Yes, I have a 2nd order BPF. Q is not very high. An all-pass such as >>Bill suggested might make sense in this case. > >The effective bandpass of your system will be tiny. You need the get the phase >close at the demod frequency, but that doesn't need an all-pass. It's customary >to trim the digital clock phase to match the analog paths. Cos is flat on top, >so small phase errors have a tiny effect on gain.Of course you have to let the sidebands through the BPF.>>>There are cmos switches rated for below 1 pC injection. And they have a >>>common-mode voltage sweet spot, where injection crosses through zero. You can >>>tweak the power supplies so's to operate there. >> >>That's an interesting technique. >> >>>There should be some clever dual-path sync demod architecture that cancels most >>>charge injection offset errors. Build two identical detectors and feed them >>>antiphase signals and take the difference, something like that. >> >>I'm thinking the glitches will only cause major troubles if there is >>nonlinearity. > >The usual hazard is DC offset, and offset vs temperature. >Oh, beware of charge-injection spikes getting into opamps, into their inputs or >their outputs. That can cause bizarre problems.That's the kind of thing I'm worried about. Op-amps like to act as detectors all on their own sometimes.>>I guess I could make a fully-differential output amplifier (or use an >>ADC driver chip) to keep the signals closely antiphase. Or (horrors) >>use a little RF tranformer with a grounded centertap. > >They don't have to be very closely antiphase. A modest amplitude or phase error >will just make a small gain change.I don't think so.. but please tell me if I missed something. If I have a 1mV signal and 1V of offset (LF noise), the average will be: 1mV + 1V - (-1mV + 1V) = 2mV signal If the 2nd one is * 0.99, then I'll have 1mV + 1V - (-0.99mV + 0.99V) = 1.99mV signal + 10mV error>>>Phemts have absurdly low capacitances and especially g-d capacitance. There may >>>be something there. >> >>Aren't they leaky? That could screw up the DC performance. > >Well, yeah, microamps maybe.Kind of a lot when you're looking for a microvolt. Are they good for anything below 100's of MHz? Apparently they still work nicely when it's a bit chilly (eg. sub-4K). How about a cheapish scope probe for probing crystal oscillator nodes?
Reply by ●September 23, 20132013-09-23
On Mon, 23 Sep 2013 12:05:25 -0700 (PDT), Lasse Langwadt Christensen <langwadt@fonz.dk> wrote:>Den mandag den 23. september 2013 12.05.47 UTC+2 skrev Spehro Pefhany: >> What's a good approach for use at 1MHz, give or take 2:1? >> >> >> >> Things like the AD630 are slow, and my usualy go-to approach (Gilbert >> >> Cell balanced demodulator with LO >> Vt) has input referred drift and >> >> offset typically in the ~1uV/K and a couple mV. I'd like both to be >> >> better by at least an order of magnitude, and preferably with some >> >> gain like the Gilbert cell things, which have 15-20dB of gain. >> >> >> >> Analog switches have negligible offset- how much trouble will all that >> >> charge injection (~4pC for a good one) cause at 1-2MHz? >> > >I wonder how well a multiplying dac would work? > >-LasseI doubt it would take kindly to swapping the reference polarity at 1MHz-- if it's in digital form anyway, might as well do it digitally.
Reply by ●September 23, 20132013-09-23
On Mon, 23 Sep 2013 14:57:56 -0400, Spehro Pefhany wrote:> On Mon, 23 Sep 2013 11:39:18 -0500, Tim Wescott > <tim@seemywebsite.really> wrote: > >>On Mon, 23 Sep 2013 06:05:47 -0400, Spehro Pefhany wrote: >> >>> What's a good approach for use at 1MHz, give or take 2:1? >>> >>> Things like the AD630 are slow, and my usualy go-to approach (Gilbert >>> Cell balanced demodulator with LO >> Vt) has input referred drift and >>> offset typically in the ~1uV/K and a couple mV. I'd like both to be >>> better by at least an order of magnitude, and preferably with some >>> gain like the Gilbert cell things, which have 15-20dB of gain. >>> >>> Analog switches have negligible offset- how much trouble will all that >>> charge injection (~4pC for a good one) cause at 1-2MHz? >> >>You've probably already considered this, but the best low-offset >>multipliers I've found all live in digital hardware, and work on a data >>stream that's been run through an ADC. >> >>Take an ADC that has good linearity, sample your signal at the absolute >>fastest that the ADC will go, decimate it if necessary in digital-land, >>then do the demodulation as a multiplication-and-sum in an FPGA, DSP, or >>ordinary processor. >> >>At your speeds, you're probably at the dividing line between a really >>hard-working DSP chip and an FPGA that's loafing along. I'd go with the >>FPGA if I could find the talent to do the implementation; if I couldn't >>then I'd flip a coin between trying to cram it into a DSP, or using my >>own inexpert FPGA skills to make it work in that realm. > > > Thanks, Tim:- > > Yup, have that concept working elsewhere in more than one design (using > FPGAs). This one is a quickie design- analog + some supervisory digital.I figured you had a good reason not to go digital, but it needed sayin' -- Tim Wescott Wescott Design Services http://www.wescottdesign.com
Reply by ●September 23, 20132013-09-23
Den mandag den 23. september 2013 22.23.49 UTC+2 skrev Spehro Pefhany:> On Mon, 23 Sep 2013 12:05:25 -0700 (PDT), Lasse Langwadt Christensen > > <langwadt@fonz.dk> wrote: > > > > >Den mandag den 23. september 2013 12.05.47 UTC+2 skrev Spehro Pefhany: > > >> What's a good approach for use at 1MHz, give or take 2:1? > > >> > > >> > > >> > > >> Things like the AD630 are slow, and my usualy go-to approach (Gilbert > > >> > > >> Cell balanced demodulator with LO >> Vt) has input referred drift and > > >> > > >> offset typically in the ~1uV/K and a couple mV. I'd like both to be > > >> > > >> better by at least an order of magnitude, and preferably with some > > >> > > >> gain like the Gilbert cell things, which have 15-20dB of gain. > > >> > > >> > > >> > > >> Analog switches have negligible offset- how much trouble will all that > > >> > > >> charge injection (~4pC for a good one) cause at 1-2MHz? > > >> > > > > > >I wonder how well a multiplying dac would work? > > > > > >-Lasse > > > > I doubt it would take kindly to swapping the reference polarity at > > 1MHz-- if it's in digital form anyway, might as well do it digitally.afaikt something like this can do it: http://www.analog.com/static/imported-files/data_sheets/AD5424_5433_5445.pdf -Lasse
Reply by ●September 23, 20132013-09-23
On Mon, 23 Sep 2013 14:07:29 -0700 (PDT), Lasse Langwadt Christensen <langwadt@fonz.dk> wrote:>Den mandag den 23. september 2013 22.23.49 UTC+2 skrev Spehro Pefhany: >> On Mon, 23 Sep 2013 12:05:25 -0700 (PDT), Lasse Langwadt Christensen >> >> <langwadt@fonz.dk> wrote: >> >> >> >> >Den mandag den 23. september 2013 12.05.47 UTC+2 skrev Spehro Pefhany: >> >> >> What's a good approach for use at 1MHz, give or take 2:1? >> >> >> >> >> >> >> >> >> >> >> >> Things like the AD630 are slow, and my usualy go-to approach (Gilbert >> >> >> >> >> >> Cell balanced demodulator with LO >> Vt) has input referred drift and >> >> >> >> >> >> offset typically in the ~1uV/K and a couple mV. I'd like both to be >> >> >> >> >> >> better by at least an order of magnitude, and preferably with some >> >> >> >> >> >> gain like the Gilbert cell things, which have 15-20dB of gain. >> >> >> >> >> >> >> >> >> >> >> >> Analog switches have negligible offset- how much trouble will all that >> >> >> >> >> >> charge injection (~4pC for a good one) cause at 1-2MHz? >> >> >> >> >> > >> >> >I wonder how well a multiplying dac would work? >> >> > >> >> >-Lasse >> >> >> >> I doubt it would take kindly to swapping the reference polarity at >> >> 1MHz-- if it's in digital form anyway, might as well do it digitally. > >afaikt something like this can do it: > >http://www.analog.com/static/imported-files/data_sheets/AD5424_5433_5445.pdf > > >-LasseWould it be completely silly to apply the input signal to the reference and drive the DAC inputs to +/- Vref? Unfortunately, the gain is only flat to about 100kHz for the -Vref code.
