Forums

lock-in amplifier

Started by scot March 5, 2013
On 3/6/2013 10:31 PM, George Herold wrote:
> On Mar 6, 9:37 pm, Phil Hobbs <pcdhSpamMeSensel...@electrooptical.net> > wrote: >> On 3/6/2013 9:30 PM, George Herold wrote: >> >> >> >> >> >>> On Mar 6, 7:18 pm, Phil Hobbs <pcdhSpamMeSensel...@electrooptical.net> >>> wrote: >>>> On 3/6/2013 7:00 PM, scot wrote: >> >>>>> On Tuesday, March 5, 2013 8:07:59 PM UTC-5, scot wrote: >>>>>> Anyone have experience with lock-in amplifiers? I am thinking of purchasing a used one but don't know enough about them. I know they are extremely effective at finding small signals buried in lots of noise but what are the downsides to a lock-in besides expense? >> >>>>>> Also, this is probably over my head but I have read about the AD630 chip from Analog Devices and supposedly that can be used to make a very low cost lock-in amplifier. Anyone ever try that? Was it worth it for the money? I should mention a friend of mine has a couple of these AD630 IC's so I would be willing to solder something up pretty quick if it's not a waste of time. >> >>>>>> Scot >> >>>>> Thanks Phil for the suggestion on the ADG1236. I read up on Lock-in's last night and learned a lot. I found this interesting article and it looks like the principles are pretty understandable. >> >>>>> http://physlab.lums.edu.pk/images/9/90/LIA.pdf >> >>>>> The article specifies a DG303A chip but I think the ADG1236 can be substituted instead. My main goal here is to learn about Lock-in's by building one myself since reading only takes you so far. What do you think? Is this a good starting point? >> >>>>> Scot >> >>>> Lock-ins are very useful for (slow) measurements in low-SNR >>>> situations--they can pull signal out of some pretty grimy-looking >>>> interference. >> >>>> Building your own analogue lock-in is an excellent project. I built my >>>> first one when I was a postdoc (about 1988) for an atomic force >>>> microscope. More recently, I've built a few just like that for >>>> customers, as part of the back ends of their measurement systems. They >>>> really are Good Medicine. (I found the ADG1236 just the other day, when >>>> I needed parts to make a baby-scale loop.) >> >>>> There are two main dangers with lock-ins. The first and more insidious >>>> one is that they can tempt you to skimp on the front end of the >>>> measurement, and so lose SNR for no particularly good reason. The other >>>> is that in the presence of drift or 1/f noise, narrowing the bandwidth >>>> doesn't help, and may make things worse. >> >>>> Cheers >> >>>> Phil Hobbs >> >>> Re: front ends >>> With a nice front end I find a DSO with an average function, and sync >>> trigger is 'good enough' lots of times. (a poor man's lockin.) >> >>> George H. >> >> That's more of a poor man's signal averager, which is a much better >> approach in the presence of 1/f noise. >> >> Cheers >> >> Phil Hobbs >>
> > Well it's not phase sensitive, but it does select frequency, so 128 > averages is like a band pass with a Q of 128(?)
Something like that. It is phase-sensitive actually, because you're triggering on some sync signal, which makes I and Q easy to tell apart. The main thing is that if you're sweeping at 1 kHz, with 1000 data points, anything below 1 kHz can cause only a baseline shift, whereas the actual data is taken from a forest of comb filters, each centered on a harmonic of the sweep frequency. Almost all of that sensitive bandwidth is well outside the 1/f noise of anything except a pHEMT.
> > Ya know I've never seen a good article on the many ways to use the > average button on a DSO. > > George H. >
You just need to get Teachspin to rebadge a Rigol or Owon scope, and write an app note about it. ;) Cheers Phil Hobbs (Just bought an Owon SDS7102, which should be here tomorrow. It's mostly for my son to use in his continuing firmware endeavours.) -- 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 USA +1 845 480 2058 hobbs at electrooptical dot net http://electrooptical.net
Phil Hobbs <pcdhSpamMeSenseless@electrooptical.net> writes:

> On 3/5/2013 8:07 PM, scot wrote: >> Anyone have experience with lock-in amplifiers? I am thinking of purchasing a used one but don't know enough about them. I know they are extremely effective at finding small signals buried in lots of noise but what are the downsides to a lock-in besides expense? >> >> Also, this is probably over my head but I have read about the AD630 chip from Analog Devices and supposedly that can be used to make a very low cost lock-in amplifier. Anyone ever try that? Was it worth it for the money? I should mention a friend of mine has a couple of these AD630 IC's so I would be willing to solder something up pretty quick if it's not a waste of time. >> >> Scot >> > > The AD630 is an ancient chip that was never that great to begin > with. Nowadays you can do a great deal better with a low charge > injection mux, e.g. the ADG1236 dual SPDT. With a bit more work, you > can do better still with a couple of dual-gate MOSFETs, which have > almost no charge injection at all.
Could you expand on that, or suggest a favorite part? I found the ADG1236 too for low charge injection. Also DG636.
> The key is to not let the > poorly-controlled channel resistance of the FETs cause too large an > error. > > Cheers > > Phil Hobbs
-- John Devereux
On Mar 7, 8:40=A0am, John Devereux <j...@devereux.me.uk> wrote:
> Phil Hobbs <pcdhSpamMeSensel...@electrooptical.net> writes: > > can do better still with a couple of dual-gate MOSFETs, which have > > almost no charge injection at all. > > Could you expand on that, or suggest a favorite part? > > I found the ADG1236 too for low charge injection. Also DG636.
Same here. Unfortunately the DG1236 does not work at cryogening temperatures, unlike e.g. the TS5 series from Texas Instruments. I have been looking for a low-injecting switch to operate in LHe, and Phil's suggestion of using dual-gste MOSFETs is good. At least such devices as the 2N7000 (from some manufacturers) and the FDV301P do work, so I might try throwing in eg. the BF998. Regards, Mikko
On Mar 6, 11:27=A0pm, Phil Hobbs
<pcdhSpamMeSensel...@electrooptical.net> wrote:
> On 3/6/2013 10:31 PM, George Herold wrote: > > > > > > > On Mar 6, 9:37 pm, Phil Hobbs <pcdhSpamMeSensel...@electrooptical.net> > > wrote: > >> On 3/6/2013 9:30 PM, George Herold wrote: > > >>> On Mar 6, 7:18 pm, Phil Hobbs <pcdhSpamMeSensel...@electrooptical.net= > > >>> wrote: > >>>> On 3/6/2013 7:00 PM, scot wrote: > > >>>>> On Tuesday, March 5, 2013 8:07:59 PM UTC-5, scot wrote: > >>>>>> Anyone have experience with lock-in amplifiers? =A0I am thinking o=
f purchasing a used one but don't know enough about them. =A0I know they ar= e extremely effective at finding small signals buried in lots of noise but = what are the downsides to a lock-in besides expense?
> > >>>>>> Also, this is probably over my head but I have read about the AD63=
0 chip from Analog Devices and supposedly that can be used to make a very l= ow cost lock-in amplifier. =A0Anyone ever try that? =A0Was it worth it for = the money? =A0I should mention a friend of mine has a couple of these AD630= IC's so I would be willing to solder something up pretty quick if it's not= a waste of time.
> > >>>>>> Scot > > >>>>> Thanks Phil for the suggestion on the ADG1236. =A0I read up on Lock=
-in's last night and learned a lot. =A0I found this interesting article and= it looks like the principles are pretty understandable.
> > >>>>>http://physlab.lums.edu.pk/images/9/90/LIA.pdf > > >>>>> The article specifies a DG303A chip but I think the ADG1236 can be =
substituted instead. =A0My main goal here is to learn about Lock-in's by bu= ilding one myself since reading only takes you so far. =A0What do you think= ? =A0Is this a good starting point?
> > >>>>> Scot > > >>>> Lock-ins are very useful for (slow) measurements in low-SNR > >>>> situations--they can pull signal out of some pretty grimy-looking > >>>> interference. > > >>>> Building your own analogue lock-in is an excellent project. =A0I bui=
lt my
> >>>> first one when I was a postdoc (about 1988) for an atomic force > >>>> microscope. =A0More recently, I've built a few just like that for > >>>> customers, as part of the back ends of their measurement systems. =
=A0They
> >>>> really are Good Medicine. =A0(I found the ADG1236 just the other day=
, when
> >>>> I needed parts to make a baby-scale loop.) > > >>>> There are two main dangers with lock-ins. =A0The first and more insi=
dious
> >>>> one is that they can tempt you to skimp on the front end of the > >>>> measurement, and so lose SNR for no particularly good reason. =A0The=
other
> >>>> is that in the presence of drift or 1/f noise, narrowing the bandwid=
th
> >>>> doesn't help, and may make things worse. > > >>>> Cheers > > >>>> Phil Hobbs > > >>> Re: front ends > >>> With a nice front end I find a DSO with an average function, and sync > >>> trigger is 'good enough' lots of times. =A0(a poor man's lockin.) > > >>> George H. > > >> That's more of a poor man's signal averager, which is a much better > >> approach in the presence of 1/f noise. > > >> Cheers > > >> Phil Hobbs > > > Well it's not phase sensitive, but it does select frequency, so 128 > > averages is like a band pass with a Q of 128(?) > > Something like that. =A0It is phase-sensitive actually, because you're > triggering on some sync signal, which makes I and Q easy to tell apart. > > The main thing is that if you're sweeping at 1 kHz, with 1000 data > points, anything below 1 kHz can cause only a baseline shift, whereas > the actual data is taken from a forest of comb filters, each centered on > a harmonic of the sweep frequency. =A0Almost all of that sensitive > bandwidth is well outside the 1/f noise of anything except a pHEMT. > > > > > Ya know I've never seen a good article on the many ways to use the > > average button on a DSO. > > > George H. > > You just need to get Teachspin to rebadge a Rigol or Owon scope, and > write an app note about it. ;) > > Cheers > > Phil Hobbs > > (Just bought an Owon SDS7102, which should be here tomorrow. =A0It's > mostly for my son to use in his continuing firmware endeavours.)
Well let us know what you think of the Owon. We bought one (probably a different model) years ago, but it now lives in Europe for trade shows over there. I didn't like the screen, it had a very restricted viewing angle. (It seemed a bit noisier than a low end tek too... but no real data.) George H.
> > -- > 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 USA > +1 845 480 2058 > > hobbs at electrooptical dot nethttp://electrooptical.net- Hide quoted tex=
t -
> > - Show quoted text -
On 03/07/2013 01:40 AM, John Devereux wrote:
> Phil Hobbs <pcdhSpamMeSenseless@electrooptical.net> writes: > >> On 3/5/2013 8:07 PM, scot wrote: >>> Anyone have experience with lock-in amplifiers? I am thinking of purchasing a used one but don't know enough about them. I know they are extremely effective at finding small signals buried in lots of noise but what are the downsides to a lock-in besides expense? >>> >>> Also, this is probably over my head but I have read about the AD630 chip from Analog Devices and supposedly that can be used to make a very low cost lock-in amplifier. Anyone ever try that? Was it worth it for the money? I should mention a friend of mine has a couple of these AD630 IC's so I would be willing to solder something up pretty quick if it's not a waste of time. >>> >>> Scot >>> >> >> The AD630 is an ancient chip that was never that great to begin >> with. Nowadays you can do a great deal better with a low charge >> injection mux, e.g. the ADG1236 dual SPDT. With a bit more work, you >> can do better still with a couple of dual-gate MOSFETs, which have >> almost no charge injection at all. > > Could you expand on that, or suggest a favorite part?
Last time I used it, it was a metal-can 3N201. ;) You drive the source, and come out the drain. Connect G2 to the source, and put the gate signal on G1. You can do somewhat different things with G2 depending on the application, but the low charge injection seems to come from the top half of the cascode cutting off before the bottom half, so that the gate charge comes back out the source instead of the drain. Unfortunately I no longer have the lab notebook that had the pictures in it. 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 03/07/2013 08:52 AM, George Herold wrote:
> On Mar 6, 11:27 pm, Phil Hobbs > <pcdhSpamMeSensel...@electrooptical.net> wrote: >> On 3/6/2013 10:31 PM, George Herold wrote: >> >> >> >> >> >>> On Mar 6, 9:37 pm, Phil Hobbs <pcdhSpamMeSensel...@electrooptical.net> >>> wrote: >>>> On 3/6/2013 9:30 PM, George Herold wrote: >> >>>>> On Mar 6, 7:18 pm, Phil Hobbs <pcdhSpamMeSensel...@electrooptical.net> >>>>> wrote: >>>>>> On 3/6/2013 7:00 PM, scot wrote: >> >>>>>>> On Tuesday, March 5, 2013 8:07:59 PM UTC-5, scot wrote: >>>>>>>> Anyone have experience with lock-in amplifiers? I am thinking of purchasing a used one but don't know enough about them. I know they are extremely effective at finding small signals buried in lots of noise but what are the downsides to a lock-in besides expense? >> >>>>>>>> Also, this is probably over my head but I have read about the AD630 chip from Analog Devices and supposedly that can be used to make a very low cost lock-in amplifier. Anyone ever try that? Was it worth it for the money? I should mention a friend of mine has a couple of these AD630 IC's so I would be willing to solder something up pretty quick if it's not a waste of time. >> >>>>>>>> Scot >> >>>>>>> Thanks Phil for the suggestion on the ADG1236. I read up on Lock-in's last night and learned a lot. I found this interesting article and it looks like the principles are pretty understandable. >> >>>>>>> http://physlab.lums.edu.pk/images/9/90/LIA.pdf >> >>>>>>> The article specifies a DG303A chip but I think the ADG1236 can be substituted instead. My main goal here is to learn about Lock-in's by building one myself since reading only takes you so far. What do you think? Is this a good starting point? >> >>>>>>> Scot >> >>>>>> Lock-ins are very useful for (slow) measurements in low-SNR >>>>>> situations--they can pull signal out of some pretty grimy-looking >>>>>> interference. >> >>>>>> Building your own analogue lock-in is an excellent project. I built my >>>>>> first one when I was a postdoc (about 1988) for an atomic force >>>>>> microscope. More recently, I've built a few just like that for >>>>>> customers, as part of the back ends of their measurement systems. They >>>>>> really are Good Medicine. (I found the ADG1236 just the other day, when >>>>>> I needed parts to make a baby-scale loop.) >> >>>>>> There are two main dangers with lock-ins. The first and more insidious >>>>>> one is that they can tempt you to skimp on the front end of the >>>>>> measurement, and so lose SNR for no particularly good reason. The other >>>>>> is that in the presence of drift or 1/f noise, narrowing the bandwidth >>>>>> doesn't help, and may make things worse. >> >>>>>> Cheers >> >>>>>> Phil Hobbs >> >>>>> Re: front ends >>>>> With a nice front end I find a DSO with an average function, and sync >>>>> trigger is 'good enough' lots of times. (a poor man's lockin.) >> >>>>> George H. >> >>>> That's more of a poor man's signal averager, which is a much better >>>> approach in the presence of 1/f noise. >> >>>> Cheers >> >>>> Phil Hobbs >> >>> Well it's not phase sensitive, but it does select frequency, so 128 >>> averages is like a band pass with a Q of 128(?) >> >> Something like that. It is phase-sensitive actually, because you're >> triggering on some sync signal, which makes I and Q easy to tell apart. >> >> The main thing is that if you're sweeping at 1 kHz, with 1000 data >> points, anything below 1 kHz can cause only a baseline shift, whereas >> the actual data is taken from a forest of comb filters, each centered on >> a harmonic of the sweep frequency. Almost all of that sensitive >> bandwidth is well outside the 1/f noise of anything except a pHEMT. >> >> >> >>> Ya know I've never seen a good article on the many ways to use the >>> average button on a DSO. >> >>> George H. >> >> You just need to get Teachspin to rebadge a Rigol or Owon scope, and >> write an app note about it. ;) >> >> Cheers >> >> Phil Hobbs >> >> (Just bought an Owon SDS7102, which should be here tomorrow. It's >> mostly for my son to use in his continuing firmware endeavours.) > > Well let us know what you think of the Owon. We bought one (probably > a different model) years ago, but it now lives in Europe for trade > shows over there. I didn't like the screen, it had a very restricted > viewing angle. (It seemed a bit noisier than a low end tek too... but > no real data.) > > George H.
Will do. This one has an 800x600 display, and otherwise similar specs to the Rigol DS1102E, for just about the same price ($400). Saelig was out of stock of the Rigol, so I switched my order. It's probably going to spend most of its time looking at SPI busses and so on, at least for the first while. 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
Phil Hobbs <pcdhSpamMeSenseless@electrooptical.net> writes:

> On 03/07/2013 01:40 AM, John Devereux wrote: >> Phil Hobbs <pcdhSpamMeSenseless@electrooptical.net> writes:
[...]
>>> The AD630 is an ancient chip that was never that great to begin >>> with. Nowadays you can do a great deal better with a low charge >>> injection mux, e.g. the ADG1236 dual SPDT. With a bit more work, you >>> can do better still with a couple of dual-gate MOSFETs, which have >>> almost no charge injection at all. >> >> Could you expand on that, or suggest a favorite part? > > Last time I used it, it was a metal-can 3N201. ;) You drive the > source, and come out the drain. Connect G2 to the source, and put the > gate signal on G1. You can do somewhat different things with G2 > depending on the application, but the low charge injection seems to > come from the top half of the cascode cutting off before the bottom > half, so that the gate charge comes back out the source instead of the > drain. > > Unfortunately I no longer have the lab notebook that had the pictures in it.
OK, thanks. Looks like something I will have to breadboard some time. I seem to recall someone (John L?) talking about using opto-fets as zero charge injection switches. But I wonder are they really? They have fets inside, with gates that charge and discharge presumably. Hmm, not sure I understand how they work, now I come to think of it. Photovoltaic mode photodiode(s) connected to the gate I guess. -- John Devereux
On Thu, 07 Mar 2013 15:26:59 +0000, John Devereux
<john@devereux.me.uk> wrote:

>Phil Hobbs <pcdhSpamMeSenseless@electrooptical.net> writes: > >> On 03/07/2013 01:40 AM, John Devereux wrote: >>> Phil Hobbs <pcdhSpamMeSenseless@electrooptical.net> writes: > >[...] > >>>> The AD630 is an ancient chip that was never that great to begin >>>> with. Nowadays you can do a great deal better with a low charge >>>> injection mux, e.g. the ADG1236 dual SPDT. With a bit more work, you >>>> can do better still with a couple of dual-gate MOSFETs, which have >>>> almost no charge injection at all. >>> >>> Could you expand on that, or suggest a favorite part? >> >> Last time I used it, it was a metal-can 3N201. ;) You drive the >> source, and come out the drain. Connect G2 to the source, and put the >> gate signal on G1. You can do somewhat different things with G2 >> depending on the application, but the low charge injection seems to >> come from the top half of the cascode cutting off before the bottom >> half, so that the gate charge comes back out the source instead of the >> drain. >> >> Unfortunately I no longer have the lab notebook that had the pictures in it. > >OK, thanks. > >Looks like something I will have to breadboard some time. > >I seem to recall someone (John L?) talking about using opto-fets as zero >charge injection switches. But I wonder are they really? They have fets >inside, with gates that charge and discharge presumably. Hmm, not sure I >understand how they work, now I come to think of it. Photovoltaic mode >photodiode(s) connected to the gate I guess.
Well? You know how reliable Larkin's advice is >:-} ...Jim Thompson -- | James E.Thompson | mens | | Analog Innovations | et | | Analog/Mixed-Signal ASIC's and Discrete Systems | manus | | Phoenix, Arizona 85048 Skype: Contacts Only | | | Voice:(480)460-2350 Fax: Available upon request | Brass Rat | | E-mail Icon at http://www.analog-innovations.com | 1962 | I love to cook with wine. Sometimes I even put it in the food.
On Thu, 07 Mar 2013 10:11:10 -0500, Phil Hobbs
<pcdhSpamMeSenseless@electrooptical.net> wrote:

> > >Will do. This one has an 800x600 display, and otherwise similar specs >to the Rigol DS1102E, for just about the same price ($400). Saelig was >out of stock of the Rigol, so I switched my order. > >It's probably going to spend most of its time looking at SPI busses and >so on, at least for the first while.
I'd be interested to know what you think of it. The display is a lot bigger and higher resolution than my Tek, and some of the models have an interesting 16-channel LA function at an interesting price point ( < $1K ).
Jim Thompson <To-Email-Use-The-Envelope-Icon@On-My-Web-Site.com> writes:

> On Thu, 07 Mar 2013 15:26:59 +0000, John Devereux > <john@devereux.me.uk> wrote: > >>Phil Hobbs <pcdhSpamMeSenseless@electrooptical.net> writes: >> >>> On 03/07/2013 01:40 AM, John Devereux wrote: >>>> Phil Hobbs <pcdhSpamMeSenseless@electrooptical.net> writes: >> >>[...] >> >>>>> The AD630 is an ancient chip that was never that great to begin >>>>> with. Nowadays you can do a great deal better with a low charge >>>>> injection mux, e.g. the ADG1236 dual SPDT. With a bit more work, you >>>>> can do better still with a couple of dual-gate MOSFETs, which have >>>>> almost no charge injection at all. >>>> >>>> Could you expand on that, or suggest a favorite part? >>> >>> Last time I used it, it was a metal-can 3N201. ;) You drive the >>> source, and come out the drain. Connect G2 to the source, and put the >>> gate signal on G1. You can do somewhat different things with G2 >>> depending on the application, but the low charge injection seems to >>> come from the top half of the cascode cutting off before the bottom >>> half, so that the gate charge comes back out the source instead of the >>> drain. >>> >>> Unfortunately I no longer have the lab notebook that had the pictures in it. >> >>OK, thanks. >> >>Looks like something I will have to breadboard some time. >> >>I seem to recall someone (John L?) talking about using opto-fets as zero >>charge injection switches. But I wonder are they really? They have fets >>inside, with gates that charge and discharge presumably. Hmm, not sure I >>understand how they work, now I come to think of it. Photovoltaic mode >>photodiode(s) connected to the gate I guess. > > Well? You know how reliable Larkin's advice is >:-}
Not even sure it was him. But do *you* know how they work? Is there charge injection? -- John Devereux