VCA Multiplying DAC.

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terkio
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VCA Multiplying DAC.

Post by terkio » Fri Sep 20, 2019 7:41 am

I am on a VCA design where I need high headroom, high accuracy ( resolution and stability )

I am thinking to use 16 bits MDACs.
DAC8812, AD5545. From TI, AD they look the same.

The data sheets give -104dB THD at 5Vpp. That is 0.0005%. How do they perform at higher levels ?
The AD5545 only mentions AC ability with Vref +12V / -12 V, the DAC8812 ??
Who knows, how close to max rating do they have a low thd ?

These MDACs are monotonic and I do need monotonicity.
The data sheets warns that the I-V op amp should be a precision one, but does not help much.
It says low ibias is needed to insure monotonicity, and mentions low Vos.
I understand about ibiais, because of the impedance seen from the op amp input that varies with the 16 bits code. However I do not see
about Vos.
IMO, Vos has no impact on monotonicity, I would like to make sure, though.
OPA 1612 seems, not good enough about ibias.
So I am thinking to use OPA 1642 for it's FET input: 20pA max ibias, ios, despite it's 3.5mV max Vos. However I wonder about Vos temperature drift that is not specified.
A higher cost alternative is OPA828 a FET input with very low Vos and drift.

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mediatechnology
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Re: VCA Multiplying DAC.

Post by mediatechnology » Fri Sep 20, 2019 8:50 am

@20 kHZ the DAC8812 only provides 78 dB attenuation.
Is that enough?
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terkio
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Re: VCA Multiplying DAC.

Post by terkio » Fri Sep 20, 2019 10:05 am

I need at least: +30dB -10dB gain range and 50 KHz a bandwidth.

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JR.
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Re: VCA Multiplying DAC.

Post by JR. » Fri Sep 20, 2019 10:11 am

I looked at MDACs back in the 80s trying to source replacements for the expensive and modest fidelity VCAs of the time. They did not look promising back then for audio gain manipulation.

Hopefully modern MDACs are better/easy to apply.

As I recall early DPOTs were not very good either, now they don't suck.

Good luck.

JR
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mediatechnology
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Re: VCA Multiplying DAC.

Post by mediatechnology » Fri Sep 20, 2019 11:24 am

You mentioned that the THD specs for the DAC8812 weren't specified for operation at reference levels >5V.
That may be because Vcc max is 5V and though the maximum for the reference is +/-18V the distortion performance of the FET(?) switches probably degrade rapidly.
It is unlikely the gates are bootstrapped.
https://ka-electronics.com

Plandemic Series: "Pland3mic - Indoctornation" https://proaudiodesignforum.com/content ... nation.mp4
Plandemic Series: "Plandemic - Doctors In Black" https://proaudiodesignforum.com/content/Plandemic.mp4

CDC: "For 6% of the deaths, COVID-19 was the only cause mentioned. For deaths with conditions or causes in addition to COVID-19, on average, there were 2.6 additional conditions or causes per death." https://www.cdc.gov/nchs/nvss/vsrr/covi ... /index.htm

terkio
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Re: VCA Multiplying DAC.

Post by terkio » Fri Sep 20, 2019 12:37 pm

I was afraid of such a limitation from the FET switches control voltage.
It's even worse, their 0.0005% thd spec is at 5V peak TO PEAK.
So the AD5545 may be better since this one mentions being AC able for Vref +12 V -12 V.
May be not, that is 24Vpp able, but they don't give a thd figure at this level.
It is quite strange to see such vagueness about the multiplying feature of products that are claimed multiplying DACs.
May be, all this hides they are only usable for best audio at levels below 7.4dBu ( 2.5 Vpeak, 1.77 Vrms ).
Running them below this level, what S/N could be expected ?

terkio
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Re: VCA Multiplying DAC.

Post by terkio » Tue Sep 24, 2019 12:01 pm

mediatechnology wrote:
Fri Sep 20, 2019 11:24 am
You mentioned that the THD specs for the DAC8812 weren't specified for operation at reference levels >5V.
That may be because Vcc max is 5V and though the maximum for the reference is +/-18V the distortion performance of the FET(?) switches probably degrade rapidly.
It is unlikely the gates are bootstrapped.
Indeed, presumably the thd distortion performance degrades rapidly at higher levels.
In that case, the suggestion of a fully balanced design is very interesting.
Twice more mDACs is not so bad cost wise because this gives twice the resolution.
With a dual 14bits mDACs one has the equivalent of a 15 bits single. The extra bit is obtained by setting the two mDacs with N and N+1 to obtain the equivalent of N+0.5.

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