Forums

Multiplexer sample and hold

Started by bitrex January 17, 2016
On Mon, 18 Jan 2016 01:43:25 -0800 (PST), Klaus Kragelund
<klauskvik@hotmail.com> wrote:

>On Sunday, January 17, 2016 at 8:17:26 PM UTC+1, John Larkin wrote: >> On Sun, 17 Jan 2016 18:41:33 +0000, John Devereux >> <john@devereux.me.uk> wrote: >> >> >John Larkin <jjlarkin@highlandtechnology.com> writes: >> > >> >> On Sun, 17 Jan 2016 11:10:28 -0500 (EST), bitrex >> >> <bitrex@de.lete.earthlink.net> wrote: >> >> >> >>> >> >>>In some old schematics for analog music synthesizers, where the >> >>> analog signal path is under microprocessor control, I see that >> >>> they use a multiplexer arrangement to route control voltages >> >>> around being driven from a single DAC, usually like a DAC08 or >> >>> something. They feed it into something like a 4051 multiplexer, >> >>> followed up with a JFET input amp like a TL084 configured as a >> >>> buffer, with a polystyrene or C0G holdup cap of a few hundred p >> >>> on the noninverting input. >> >>> >> >>>Is this an approach that continues to make any sense from a cost >> >>> perspective in year of our Lord 2k16, as we have serial input i2c >> >>> DACs with multiple outputs available, or processors with tons of >> >>> pins that can hardware PWM? >> >> >> >> That's not common any more. Charge injection and drift limit accuracy, >> >> and buffered multichannel DACs are small and cheap. >> > >> >It works fine where you only need "DC" or low frequencies. Then you can >> >use a NPO and quite a high value (10-100nF say). >> > >> >I used it for a 8-channel 0-10V output where it was just going to a PLC >> >monitoring slow process variables. There was an IRQ that drove the 4051 >> >address lines and updated the next output, every 100us or whatever. >> >> Sure it works, but is it sensible to replace one octal DAC chip with a >> DAC, a mux, eight caps, and eight opamps? Less often than in the past. >> > >Yes. I've used it in a design where we saved a great deal of money doing that. I have also used a simelar technique to reduce power consumption of a reference using S/H low current amp. > >Cheers > >Klaus
I have a medical application chip design where the bandgap is fired up, sampled-and-held, then put back to sleep, to significantly save power. ...Jim Thompson -- | James E.Thompson | mens | | Analog Innovations | et | | Analog/Mixed-Signal ASIC's and Discrete Systems | manus | | San Tan Valley, AZ 85142 Skype: skypeanalog | | | 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.
whit3rd <whit3rd@gmail.com> writes:

> On Sunday, January 17, 2016 at 10:41:41 AM UTC-8, John Devereux wrote: > >> >>In some old schematics ... [ mux a DAC output to multiple] >> >> ... a TL084 configured as a >> >> buffer, with a polystyrene or C0G holdup cap of a few hundred p >> >> on the noninverting input. > >> It works fine where you only need "DC" or low frequencies. Then you can >> use a NPO and quite a high value (10-100nF say). > > Why does NPO matter here? You have a reset every few milliseconds, anyhow, > and it's hard to believe there's any microphonic problem or drift > issue.
Yes you could probably use X7R. I observed microphony from X7R before, so I usually specify NPO for this sort of thing but it was probably superstition for that application. (10nF has a negligible cost difference anyway for us).
> More to the point, if you're gonna do a dozen of these, why not pester the > manufacturers to come up with a purpose built track/hold chip, > it just takes plus/minus power, capacitor, input, output, and strobe.
I can't see that being a productive use of my time :)
> That way, the leakage and pickup on the full mux-to-amp trace doesn't noise up > the output. As I read it, that trace is just floating most of the > time.
Well it has a reasonable size cap hanging off it so it is not really floating. -- John Devereux
"Jim Thompson"  wrote in message 
news:ld0q9bdndsomk9ff8kfhienrmpvlnjbtqg@4ax.com...
> I have a medical application chip design where the bandgap is fired > up, sampled-and-held, then put back to sleep, to significantly save > power.
Or there's those floating-gate voltage references, which are sampled at manufacture time and that's it. ;-) Tim -- Seven Transistor Labs, LLC Electrical Engineering Consultation and Contract Design Website: http://seventransistorlabs.com
On Mon, 18 Jan 2016 02:14:56 -0500, "Tom Del Rosso"
<fizzbintuesday@that-google-mail-domain.com> wrote:

>John Larkin wrote: >> >> That's not common any more. Charge injection and drift limit accuracy, >> and buffered multichannel DACs are small and cheap. > >Is sample and hold still used for ADC?
Delta-sigmas don't need a s/h, and most SAR ADCs these days are capacitive ladder designs, which have inherent sample/hold. Some communications ADCs have s/h bandwiths far above their sample rate. That can be useful. -- John Larkin Highland Technology, Inc lunatic fringe electronics
John Larkin wrote:
> On Mon, 18 Jan 2016 02:14:56 -0500, "Tom Del Rosso" > <fizzbintuesday@that-google-mail-domain.com> wrote: > >> John Larkin wrote: >>> >>> That's not common any more. Charge injection and drift limit >>> accuracy, and buffered multichannel DACs are small and cheap. >> >> Is sample and hold still used for ADC? > > Delta-sigmas don't need a s/h, and most SAR ADCs these days are > capacitive ladder designs, which have inherent sample/hold.
Capacitive ladder? You mean C...C/2 instead of R...2R? My knowledge is 30 years out of date. I never heard of a capacitive ladder back then.
> Some communications ADCs have s/h bandwiths far above their sample > rate. That can be useful.
So they interleave multiple s/h and ADC channels fed from one signal? --
bitrex <bitrex@de.lete.earthlink.net> Wrote in message:
> Tim Wescott <tim@seemywebsite.com> Wrote in message: >> On Sun, 17 Jan 2016 11:10:28 -0500, bitrex wrote: >> >>> In some old schematics for analog music synthesizers, where the >>> analog signal path is under microprocessor control, I see that they use >>> a multiplexer arrangement to route control voltages around being driven >>> from a single DAC, usually like a DAC08 or something. They feed it into >>> something like a 4051 multiplexer, followed up with a JFET input amp >>> like a TL084 configured as a buffer, with a polystyrene or C0G holdup >>> cap of a few hundred p on the noninverting input. >>> >>> Is this an approach that continues to make any sense from a cost >>> perspective in year of our Lord 2k16, as we have serial input i2c DACs >>> with multiple outputs available, or processors with tons of pins that >>> can hardware PWM? >> >> I have not found it so -- compare the cost of good capacitors with the >> cost of good DAC channels and make your own conclusions, though. >> >> -- >> www.wescottdesign.com >> > > Will do. For the thing I'm thinking about I think I would need on > the order of a dozen outputs. > > -- > > > ----Android NewsGroup Reader---- > http://usenet.sinaapp.com/ >
I'd be interested in what you finally decide, and why. -- www.wescottdesign.com ----Android NewsGroup Reader---- http://usenet.sinaapp.com/
On 01/17/2016 12:34 PM, bitrex wrote:
> Tim Wescott <tim@seemywebsite.com> Wrote in message: >> On Sun, 17 Jan 2016 11:10:28 -0500, bitrex wrote: >> >>> In some old schematics for analog music synthesizers, where the >>> analog signal path is under microprocessor control, I see that they use >>> a multiplexer arrangement to route control voltages around being driven >>> from a single DAC, usually like a DAC08 or something. They feed it into >>> something like a 4051 multiplexer, followed up with a JFET input amp >>> like a TL084 configured as a buffer, with a polystyrene or C0G holdup >>> cap of a few hundred p on the noninverting input. >>> >>> Is this an approach that continues to make any sense from a cost >>> perspective in year of our Lord 2k16, as we have serial input i2c DACs >>> with multiple outputs available, or processors with tons of pins that >>> can hardware PWM? >> >> I have not found it so -- compare the cost of good capacitors with the >> cost of good DAC channels and make your own conclusions, though. >> >> -- >> www.wescottdesign.com >> > > Will do. For the thing I'm thinking about I think I would need on > the order of a dozen outputs. >
If you can use the capacitors barefoot rather than having to buffer them, it's often a win. (I'm thinking about nulling offset voltages on CMOS or JFET parts, for instance.) Some recent-vintage low-voltage muxes have really low charge injection, like 1 pC, so that you could use a 4-cent 33-nF NP0 cap up to like 16 bits. 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, 18 January 2016 20:17:02 UTC+1, Phil Hobbs  wrote:
> On 01/17/2016 12:34 PM, bitrex wrote: > > Tim Wescott <tim@seemywebsite.com> Wrote in message: > >> On Sun, 17 Jan 2016 11:10:28 -0500, bitrex wrote: > >> > >>> In some old schematics for analog music synthesizers, where the > >>> analog signal path is under microprocessor control, I see that they use > >>> a multiplexer arrangement to route control voltages around being driven > >>> from a single DAC, usually like a DAC08 or something. They feed it into > >>> something like a 4051 multiplexer, followed up with a JFET input amp > >>> like a TL084 configured as a buffer, with a polystyrene or C0G holdup > >>> cap of a few hundred p on the noninverting input. > >>> > >>> Is this an approach that continues to make any sense from a cost > >>> perspective in year of our Lord 2k16, as we have serial input i2c DACs > >>> with multiple outputs available, or processors with tons of pins that > >>> can hardware PWM? > >> > >> I have not found it so -- compare the cost of good capacitors with the > >> cost of good DAC channels and make your own conclusions, though. > >> > >> -- > >> www.wescottdesign.com > >> > > > > Will do. For the thing I'm thinking about I think I would need on > > the order of a dozen outputs. > > > If you can use the capacitors barefoot rather than having to buffer > them, it's often a win. (I'm thinking about nulling offset voltages on > CMOS or JFET parts, for instance.) > > Some recent-vintage low-voltage muxes have really low charge injection, > like 1 pC, so that you could use a 4-cent 33-nF NP0 cap up to like 16 bits. >
Also, you do not need 8 opamps, just one fast opamp, and switching of the feedback nodes also Cheers Klaus
On Monday, January 18, 2016 at 11:17:02 AM UTC-8, Phil Hobbs wrote:

> If you can use the capacitors barefoot rather than having to buffer > them, it's often a win. (I'm thinking about nulling offset voltages on > CMOS or JFET parts, for instance.) > > Some recent-vintage low-voltage muxes have really low charge injection, > like 1 pC, so that you could use a 4-cent 33-nF NP0 cap up to like 16 bits.
It's a tad discomfiting, though, that a mux limits the output voltage range you can achieve. I'd really like the full common mode range of a +/- 12V op amp available, if that's possible. The op amp buffer can apply gain and offset where the range is important.
On 01/18/2016 10:33 AM, John Devereux wrote:
> whit3rd <whit3rd@gmail.com> writes: > >> On Sunday, January 17, 2016 at 10:41:41 AM UTC-8, John Devereux wrote: >> >>>>> In some old schematics ... [ mux a DAC output to multiple] >>>>> ... a TL084 configured as a >>>>> buffer, with a polystyrene or C0G holdup cap of a few hundred p >>>>> on the noninverting input. >> >>> It works fine where you only need "DC" or low frequencies. Then you can >>> use a NPO and quite a high value (10-100nF say). >> >> Why does NPO matter here? You have a reset every few milliseconds, anyhow, >> and it's hard to believe there's any microphonic problem or drift >> issue. > > Yes you could probably use X7R. I observed microphony from X7R before, > so I usually specify NPO for this sort of thing but it was probably > superstition for that application. (10nF has a negligible cost > difference anyway for us). > >> More to the point, if you're gonna do a dozen of these, why not pester the >> manufacturers to come up with a purpose built track/hold chip, >> it just takes plus/minus power, capacitor, input, output, and strobe.
Like an LF398? ;) 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