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/
Multiplexer sample and hold
Started by ●January 17, 2016
Reply by ●January 17, 20162016-01-17
bitrex <bitrex@de.lete.earthlink.net> Wrote in message:> > 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/ >Just an aside, I have a stock of a bunch of 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 -- ----Android NewsGroup Reader---- http://usenet.sinaapp.com/
Reply by ●January 17, 20162016-01-17
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 propertieshttp://www.erseaudio.com/Products/PulseX
Reply by ●January 17, 20162016-01-17
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
Reply by ●January 17, 20162016-01-17
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.The under 1000pF region would let you use C0G caps, which are inexpensive and don't have a D.A. distortion problem.> 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've analyzed it for some Arduino controllers, with limited PWM output lines (and also in applications where I've used most of the pins). You set an interrupt at the end of each PWM cycle, and change the multiplexer switch and change the PWM setting to the next channel before restarting the timer. If you increase the PWM timer input clock to 1MHz, http://playground.arduino.cc/Main/TimerPWMCheatsheet and are using 8-bit PWM, each PWM cycle takes 256 us. Using an 8 channel multiplexer, the period time is 2ms, and you need a say 10 to 20ms filter on each channel. Some may like the idea, but I wasn't encouraged. With PWMing, there's always a speed / resolution tradeoff. There are several ways to improve the speed and resolution of the PWMing. One is to dither (change the PWM level by one LSB, over a number of consecutive PWM-channel periods), ST's AN4507. http://www.st.com/st-web-ui/static/active/en/resource/technical/document/application_note/DM00119042.pdf argg, what a painful link! Another is to use two PWM channels, and combine them with resistor ratios. This is possible because the MSB channel has perfect single LSB steps, due to the crystal clock. http://hackaday.com/2013/01/17/making-better-noises-with-dual-pwm/ -- Thanks, - Win
Reply by ●January 17, 20162016-01-17
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/
Reply by ●January 17, 20162016-01-17
Winfield Hill wrote...>Another option to deal with inadequate PWM channels is to add a multichannel PWM chip. These are popular because they're so useful with LED displays. E.g., NXP's PCA9635, 16 bits, 16 channels LTC's LT8500, 12-bits, 48 channels -- Thanks, - Win
Reply by ●January 17, 20162016-01-17
On 18/01/2016 03:16, bitrex wrote:> bitrex <bitrex@de.lete.earthlink.net> Wrote in message: > > Just an aside, I have a stock of a bunch > of 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 >If anyone wants NOS Rifa (Swedish?) polystyrene capacitors, sometimes at a very low price then Rockby have some: http://rockby.com.au/ They are good for charge amplifiers etc. as they have extremely low leakage and low dielectric absorbtion. I built a peak detector using a polystyrene capacitor and a LMC662 and some PN4117As used as diodes. It seems to drift about half a millivolt per hour, IIRC that works out to be about 5000 electrons per second.
Reply by ●January 17, 20162016-01-17
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. We sometimes make slow DACs from one pin of an FPGA, delta-sigma and an RC lowpass filter. That's an alternate to PWM. I guess that software bit-bang delta-sigma is not a totally crazy idea. -- John Larkin Highland Technology, Inc lunatic fringe electronics
Reply by ●January 17, 20162016-01-17
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.> We sometimes make slow DACs from one pin of an FPGA, delta-sigma and > an RC lowpass filter. That's an alternate to PWM. I guess that > software bit-bang delta-sigma is not a totally crazy idea.I don't know why the hardware is not more common to do that, in microcontrollers. PWM can be extremely accurate. Market it as a 24 bit DAC. -- John Devereux
