AD5761: Arduino Library and S/H Mux--one worked, the other sucked

 

Two goals since last time: see if I could create an IC library for Arduino sketch, which was surprising easy, and breadboarding an Arduino based bipolar audio Sample &Hold multiplexer, which I never got working. 

A basic sample and hold circuit, aka S/H or just SH. The HOLD COMMAND is provided by a microcontroller switching multiples of this S/H circuit one after another in rapid succession, allowing a single DAC to produce multiplexed output voltages.

At the bench I used an AD5761 16 bit bipolar DAC IC (previous post here), an Arduino Micro, a library for the DAC I wrote for Arduino Sketch (get the library on github, here), and, for the mux switch, the ADG413 analog switch featured in previous post here.

Writing the sketch library took a lot less time than anticipated.  

I used the Helge Langehaug's sketch here as a starting point and combined it with embedded C code created previously--post is here. 

To get the C library working with an Arduino sketch IDE:

• I put the IC C code for the AD5761 into a single .h file: AD57651
• I put the .h file into the same INO directory and the setup() n' loop() Arduino .ino sketch file.
• I #included the library file at the top of the sketch's INO file
• I compiled.

Worked first time--after initializing the DAC I could enter values into an AD6751_out() function and sure enough, correct voltages were present on my scope. 

You can get the working library file and a sample .ino sketch on github, here.

Whole thing took less than an evening to craft. Joy!

With confidence soaring I used the same library to create a 16-bit four-channel sample and hold. 

It's the idea found in this previous post, but employing Arduino sketch .ino/.h files instead of embedded C.

Guess what: the AD5761/ADG413 based S&H circuit never worked. At all.

I could get pretty steady sampled DC voltages out of the circuit, but the voltages at output were never close to being accurate, especially when voltages went below ground.  

I speculated that the issue was a combination of trying to breadboard the design (I read that accurate S/H circuits need to have caps and buffers close to one another which didn't happen here), using the AD5761 in bipolar mode instead of 0-10V, which I hadn't tried in a mux design before, and the lack of precise timing in the world of Arduino--I constrained my sketch to use only delay(), digitalWrite(), Serial.begin() function calls and the rest, in the best Arduino fashion; I could have called out Atmel registers in my code and slammed away, but I might have well have used embedded C for that.

And/or some really stupid mistake(s) I never figured out.  

After a too many evenings (four? five?) I gave up; easier to use unipolar dacs then level shift them with op amps.  

Or, use 3 AD5761's in parallel.

Or, buy a 4- or 8-channel DAC with the mux built in.  

Or, whatever.....

Thinking I had an issue with the ADG413 I tried a CD4051 Mux. Nope, that sucked too.  

The S/H worked, sort of, but the voltages were never close to accurate. I stripped things out and tried to use a Salae logic analyzer to sort the issue. Nope, it wasn't SPI.   

OMG, too many wires, too much mess.  

Overall: breadboarding is NO FUN. 

I am probably returning to ditching breadboards and getting small quantities of PCB's for experimentation from the blog's faithful and patient sponsor, PCBWAY. 

If said PCB's don't work because my design is crap, well, I will pitch them and try again.  

Even with the tariffs, I think time- and money-wise I would have come out WAY ahead this time. 

Overall a win in library land, but this was a lot bench time spent chasing what I thought was a cool idea with zilch to show for it. 

Enough! See ya next time.

Analog Devices ADG413--Incredible Audio Switch IC

Two of my favorite social media (S&M) things: IMSAI Guy's Chip of the Day vlog and finding a great surplus electronics deal on the web. 

This time I do both: my own chip of the day (more like chip of the quarter?  I blame my day job...): an audio/video frequency switch IC I found online at a great surplus price, Analog Device's ADG413BR.

MUX AND FLUX

Audio/digital switch IC's--MUX'es or "multiplexers"--are extremely useful for what we do. The classics contain them; inside Roland's Jupiter 8 (schematic here) and Oberheim's OBXa (here) you find CD4053 and CD4051 mux IC's. 

Details about MCU control of this IC family can be found here and here. 

The ADG413 is a 4-channel switch with superb specs: 50V rail-to-rail supply range compared to the CD4051's 20V, for instance.  

The problem with these high performance switch IC's is their expense. Digikey US sells the ADG413BRZ for $9.08USD in small quantities, compared to 79c each for 4051's on Amazon. 

I'll stick with CD4051's?

Not so fast!! An AudioDiWHY favorite, The Electronic Goldmine, had ADG413BR's on sale recently for $1.79USD each. The 413BR is an obsolete version of the current BRZ IC. The only difference I could see was that the surplus parts were not RoHS compliant. 

(Better to use non-ROHS parts than throw them away--but if they have to go, please get rid of them sensibly....)

An aside: I love The Electronic Goldmine. Hobbyist-friendly surplus stores are a dying breed with many of the great ones like Haltek long gone. Goldmine keeps fighting the good fight. Sign up for their 4AM daily email flyer (here)--good way to start your work day. 

Lifetime buy....

The IC's arrived quickly. I got them ready for the bench.

SOIC: an adapter was needed for breadboarding

Ready to roll....

QUICK WORD FROM THIS BLOG'S SPONSOR

Many thanks to PCBWAY for sponsoring the AudioDiWHY blog and for all the help they've provided over the years. You can help out this blog in a big way by checking out PCBWAY, here. 

As soon as I get my first ADG413 based PCB laid out I'll send the gerber to PCBWAY along with any other fabrication needs, including 3D printing and assembly. 

PCBWAY is super fast, super friendly, and does great work. Please consider using PCBWAY for your next audio project. 

MAKING BALLOON ANIMALS

With two ADG413's soldered up, time to see if I could lay out basic switch circuits.

I still hate breadboarding....

....but I'm slowly getting better. I have found extreme patience, along with using a quality breadboard and checking my work early and often, gets the job done.

Using the 413 to switch LED's off and on (doh). Seems working!

 

Wiring used for ADG413 tests. "IN" represents logic inputs. Yes, the 10K pulldown was necessary; I used 12V for Vdd, GND for VSS, and 5V for logic but found the IC accepted a surprisingly wide range of voltages for rails and logic. Nice! Also, Pins 6/7 are one of 4 I/O pairs and for all I/O audio or DC voltage can flow source to drain or drain to source, makes no difference. 

For breadboarding sanity I used a CD4069 hex inverter for ADG413 pins "IN2" and "IN3". Using an MCU control I'd do this inversion in software.

 A few ideas....

The ADG413 let me create common switch configurations without needing a lot of logic signals:

1-to-4 multiplexer:

Dual SPDT's:

4 SPST's:

DPDT:

1:8 multiplexer using a 74HC595. Use more IC's and get as many channels as you want.

Good tutorial for the 74HC595 IC is here.

OUTTRO

OK enough for now. I bought a lot of ADG413's so (hopefully) they make it into future designs--hours of fun! 

In the meantime, ROHS well, AI well, and above all: don't breathe the fumes.

Dual 13700-based LFO: Mo' Hotta, Mo' Frya

Readers--this is a work in progress, you can see the schem and pcb on the one page website here.

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Hidee Ho! This time I tried to turn a working OTA LFO design (post here) into a dual LFO/small PCB, based on a SOIC SMD LM13700 OTA chip. 

Guess what?  Damn thing didn't work, the LM13700 kept heating up, and I had no idea why. Update 9-14-25 ModWiggler to the rescue! got it working; update at the bottom of this post.

"Seems not working"

 Started as a PCB n' stencil, from the blog's sponsor PCBWAY

Lately I have been using SMD non-framework stencils extensively; speeds up fabrication

SPONSOR TIME....

PCBWAY has been instrumental in keeping this blog up off the ground.  

Serene, my totally friendly handler at PCBWAY, told me to tell you: this month they are having specials in addition to their usual great pricing: 

1. Purple Solder Mask: In September, it's free--they won't charge an extra fee for purple cool color PCB's. That's to say, for 10 purple 2-layer 100x100mm PCBs, it's only $5USD, rather than $38.43USD.

2. TPU(FDM) in 3D printing: TPU is good for you! This super-useful material starts from $12.45USD, but in September, it's priced from $7.96. The heavier the weight, the bigger the discount, which can be up to 80% off. Go Getim!

And as always--PCB has all sorts of cool services tailored to what we do; you can help this blog immensely by checking PCBWAY out via the link here. Thanks!

USELESS (?) BUILD PHOTOS

Back to the blown up dual LFO....

Maybe if I look at these pictures long enough I'll find a build fault. but really, it all looks pretty good to me.

Glooped paste and picked/placed parts pre-bake--looking good I think

As usual, I heated up the plate to 215C, let it sit a minute, then back down to 150C. I am always surprised how well this works and how forgiving the process is. 

Next I soldered in THT parts like trimmers and a few caps:

Finally soldered the edge connector:

Test time...nope! 

My power supply immediately went haywire--the voltages maxed out at about .25 to 2V while the current went to the 30mA limit on each rail. Not good. 

NASA we have a short--the 13700 got really hot, quickly. 

Removing the 13700 the short went away but (obviously) the LFO's didn't work.

So why doesn't this work (besides having removed the LM13700) ??

Was the 13700 a cheap fake? I replaced the LM13700 with chips from two different vendors (Digikey, Tayda), same problem; I removed the PNP transistors thinking maybe I had a I-abc issue, nope.  

I spent a couple more evenings troubleshooting, wishing the design was through-hole (I usually am glad it's not) so I could lift up IC pins. No love.

There was a time in my DiWHY journey where I would have kept slamming away on this until fixed, but I have built so many LFO's at this point that I realized: the hell with it, I can redesign it, pitch it, and/or move on. 

Above all, I had to make sure I was having fun, and more evenings troubleshooting this didn't feel like it'd be that.

I posted to the modwiggler DIY forum to see if those folks had ideas--maybe they'd save my bacon, they had before. 

If not, this one might be one and done for the dumpster.

update 9-13-25: Modwiggler to the rescue! In short order synthiq saw the problem:

If you look at the schematic (here), the power rails to the quad op amps are flipped!!!

That's at least part of the problem, right? 

Not sure why this foobar heated up the OTA and didn't smoke the TL074's, but, things definitely wouldn't work as-is. 

I wanted to test the rest of the design. I removed the TL074's and put on new ones flipped 180 degrees (quad op amps are fully symmetrical--so this counter-intuitive fix worked! Flipping quad op amps to solve audioDiWhy stupidity is another awesome tip from MW's resident genius "guest").  

Finally, I used trimmers TR1 and 2 to set the frequencies to about 200hz, then used 4.7K for R21 and R14, and 39K for R7 and R20, to get the output waveforms closer to 10V P/P. Finally tested CVs at input --which modulated the LFO's frequencies. HA!! WORKS!!!

Post TL074 flip: no more short circuits--made me very happy indeed.

Battle damage post-fix. Lots of flux spoojim visible. As I fix repair SMD boards I find myself using no-clean flux more and more. For SMD work it's crucial, the flux inside fine gauge solder doesn't get it done. No-clean leaves a mess; I honor it by not cleaning.

Testing post-fixes. I will lay this board out again.Flipping the TL074's is a clever fix but energizes my OCD buzzer.

I will have another round of boards fabricated and try again. Stay tuned, and don't breathe the fumes.

CV Generator Part II--"Seems Working"

Readers: If you'd like to build the CV Generator featured in this post, please go to PCBWAY's Community pages--gerber file; KiCAD 9 project/pcb/schematic/library files, B.O.M.'s, and more, are here.  

You can also help out this blog immensely by checking out PCBWAY using the link here. Thanks!

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Just got back from a long and much needed vacation with the psychiatrist wife--back home and back to it. 

This time I improved the ATTINY 1624 based CV generator from this previous post--good news: hey, it works.

Synch issues with my camera made the OLED appear to drop lines of text but in real life the super cheap OLED display looks good.

Reviewing its features: 

• Three -10 to +10 DC CV outputs, independently controlled from potentiometers (not rotary encoders). I like the way pots feel for this application; in general I dislike membrane switches and rotary encoders.
• A 5V momentary switch for creating triggers with a dedicated output and LED.
• Practical use of the ATTINY1624's super affordable high resolution mux'd AD conversion (details here towards the bottom of the post)--to see if I could get >12 bit oversampling using the analogReadEnh function in sketch (it works!)....This is killer feature that's surprisingly under-documented.
• Eurorack footprint--this is going to go into my bench rack.
• Test Points. TP1 can be used to test the MCU via a blink subroutine, or maybe some future expansion. TP2 should be adjusted to 2VDC using the 20K trimmer for bias offset for the final buffer stages.
• A TX/GND serial port. For future expansion? Who knows. But I figured why not.
• A port for UPDI programming. See previous post here (halfway down the post) and IMSAI guy video here.  

For firmware I used Arduino Sketch and IDE and the Greiman SSD1306ASCii library (here).  Firmware creation for the generator was pretty easy. By bringing analog data from the pots in at 16 bits, I could ditch a bunch of LSB's and get reasonable readings on the OLED that didn't jump around a lot. 

Some simple math converted the right shifted 16-bit value into a x = -10 to10 value for the OLED:

float scaleToPlusMinus10(unsigned int x2){

  x2 = x2 >> 8;

  return ((-10 + (float)x2 * 20.0 / 256) + .5  ;

}

The module consisted of 2 PCB's provided by this blog's patient and helpful sponsor PCBWAY: a main board with hardware on the front and the SMD components (1206 and SOIC--not too tiny) on the back, and an 8HP Eurorack front panel.  

I did a lot of the heavy lifting for the design during the initial build (post here). So this "rev 2" is a prettier version with a few improvements.

And--thankfully my measurements this time around were correct; everything fit.

USELESS BUILD PHOTOS

Always happy to get new PCB's from my sponsor, PCBWAY.  Even with the tariffs these guys are still way cheaper and faster than anything I can find in North America. Go A's!

I used an SMD stencil, solder paste, and a hotplate. This has become my go-to assembly process for most of what I do these days--it's fast....with some practice SMD hand assembly has become easier for fab, mods and troubleshooting vs. through-hole.  

I gloofed on a bit too much paste but fortunately it's a pretty forgiving process.

For the hotplate, I heated it up to 210C and it sat there for a couple of minutes, then cooled it to 150C.  This was all done manually from the plate's front panel. There appears to be way to automate this--there is some sort of temperature curve firmware in the hotplate--but I can't find any instructions for this in English. Sorry.

The hot plate process is surprisingly forgiving but I ended up with a few solder blobs on the SOIC IC's. Fortunately these can be quickly fixed with solder wick. 

Firmware test....

For the OLED I de-soldered its edge pins and used longer pins instead. This allowed the display to stand off from the PCB an appropriate distance. After putting items into place I tacked soldered the edge connector pins, removed the front panel PCB, and finished soldering.

As far as accuracy--the completed module is pretty good but not perfect. For what I do--cranking CV into whatever module I am messing around with on the bench it is definitely good enough. For better accuracy I'd probably need to use a rotary encoder and a 16-bit DAC. Maybe a future project?

Overall, fun times; I might build a few more of these. 

Until next time, don't breathe the fumes.

3 Channel Bipolar CV generator Part I: Didn't Fit; Built it Anyway

Readers: this is a work in progress but you can get the files associated with this post from Github, here. 

(Update--Euro 'version 2' of this module is working, post is here).

===========

Back again! 

I wanted a synthesizer module that accurately output and displayed 3 independent control voltages ranging from -10V to 10V DC.  

The early prototype used a 3D printed front panel conforming to PAIA Frac format. Camera Synch issues meant my digitial camera couldn't capture all 4 lines of the working OLED but in real life it works.

There were a lot of ways I could have done this--using a multiplexed bipolar digital to analog converter (post for DAC here; multiplexer here) but instead employed the 16-bit ADC features of the impressive ATTINY 1624 MCU, covered briefly in this previous post. Overall, design for version 0.1 was simple; I used the pots configured as voltage dividers, buffered by op amps, took the 0-5V wiper voltages from the pots and connected that to ADC inputs of the ATTINY, and had the MCU feed a 128 x 64 OLED display to convey the 3x voltages.

Finally I used inverting op amps to get 0-5V DC from the pots to the -10 to 10VDC needed at output:  

One of 3 identical buffer subcircuits. A simulation of this simple inverting buffer/amplifier can be found here; I have posted the files for this prototype at Github, here. When the design is further along I will post files to the PCBWAY Community.

CONSTRUCTION

I laid out the design in KICAD 9 and used Arduino Sketch to (quickly) create the firmware. Because the ATTINY isn't blessed with buttloads of flash memory I used Bill Greiman's memory-friendly SSD1306ASCii library (here) to provide functions needed for the OLED. 

I sent the gerber file to this blog's trusty and most-excellent sponsor, PCBWAY. Very soon the boards were back. They do great work! You can help out the blog by checking them out here.

The prototype used 1206 SMD components, a stencil, and a Eurorack formatted front panel. After years of trying different workflows, this combination seemed a decent mix for fast prototyping, flexibility in terms of modification, and small size. 

After unbagging and inspecting components I discovered a major mistake: the PCB was too tall to fit into a Eurorack or Frac enclosure:

I contemplated pitching the entire thing and starting again, but instead consulted the guys in my audio geek discord group. Could this be fixed? No, but, one member suggested I try to build it anyway, to shake out the first batch of mistakes, before going to Version 2. Good idea....

I got building....

Used my $35USB hotplate for SMD, works great.

 

Original design incorporated Eurorack power, but I adapted this to Frac Power using edge connectors, 22 gauge hookup wire, and shrink tubing. This allowed a fair amount of twisting and bending without breaking wires or shorting connections.

Same idea for the rest of the hookup wire....

Initial tests showed way too much current draw for the positive rail--should have been more like 4-5mA, but here it's 230mA. I pretty quickly traced this to a 78L05 regulator being installed upside down, due to using the wrong Kicad footprint--I used the 3N3904 footprint, not 78L05, so Pin 1 and 3 were flipped. Whoops. I pulled the 78L05 using a Hakko FR301, flipped it, and resoldered. Surprisingly this snafu didn't blow up the MCU, nor the regulator.  

With the power problems sorted, things seemed to work--albeit not terribly accurately -- off by as much as 5% at times--but OK for the first attempt.

I laid out a quick-and-dirty front panel for the PCB using FREECAD 1.01.

....then mounted the PCB to the 3D printed front panel:

Version 0.1, unpowered, in the flesh...

Version 0.1 seen through the lens of open source AI (ComfyUI + Flux Context Dev with a 12GB Lenovo/Nvidia RTX2000 GPU).  This first and only pass through the AI algo took 50 seconds and beat the holy crap out of what I could have done with an hour of organic intelligence and Photoshop--if this doesn't scare the hell out a tech + graphics professional, I don't know what does.

OK, quick post but enough for now. I will keep plugging away at this project and hope to have a revision with better accuracy in the coming weeks. Update 8-28-25: Eurorack "rev 2" version of the generator is complete and works....please see the post here.

So--until next time: don't breathe the AI.