Forums

Multiplexer sample and hold

Started by bitrex January 17, 2016
DecadentLinuxUserNumeroUno wrote:
> On Sun, 17 Jan 2016 11:16:10 -0500, bitrex wrote: > > snip aside >>> >> Just an aside, I have a stock of a bunchof polystyrene caps I >> bought several years ago for some reason. They're quite physically >> large for the capacitance they provide, but they are pretty good at >> having close to ideal properties > > > http://www.erseaudio.com/Products/PulseX
I see that they crow about +/- 3%..there are other makers that does 2% or better...
On Monday, January 18, 2016 at 1:55:11 PM UTC-8, Phil Hobbs wrote:
> On 01/18/2016 10:33 AM, John Devereux wrote: > > whit3rd <whit3rd@gmail.com> writes:
> >> 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? ;)
Well, it's too expensive, and it hasn't the ability to add an offset and gain, and... it makes the octal DAC look good. My bad, I shouldn't ask for a magic bullet without stringent specs for ballistic accuracy. A tube of 4051's I can buy and put on the shelf until I need one. Ditto TL061 or TL062. Not so for the LF398 (nor any other track/hold); they're not really affordable.
On 01/19/2016 01:56 AM, whit3rd wrote:
> On Monday, January 18, 2016 at 1:55:11 PM UTC-8, Phil Hobbs wrote: >> On 01/18/2016 10:33 AM, John Devereux wrote: >>> whit3rd <whit3rd@gmail.com> writes: > >>>> 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? ;) > > Well, it's too expensive, and it hasn't the ability to add an offset and gain, and... > it makes the octal DAC look good. My bad, I shouldn't ask for a magic bullet > without stringent specs for ballistic accuracy.
The LF398 never really made it to jellybean status, and today it's far inferior to things you can cobble together for pennies.
> > A tube of 4051's I can buy and put on the shelf until I need one. Ditto TL061 or TL062. > Not so for the LF398 (nor any other track/hold); they're not really affordable.
Agreed. OTOH the DG2042 or ISL8454x are a bit on the pricy side too ($2). 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
Tim Wescott  <Tim@seemywebsite.com> Wrote in message:
> 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/ >
I was scoping the usual supplier sites, and octal 8 bit SPI DACs are pretty cheap, around $2 something in quantity (Maxim tho, and JL says friends don't let friends use Maxim.) I think from a board space and parts perspective it's going to be hard to beat just using two of those vs. some multiplexer ararrangement. I really would like 10 bits, and when you start going up in bit depth the cost starts jumping up quite a bit to the point it makes you wonder again if you should multiplex. But, I don't really need high accuracy, just higher resolution to limit "stepping" effects when values are rapidly changed between significantly different levels. And well well look at this http://www.embedded.com/design/configurable-systems/4006431/Sigma- delta-techniques-extend-DAC-resolution -- ----Android NewsGroup Reader---- http://usenet.sinaapp.com/
On Tue, 19 Jan 2016 10:47:52 -0500, bitrex wrote:

> Tim Wescott <Tim@seemywebsite.com> Wrote in message: >> 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/ >> >> > I was scoping the usual supplier sites, and octal 8 bit SPI DACs > are pretty cheap, around $2 something in quantity (Maxim tho, and JL > says friends don't let friends use Maxim.) I think from a board space > and parts perspective it's going to be hard to beat just using two of > those vs. some multiplexer ararrangement. > > I really would like 10 bits, and when you start going up in bit > depth the cost starts jumping up quite a bit to the point it makes you > wonder again if you should multiplex. > > But, I don't really need high accuracy, just higher resolution to > limit "stepping" effects when values are rapidly changed between > significantly different levels. > > And well well look at this > > http://www.embedded.com/design/configurable-systems/4006431/Sigma- > delta-techniques-extend-DAC-resolution
But are you sure that guy knows what he's talking about? If you do that, make sure that you have the processor bandwidth to support it, and that you can sample the DAC updates out at an even sampling rate, and that the bandwidth of the stage following the DAC is low enough that you get the average out, instead of an output that's madly jumping around. If the size of the step is acceptable but the suddenness isn't, and if the following stages are high impedance, then just put an RC filter after each sensitive DAC output. That'll prevent pops, while still being a low- component count solution. -- Tim Wescott Wescott Design Services http://www.wescottdesign.com
On 20/01/2016 01:56, Phil Hobbs wrote:
> On 01/19/2016 01:56 AM, whit3rd wrote: >> On Monday, January 18, 2016 at 1:55:11 PM UTC-8, Phil Hobbs wrote: >>> On 01/18/2016 10:33 AM, John Devereux wrote: >>>> whit3rd <whit3rd@gmail.com> writes: >> >>>>> 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? ;) >> >> Well, it's too expensive, and it hasn't the ability to add an offset >> and gain, and... >> it makes the octal DAC look good. My bad, I shouldn't ask for a >> magic bullet >> without stringent specs for ballistic accuracy. > > The LF398 never really made it to jellybean status, and today it's far > inferior to things you can cobble together for pennies. > >> >> A tube of 4051's I can buy and put on the shelf until I need one. >> Ditto TL061 or TL062. >> Not so for the LF398 (nor any other track/hold); they're not really >> affordable. > > Agreed. OTOH the DG2042 or ISL8454x are a bit on the pricy side too ($2). > > CHeers > > Phil Hobbs > >
Mostly I prefer analog switch chips with internal circuitry more like the 4016 than the 4066 style, because driving the back-gates (wells) under the transistors *using charge stolen from the signal pins* just to get lower ron seems like a bad deal in most of my applications. This is especially true if the manufacturer didn't correctly implement break-before-make on the back-gate switching so that the back-gate is disconnected from the signal pin *before* it is connected to the supply rail, and vice versa. Of course if the control input of the switch is just supplied with a constant logic level rather than being repeatedly switched, then it doesn't matter, but that is rarely the case for me. Most people seem to just look at the chip with the lowest on-resistance and think "ooh-shiny", even though their circuit may not care about on resistance as much as signal-dependent charge injection (e.g. S/H, or many mixers). I'm not sure whether you can get the 4016-style (with back-gates just tied to the supply rails) in a MUX configuration like 4051 etc. Chris
On 01/19/2016 07:35 PM, Chris Jones wrote:
> On 20/01/2016 01:56, Phil Hobbs wrote: >> On 01/19/2016 01:56 AM, whit3rd wrote: >>> On Monday, January 18, 2016 at 1:55:11 PM UTC-8, Phil Hobbs wrote: >>>> On 01/18/2016 10:33 AM, John Devereux wrote: >>>>> whit3rd <whit3rd@gmail.com> writes: >>> >>>>>> 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? ;) >>> >>> Well, it's too expensive, and it hasn't the ability to add an offset >>> and gain, and... >>> it makes the octal DAC look good. My bad, I shouldn't ask for a >>> magic bullet >>> without stringent specs for ballistic accuracy. >> >> The LF398 never really made it to jellybean status, and today it's far >> inferior to things you can cobble together for pennies. >> >>> >>> A tube of 4051's I can buy and put on the shelf until I need one. >>> Ditto TL061 or TL062. >>> Not so for the LF398 (nor any other track/hold); they're not really >>> affordable. >> >> Agreed. OTOH the DG2042 or ISL8454x are a bit on the pricy side too >> ($2). >> >> CHeers >> >> Phil Hobbs >> >> > > Mostly I prefer analog switch chips with internal circuitry more like > the 4016 than the 4066 style, because driving the back-gates (wells) > under the transistors *using charge stolen from the signal pins* just to > get lower ron seems like a bad deal in most of my applications. This is > especially true if the manufacturer didn't correctly implement > break-before-make on the back-gate switching so that the back-gate is > disconnected from the signal pin *before* it is connected to the supply > rail, and vice versa. > > Of course if the control input of the switch is just supplied with a > constant logic level rather than being repeatedly switched, then it > doesn't matter, but that is rarely the case for me. Most people seem to > just look at the chip with the lowest on-resistance and think > "ooh-shiny", even though their circuit may not care about on resistance > as much as signal-dependent charge injection (e.g. S/H, or many mixers). > > I'm not sure whether you can get the 4016-style (with back-gates just > tied to the supply rails) in a MUX configuration like 4051 etc.
Not sure how much that sort of consideration means these days, because even apparently simple parts have a bazillion transistors inside. Studying the shape of the charge injection vs voltage curve is a good idea, for sure, but since the ON resistance of a modern switch is usually a good two orders of magnitude better than a CD4016, with 1-2 pC of charge injection, you can usually just jack up the hold capacitor value a bit and have a big win. 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 Sunday, January 17, 2016 at 10:11:00 AM UTC-6, 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? > > -- > > > ----Android NewsGroup Reader---- > http://usenet.sinaapp.com/
I know this is a bit old post, but I am researching this topic as well, and would like to add a little idea which is also doable and relatively cheap. Its about the R-2R topology dacs. For multichannel application dtype octal transparrent latches could be used with the benefit of having parallel input (fast speed), where the application microcontroller IO count permits. The latches can be paralleled to any amount necesary, as well as the bit width. Otherwise serial to parallel shift registers could do the same, driven by fast SPI. Another idea which I saw somewhere was implementing multichannel PWM with serial to parallel shift registers. Driven fast enough, with proper pwm channel data encoding this also could work. Just my 2 cents. Greetings to all!