digital attenuator

[Pars]

Also, is there a schematic for this around?

[kevin gilmore]

only for the bipolar version do you need 10k, otherwise anything works.

 

schematic, nope never did one, 2 x pcf8574 driving 2 x uln2803 latching relays.

[kevin gilmore]

for the V2 attenuator, there was one missing wire, will fix soon.

 

 

fixed

 

http://gilmore.chem.northwestern.edu/boards/attenuatorsmtv2.zip

 

http://gilmore.chem.northwestern.edu/attenuatorsmtv2.pdf

Edited July 24, 2015 by kevin gilmore

[Laowei]

Deleted.

Bought a set.

Edited July 25, 2015 by Laowei

[Pars]

The boards will support both 2512 and 1206 sizes.

Is this the case with the group buy boards?

[Kerry]

The final version of these boards did not have 2512 pads, though they do support through hole resistors. 

[kevin gilmore]

the V2 version of the attenuator is surface mount only resistors, and the chips that drive the relays

need different software.

 

the original boards while not hardware compatible with amb's attenuator are in fact software

compatible with the lcduino.  relays with 2 coils were used.

 

the v2 boards use single coil relays and pull up resistors, same as amb's attenuator.

but the driver chips are different and require 32 bits clock + data

Edited July 25, 2015 by kevin gilmore

[Pars]

Looking at Kerry's previous 10K resistor values BOM and comparing to amb's calculator, I had some questions. Here is Kerry's listing

Input Z = 10K

562R RS0 2 
169k RP0 2 
1k1  RS1 2 
82k5 RP1 2 
2K05 RS2 2 
38k3 RP2 2 
3k65 RS3 2 
16k9 RP3 2 
6K04 RS4 2 
6k65 RP4 2 
8k45 RS5 2 
1k87 RP5 2 
9k76 RS6, RS7 4 
255R RP6 2 
6R00 RP7 2 
10K  RT  2

From amb's calculator, I had a couple of differences, but I'm not sure I set it up correctly as far as input impedance for the multiamp, etc.

 

With Rload = 1M

 

 

with Rload = 10K

 

 

I also ran it with Rload as 100K, which might be more realistic. Results were the same except for an 11.0K Rt

Edited July 25, 2015 by Pars

[Kerry]

I might have substituted some values based on availability. To long ago to remember.

A number of Kevin's amps have 500k input to ground resistors into the fets so basically the attenuator will see the 500k.

I probably used the default 1M though.

[Pars]

Thanks Kerry. The calculator values as I have them using either 100K or 1M input matches what you had (E96), other than RT which is 10K in yours.

 

The multiamp doesn't have an input to ground resistor, though most of Kevin's amps do. I had wondered whether this one should actually have one as you mentioned, as it does have low input impedance when using the bipolar THAT chips. I'm using FETs in mine, but thought I would set the digital attenuator up in case I were to use the bipolar.

 

A number of the thin film IRC/TT values are not stocked by Mouser (both for the 25K and the 10K). I have found most if not all of the values, but many are Vishay TNPW instead. The 1.87K is a problem (KOA Speer only), and the 6.0R isn't available (6R04 is in a 1%).

 

I'll post the part numbers from both Mouser and Digikey once I have them ironed out.

 

Is the V2 board set up for 1206? It seems like availability is much better the smaller you go. 0805 seems to hit most of them, as does 0402 , which seems silly small. I've worked with 0805 before and they aren't too bad.

Edited July 25, 2015 by Pars

[Kerry]

Times are changing

When we first looked at the resistor values, 1206 had the most matches. I don't mind 0805 and work with that all the time.

Were you looking at the thin film?

Edit: It might be harder to use the smaller resistors with the current layout.

Edited July 25, 2015 by Kerry

[kevin gilmore]

the new board uses 1206

 

I might be able to lay 0805 on top of the 1206, not sure how great an idea that would be.

 

other than the connectors, the entire board is surface mount only.

 

I could even do the connectors as surface mount, also probably not a good idea.

 

edit: I take that back, the capacitors are thru hole... but I could fix that too.

Edited July 25, 2015 by kevin gilmore

[Kerry]

If anyone is interested in the original group buy assembled boards. I have a set I'd be willing to sell.

 

Please PM me if you're interested.  Same price as the original group buy.  They are the original 24K version.

Edited July 26, 2015 by Kerry

[Pars]

Times are changing

When we first looked at the resistor values, 1206 had the most matches. I don't mind 0805 and work with that all the time.

Were you looking at the thin film?

Edit: It might be harder to use the smaller resistors with the current layout.

Yes, 0.1% thin film was my target resistor. The IRC/TT PFC was my initial target. I have had to go with the Vishay TNPW for many of the values. I was also going to look at the Panasonic ERA series from Digikey as well.

 

Kevin, it looks like you are running traces in between the pads on the board pdf I looked at for the V2, so I could see where going smaller on the resistors could be a problem. Hopefully the kids will keep stocking reasonable sizes such as 1206. A newer IRC series they advertised on Mouser comes in 0805 as the largest package. I'm not sure what size the caps were on two of the PSU daughterboards in the Panasonic plasma I have, probably 0402 or so, but they were awfully damn small when I fixed it.

[kevin gilmore]

it does all fit, not by much

 

http://gilmore.chem.northwestern.edu/attenuatorsmtv4.jpg

 

50 ohm resistors and smt caps added

Edited July 26, 2015 by kevin gilmore

[Kerry]

For fun, here is a 3D rendering of the new boards...

 

 

 

Edit:

Don't let the size above fool you.  These are 3" x 2".

 

I think they should even be small enough for Birgir ​

Edited July 26, 2015 by Kerry

[spritzer]

  These will be used in some future amps. 

[luvdunhill]

I assume those aren't gull wing parts, but j-hook? I might extend the lands a bit more regardless as it really helps hand soldering these parts.

Edited July 26, 2015 by luvdunhill

[kevin gilmore]

yes they are the gull wing parts, and the layout is exactly as specified in the pdf.

[luvdunhill]

yes they are the gull wing parts, and the layout is exactly as specified in the pdf.

Ok. Just speaking from experience, having longer lands with these parts is nice to have. I generally do this with SMD capacitors, etc. as well.

[Kerry]

I just ordered a few so I can test the board out

 

I'm going to try using a hot air gun and solder paste for the MAX4820 chips since they have a pad underneath.  Maybe see how that works for some of the other SMD parts.

[Pars]

I got the Arduino Uno board (part # 782-A000049) on Mouser. I would also recommend using a rotary encoder, since there are already coding examples for using one on this thread. I don't have a part number for the rotary encoder handy. Maybe someone else can supply that.

The Arduino Uno you reference above is MD'd. The 782-A000073 is the listed replacement and seems to be the newer equivalent. Is this the only thing, other than a rotary encoder(s) needed to control the attenuators? Does the Arduino board supply the power to the attenuators, or do I need to accommodate them with the amp or accessory PSU?

 

Sorry, new to this. Thanks in advance for any pointers!

[lkong]

The Arduino Uno you reference above is MD'd. The 782-A000073 is the listed replacement and seems to be the newer equivalent. Is this the only thing, other than a rotary encoder(s) needed to control the attenuators? Does the Arduino board supply the power to the attenuators, or do I need to accommodate them with the amp or accessory PSU?

 

Sorry, new to this. Thanks in advance for any pointers!

Uno needs 5V PS too, it also has pins to output 5V. So you only need one PSU for both.

Basically any Arduino board should work. I was wondering if a teensy or arduino nano would do the same trick.

[Kerry]

The original boards will require a 12v supply for the relays. You can feed the uno from that as it has built in regulators to get it to 5v but you need to be careful how much current you consume off the 5v line. So you'll need just the 12v supply. I'll need to confirm the current requirements.

The new boards require 5v to both the digital section and the relays. Each relay requires 116mA to activate. Technically you can switch all 16 at once, but you'll need to make sure your 5v supply can handle it. I would not take the 5v for the attenuator from the uno in this case though one supply could feed both.

Edited July 27, 2015 by Kerry

[Pars]

Thanks Kerry. I had thought of using something like a Tread regulating to 12V off of the main amp PSU 15-20V. Since the v1 board uses dual latching relays, I'm not certain how to calculate the current requirements of potentially 16 relays. Max coil current (12V) is 16.7mA from the relay datasheet, so absolute worst case would be around 270mA for 16 relays all switching at the same time. Maybe I will need to do a separate PSU for the attenuator so as not to load the amp PSU.

 

p.s. I went back thru the thread and looked at your setup in post 208 which makes things clearer. Also had wondered what the 3 pin jumper was for, and found the post regarding that (which makes sense... DUH )

Edited July 27, 2015 by Pars