Digital gain control w/ analog VCA: PTAT

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Igor Dockx
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Digital gain control w/ analog VCA: PTAT

Post by Igor Dockx » Mon Oct 30, 2017 3:18 pm

Hi,

I am looking into controlling eight VCA's by a single-chip 8-channel DCA with 5V supply.

I digested what I could find about the subject on this forum, and came up with the following existing balanced-drive schematic, which I simulated and proves to be working according to my requirements:
DAC VCA balanced drive.png
The value of R3 and R6 is chosen to have a 100dB VCA control range; Roffset shifts the range by 10dB to get +10dB gain when the DAC spits out 0V; the rescaled control voltage constant = 50mV/dB at the DAC output. (0V @DAC = +10dB, +5V @DAC = -90dB).

Since the eight VCA's and surrounding electronics are to be integrated into a mixing console, which heats up, I'd like to look into incorporating PTAT temperature compensation, for example by using a current source.

What would be the easiest way to incorporate PTAT compensation, preferably for all eight channels at once? The DAC sits at least 5m away from the console, so there will be some cable length to get the PTAT reference voltage to the DAC's ref pin.

Wayne has touched the subject in following post, but i'd love to see it discussed a bit more in-depth... viewtopic.php?f=6&t=473&hilit=DAC+outpu ... t=10#p5300

Thanks,
Igor

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mediatechnology
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Re: Digital gain control w/ analog VCA: PTAT

Post by mediatechnology » Mon Oct 30, 2017 3:56 pm

Gary Hebert sent me this circuit many years ago.
It provides a bipolar voltage-output PTAT using an LM334 as a current sink with PTAT "error."

Image
PTAT using an LM334 current source having bipolar outputs

This one was designed for a 9V circuit and referenced to Vcc/2.
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Igor Dockx
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Re: Digital gain control w/ analog VCA: PTAT

Post by Igor Dockx » Tue Oct 31, 2017 2:58 pm

If I understand correctly, at VCA gains different from 0dB, the VCA's gain drops as temperature rises.

The solution proposed by That Corp in their Design Note 02 does connect between the DAC's output and the rescale buffer, and thus seems to be needed per DAC channel...

Since all eight VCA's in my application do need the same 10dB offset, provided by injecting a 30,5µA offset current at the R3 / R6 / C3 node thru Roffset, am I right in assuming that making this offset current a PTAT temperature-dependent current would be a solution?

Since this is a fixed current, equal for all eight VCA's, can I generate it once and feed it to the eight rescale buffers?

Thanks!

Igor

Igor Dockx
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Re: Digital gain control w/ analog VCA: PTAT

Post by Igor Dockx » Sun Nov 05, 2017 1:18 pm

Okay, I'm not a designer and I'm not familiar working with current mode devices...

But here's an attempt:
DAC VCA balanced drive PTAT compensation draft.png
The LM334 current source produces -10V at the output of U4 at room temperature; R8 turns this into a current of about -30,5µA which shifts the VCA's gain range.

The LM334 generates a current with ≈+0.33%/°C temperature dependence.

Question: What does U4 do to this temperature dependence? Does the same temperature dependence still apply to the current through R8?

Looking forward to your comments...

Thanks,

Igor

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mediatechnology
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Re: Digital gain control w/ analog VCA: PTAT

Post by mediatechnology » Sun Nov 05, 2017 3:32 pm

What does U4 do to this temperature dependence? Does the same temperature dependence still apply to the current through R8?
U4 converts the temp-dependent current to voltage. R8 converts the temp-dependent voltage back to current. You should be able to apply LM334 current directly without the current to voltage conversion of U4 if you can scale it properly.

Could you just read the temperature with an A/D channel and adjust the DAC scaling/offset appropriately? That assumes of that the console's VCAs are at the same internal temperature.
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Igor Dockx
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Re: Digital gain control w/ analog VCA: PTAT

Post by Igor Dockx » Mon Nov 06, 2017 6:19 am

Thanks Wayne,

U4 also inverts the current. I haven't found a way to make the LM334 sink current, as to be able to omit U4...

In my application, the DAC's live in a 19" rack a couple of meters away from the console which holds the VCA's
I'll have to double-check, but the DAC rack is probably quite stable temperature-wise.

My idea was to place the LM334 close to the VCA's in the console, which heats up after a while.

I have one 8-ch DAC chip per block of eight faders. The DAC voltages are transported via an IDC ribbon to the VCA cards.
Ideally, I'd use one LM334 per block of eight VCA's. So, have the LM334 generate enough current to offset eight VCA's, temperature dependent.

Best Igor

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mediatechnology
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Re: Digital gain control w/ analog VCA: PTAT

Post by mediatechnology » Mon Nov 06, 2017 8:58 am

Igor Dockx wrote:
Mon Nov 06, 2017 6:19 am
Thanks Wayne,

U4 also inverts the current. I haven't found a way to make the LM334 sink current, as to be able to omit U4...
The LM334 is essentially a two-terminal device. No reason it cant sink current.
Igor Dockx wrote:
Mon Nov 06, 2017 6:19 am
In my application, the DAC's live in a 19" rack a couple of meters away from the console which holds the VCA'sI'll have to double-check, but the DAC rack is probably quite stable temperature-wise.

My idea was to place the LM334 close to the VCA's in the console, which heats up after a while.
Remotely locate the LM334 in the console and send the reading to the DAC rack. That would seem to eliminate a lot of extra op amps.
If there's one U4 per bucket of 8 VCAs then I guess its not so bad.
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diffuse371
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Re: Digital gain control w/ analog VCA: PTAT

Post by diffuse371 » Mon Nov 13, 2017 11:18 pm

For temperature compensation of the kT/Q term of a VBE you need to multiply by the PTAT reference. All you have at the moment is a constant that is compensated. You are not compensating the voltage control coming from the DAC.

For a pure voltage controlled amplitude application a resistor with temperature dependence of +3300PPM can be used to multiply by PTAT.

Else, you need to multiply the PTAT term generated by the LM334 using an IC like an AD633. This works very well, although the cost may be hard to justify. Watch out for input and output offsets on the AD633.

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mediatechnology
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Re: Digital gain control w/ analog VCA: PTAT

Post by mediatechnology » Tue Nov 14, 2017 5:32 am

I was thinking he could take the analog PTAT measurement and perform the multiplication in software.
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diffuse371
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Re: Digital gain control w/ analog VCA: PTAT

Post by diffuse371 » Tue Nov 14, 2017 7:13 am

That's a nice idea. 8 AD633s is hard to justify the cost of. You need to be careful about gain staging too, but it's easier that than hand trim the offsets.

I can confirm that the LM234 is a good performance technique for compensating gain control elements in timed circuits like VCOs or filters though.

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