Precision synchronous demodulator

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.

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.

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.

Phemts have absurdly low capacitances and especially g-d capacitance. There may
be something there.


-- 

John Larkin                  Highland Technology Inc
www.highlandtechnology.com   jlarkin at highlandtechnology dot com   

Precision electronic instrumentation
Picosecond-resolution Digital Delay and Pulse generators
Custom timing and laser controllers
Photonics and fiberoptic TTL data links
VME  analog, thermocouple, LVDT, synchro, tachometer
Multichannel arbitrary waveform generators

Jasen Betts <jasen@xnet.co.nz> wrote:

> On 2013-09-23, Adrian Tuddenham <adrian@poppyrecords.invalid.invalid> wrote:
> > 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? 
> >
> > Has anyone tried using a fully-balanced system to null the charge
> > injection?
> 
> You mean like a diode ring modulator?

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.


-- 
~ Adrian Tuddenham ~
(Remove the ".invalid"s and add ".co.uk" to reply)
www.poppyrecords.co.uk

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. 

>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.  

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.  

>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. 

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.

>
>>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.


>
>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.

>
>>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.


-- 

John Larkin                  Highland Technology Inc
www.highlandtechnology.com   jlarkin at highlandtechnology dot com   

Precision electronic instrumentation
Picosecond-resolution Digital Delay and Pulse generators
Custom timing and laser controllers
Photonics and fiberoptic TTL data links
VME  analog, thermocouple, LVDT, synchro, tachometer
Multichannel arbitrary waveform generators

On 09/23/2013 07:26 AM, Adrian Tuddenham wrote:
> 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?
>
> Has anyone tried using a fully-balanced system to null the charge
> injection?
>
>

Sure, all the time.  The late lamented Si8601 quad MOSFET was amazing 
for that.

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

On Monday, September 23, 2013 6:05:47 AM UTC-4, 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.

Fun,  I don't have any suggestions though. :^(  
You'd think someone would make a faster AD630.. but I couldn't find one on digikey.  (maybe I don't have the right search term.)  
For the Gilbert cell I assume you're talking about an analog multiplier.
I've used the AD734.. good to ~10MHz, you could trim out the offset with a (gack) pot., but the drift remains.       
>  
> Analog switches have negligible offset- how much trouble will all that 
> charge injection (~4pC for a good one) cause at 1-2MHz? 

So rolling your own switched gain stage.  I assume there is a preamp so you can keep the impedance down... still it looks like ~uA's of current. 
 
Jamming on JL's dual path approach, maybe have one path that does the charge injection offset, (grounded input) that later gets subtracted from the signal.
(Or a single path, but every once in a while ground the input and measure the offset?  Kinda a double lockin)  

Any idea how they do it in fast lockins?  (SRS or Zurich)

George H.
> 
> Best regards,  
> Spehro Pefhany
> 
> -- 
> "it's the network..."                          "The Journey is the reward"
> speff@interlog.com             Info for manufacturers: http://www.trexon.com
> Embedded software/hardware/analog  Info for designers:  http://www.speff.com

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.

-- 

Tim Wescott
Wescott Design Services
http://www.wescottdesign.com

Le Mon, 23 Sep 2013 11:31:12 -0400, Spehro Pefhany a écrit:

> 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.
> 
>>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.
> 

I mentioned exactly that a few years back here, when doing my pV/rtHz 
preamplifier.
Could bring charge injection into the fC range.

For some other interesting aspects, have a look at the 
"Win, where do they all come from ? (e-)" thread, back in 2006.




-- 
Thanks,
Fred.

On 9/23/2013 5:05 AM, 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?

Make balanced mixer with analog switches or FETs. Those things work very 
well.
Further improvement is using X^3 balanced mixer rather then X^2; so your 
LO would be running at 1/2 frequency of the signal. Those mixers are 
good as they don't have any X^2 rectification effect neither on signal 
nor on LO input.

Vladimir Vassilevsky
DSP and Mixed Signal Designs
www.abvolt.com

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.

I know a good contract FPGA guy in San Diego.


-- 

John Larkin         Highland Technology, Inc

jlarkin at highlandtechnology dot com
http://www.highlandtechnology.com

Precision electronic instrumentation
Picosecond-resolution Digital Delay and Pulse generators
Custom laser drivers and controllers
Photonics and fiberoptic TTL data links
VME thermocouple, LVDT, synchro   acquisition and simulation