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Stepped Differential Attenuator
Stepped Differential Attenuator
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Old 14th September 2021, 09:16 PM   #1
wtnh is offline wtnh  United States
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Default Stepped Differential Attenuator

I got tired of cobbling together resistors to attenuate amplifier signals when doing distortion measurements. For low-level measurements, having a rats nest of parts and wires on the bench was bad from a noise standpoint as well.

So I decided to put together a fully differential stepped attenuator. I wanted:
  • Either fully differential or single-ended use (switchable)
  • XLR (mini) and BNC connectors
  • Coarse and fine adjustment
I looked around for an easy way (as opposed to something fancy like relays and R2R networks) to do this and came across Joe Broskie's clever idea of using a combination of shunt resistors for coarse adjust and a series string for fine steps. He sells a kit to do this on his Glass-Ware store. Currently, they appear to be out of stock.

The first picture shows the completed unit in a small aluminum case. The second shows the schematic with resistor values and the third shows the internal (somewhat messy) point-to-point wiring. The resistors are mounted directly on the switches.

For the shunt resistors, I used 1%. For the input and series string resistors, I used 0.1% in order to keep common-mode errors low in balanced mode.

Note that the attenuator has an in-built 3 dB insertion loss due to the shunt arrangement, so I have to mentally add in -3 dB when doing measurements. Not a big deal.

The front panel was designed in KiCad and sent off to JLCPCB. I specified aluminum for the substrate. They charged me US$0.50 each (plus shipping) and had them to me in a week. This is a great way to get nice-looking panels done on the cheap.

Cheers!
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File Type: png Screen Shot 2021-09-14 at 2.11.23 PM.png (181.4 KB, 178 views)
File Type: png IMG_1290.png (548.7 KB, 171 views)

Last edited by wtnh; 14th September 2021 at 09:18 PM.
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Old 15th September 2021, 12:12 AM   #2
1audio is offline 1audio  United States
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Stepped Differential Attenuator
The panel option from JLBPCB looks really interesting.

The impedances look low enough that you won't get serious response issues. The easy balanced/unbalanced is a nice trick, especially if you have an isolated floating source.
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Old 15th September 2021, 08:12 AM   #3
phofman is offline phofman  Czech Republic
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I wonder how you ordered the panel on JLCPCB. Aluminum substrate, white PCB color -> black silkscreen, no solder mask, removed order number? Thanks!
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Old 15th September 2021, 01:23 PM   #4
wtnh is offline wtnh  United States
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Exactly right - except I forgot to say no order number.

The aluminum looks nice, but you could also specify a white solder mask on the front with black lettering.

Here is a better picture of the panel.
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Last edited by wtnh; 15th September 2021 at 01:26 PM.
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Old 16th September 2021, 09:28 AM   #5
phofman is offline phofman  Czech Republic
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Perfect, thanks!
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Old 16th September 2021, 10:11 AM   #6
HpW is offline HpW
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Quote:
Originally Posted by wtnh View Post
For the shunt resistors, I used 1%. For the input and series string resistors, I used 0.1% in order to keep common-mode errors low in balanced mode.

Cheers!
What about freq. response and required freq. compensation cap's?

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Old Yesterday, 10:32 PM   #7
wtnh is offline wtnh  United States
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The frequency response is pretty much ruler-flat to 20 Hz to 20KHz at all attenuation steps. This is using short, low capacitance cables and a low impedance output from my sound card. I think that since the resistors are all relatively low value, there is not much roll-off in the audible band.

For my use case, it is a moot point regardless, since I tend to do a calibration loop with the attenuator, notch filter, and soundcard in series, then switch to a low distortion oscillator (Victor's) feeding a DUT and subtracting out the calibration anomalies.
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