[sdiy] Niche ACM-1 how's this thing work?

brianw brianw at audiobanshee.com
Wed Mar 22 22:25:58 CET 2023

On Mar 22, 2023, at 1:11 AM, Roman Sowa <modular at go2.pl> wrote:
> That's right not every DAC goes up with bandwidth to the same frequency, but I have yet to see a DAC that cannot reach even 20kHz. Usually it's in range of 100s of kHz up to several MHz, and all of that in low speed DACs.
> For example out of 600 1-channel >=12-bit DACs in stock at mouser sorted by price, the cheapest ones offer from 50 to 500kHz with -3dB bandwidth.

You're citing the specs for the DAC output. Those are indeed adequate, but it's a different pin.

I'm talking about the specs for the Vref input pin. Those are often designed to reject noise on the reference voltage. Some chips even have an extra pin for a large capacitor to filter Vref. Others have an internal filter or both internal and external. In other words, the bandwidth is completely unrelated to the DAC signal output.

Keep in mind that even something as simple as an op-amp can have different specs (voltage range, bandwidth) for input versus output.

> Maybe there's one peculiar DAC having problems with AC at Vref (I can't see a reason why it should have) but why bother looking for it since $1 part already does the job. And if I want better performance I can go straight to DAC7811, which is real multiplying DAC and even offers low glitch on code changes.

Yes. A real multiplying DAC (MDAC) will be designed so that the Vref input response has full bandwidth for the signal types expected.

> So yes, I will say ANY DAC

A standard DAC will be designed to limit the bandwidth of Vref as much as possible. Two different things: MDAC versus DAC.

> Roman
> W dniu 2023-03-21 o 17:25, brian pisze:
>> Not every DAC has a flat, 20 kHz bandwidth response to signals applied to its Vref input. Hell, not even every op-amp guarantees flat audio response across the full range of inputs. Fortunately, for op-amps, we have over 10,000 different models to choose from. Not so much for DAC chips. So, you can't really use "any" DAC chip for this, without even mentioning the type of DAC.
>> Brian
>> On Mar 21, 2023, at 3:25 AM, Roman Sowa <modular at go2.pl> wrote:
>>> You can make multiplying DAC out of 2 any DACs with external reference input. Both DACs must be controlled by the same SPI. The Vref of 1st DAC is driven with your input of interest, say mxer's channel audio, but with added voltage offset to midscale so the signal fits within DAC's ratings. The Vref of 2nd DAC takes only the offset itself. Then the output of 2nd DAC is reversed and added to the output of 1st DAC.
>>> Despite the hassle and complexity it's still cheaper than "real" multiplying DAC.
>>> Roman
>>> W dniu 2023-03-20 o 23:29, Paul Perry via Synth-diy pisze:
>>>> This reminds me that I can't seem to find any cheap available multiplying DACs. Any suggestions? Are they abandoned technology?
>>>> paul perry Melbourne Australia

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