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low-distortion gain control

P

Pooh Bear

Jan 1, 1970
0
Winfield said:
What do you think of TI's dac8811 as a low-distortion gain control
(less than 0.0025% at 35kHz)? What opamp would you use with it?
http://focus.ti.com/docs/prod/folders/print/dac8811.html

Looks good. Been a long time since I used an MDAC but they're handy.
Only fly in the ointment is the 12nV/rtHz noise. Quieter would be nicer.

For audio - indeed even 35kHz I'd probably choose the 20 cent 5532 dual
op-amp. For DC < whoops ! > precision you'd want something finer. TI's
Burr Brown division also makes some fine op-amps but I just get to use
commodity parts these days, so I'm not the man to advise.

Graham
 
B

Ban

Jan 1, 1970
0
Winfield said:
What do you think of TI's dac8811 as a low-distortion gain control
(less than 0.0025% at 35kHz)? What opamp would you use with it?
http://focus.ti.com/docs/prod/folders/print/dac8811.html

I'm a little bit concerned with the zipper-noise, that would be heard when
using it for audio. For unipolar instrumentation it seems to be OK. OP627,
fast enough that the glitches(fig.9 -30mV) don't upset the input.
If you want a good audio gain control have a look at PGA2311 or PGA2310
http://focus.ti.com/docs/prod/folders/print/pga2311.html
These are classes better, stereo, much more dynamic and control range and
you can change the gain at 0-xssings, which avoids the zipper-noise.
 
P

Pooh Bear

Jan 1, 1970
0
Ban said:
I'm a little bit concerned with the zipper-noise, that would be heard when
using it for audio. For unipolar instrumentation it seems to be OK. OP627,
fast enough that the glitches(fig.9 -30mV) don't upset the input.
If you want a good audio gain control have a look at PGA2311 or PGA2310
http://focus.ti.com/docs/prod/folders/print/pga2311.html
These are classes better, stereo, much more dynamic and control range and
you can change the gain at 0-xssings, which avoids the zipper-noise.

For pure audio the PGA range is indeed the preferred choice.

Actually, as I understand it, it doesn't matter where gain changes are made.
If you make an instantaneous gain change at Vpk then you get the largest
deltaV. Zero-crossing yields zero deltaV but yields instead the highest
possible delta (dV/dt). I'll bet Mr Fourier says they're all but equivalent.
I.e. - no free lunch.

Ramping gain up / down exponentially / log - whatever should be quietest for
zipper noise. Looks like I'm going to have to work on this myself soon.

Graham
 
M

martin griffith

Jan 1, 1970
0
Looks good. Been a long time since I used an MDAC but they're handy.
Only fly in the ointment is the 12nV/rtHz noise. Quieter would be nicer.

For audio - indeed even 35kHz I'd probably choose the 20 cent 5532 dual
op-amp. For DC < whoops ! > precision you'd want something finer. TI's
Burr Brown division also makes some fine op-amps but I just get to use
commodity parts these days, so I'm not the man to advise.

Graham
Going by the TI data sheet, the 5534/2 aint that good at 35k, 0.004%
approx, at AV=1 It's almost good enough for my hearing though,.

The AD797 claims to be 0.0003% and the LT1115 dont show graphs going
up above 20K.




martin
 
M

martin griffith

Jan 1, 1970
0
For pure audio the PGA range is indeed the preferred choice.

Actually, as I understand it, it doesn't matter where gain changes are made.
If you make an instantaneous gain change at Vpk then you get the largest
deltaV. Zero-crossing yields zero deltaV but yields instead the highest
possible delta (dV/dt). I'll bet Mr Fourier says they're all but equivalent.
I.e. - no free lunch.

Ramping gain up / down exponentially / log - whatever should be quietest for
zipper noise. Looks like I'm going to have to work on this myself soon.

Graham
I think I saw that someone had designed an audio compressor around
the 2310, possibly on the prodigi forum.
But long time ago, I was trying to use parallel DACS in Eq's, the
biggest problem was charge injection zipper noise, especially when the
MSB flips. I wonder how TI do it?


martin
 
W

Winfield Hill

Jan 1, 1970
0
martin griffith wrote...
Going by the TI data sheet, the 5534/2 aint that good at 35k, 0.004%
approx, at AV=1 It's almost good enough for my hearing though,.

The AD797 claims to be 0.0003% and the LT1115 don't show graphs going
up above 20K.

I could probably live with 40ppm rather than 25pm (this isn't an audio
app, but rather for instrumentation), but what would be a good choice,
better than the 5532 and not as painful to use as an AD797 or LT1115?
I don't want high Ib, In, Is, Pd, nor do I need the 0.9nV input noise.
But 20V/us and 110MHz GBW should help insure low distortion at 30kHz.

- an hour passes -

I just looked at the LT1468 suggested in the appnote in Fred's post,
it's an improvement over the AD797. Consider, same 2.5ppm distortion
at 30kHz, yet 3nA Ib instead of 250nA, and half the supply dissipation.
I mean, 1/4 watt in an 8-soic is pretty severe, if it's not necessary,
and I'll have 30 of them in this project.

Both opamps use the bootstrapped-output-stage-driver trick, including
the distortion-canceling capacitor Analog Devices talks about (page 8),
but with opposite polarity transistors (a more detailed schematic of
the LT1468 appears in George Feliz' 3-page article in Linear Technology
Magazine, Nov 1998). I also like the LT1468's more reasonable price.

Next I'll examine the pga2310 and pga2311 suggested by ban (although
zipper-noise doesn't worry me, in my instrument the gain settings will
be fixed), one of those may trump the dac8811 + LT1468 combination.
At first glance the 30kHz distortion looks 10x higher, but we'll see.
 
K

Ken Smith

Jan 1, 1970
0
martin griffith said:
The AD797 claims to be 0.0003% and the LT1115 dont show graphs going
up above 20K.

Try the LT1028's data sheet. The LT1115 and the LT1028 are really the
same part.
 
B

Ban

Jan 1, 1970
0
Winfield said:
martin griffith wrote...

I could probably live with 40ppm rather than 25pm (this isn't an audio
app, but rather for instrumentation), but what would be a good choice,
better than the 5532 and not as painful to use as an AD797 or LT1115?
I don't want high Ib, In, Is, Pd, nor do I need the 0.9nV input noise.
But 20V/us and 110MHz GBW should help insure low distortion at 30kHz.

- an hour passes -

I just looked at the LT1468 suggested in the appnote in Fred's post,
it's an improvement over the AD797. Consider, same 2.5ppm distortion
at 30kHz, yet 3nA Ib instead of 250nA, and half the supply
dissipation. I mean, 1/4 watt in an 8-soic is pretty severe, if it's
not necessary, and I'll have 30 of them in this project.

Both opamps use the bootstrapped-output-stage-driver trick, including
the distortion-canceling capacitor Analog Devices talks about (page
8), but with opposite polarity transistors (a more detailed schematic
of the LT1468 appears in George Feliz' 3-page article in Linear
Technology Magazine, Nov 1998). I also like the LT1468's more
reasonable price.

Next I'll examine the pga2310 and pga2311 suggested by ban (although
zipper-noise doesn't worry me, in my instrument the gain settings will
be fixed), one of those may trump the dac8811 + LT1468 combination.
At first glance the 30kHz distortion looks 10x higher, but we'll see.

I wonder because the graphs show at 20kHz less distortion than the 8811.
If you want to use a fine external amp the Wolfson WM8816 is similar to the
TI device, but requires external opamps. The big question if you want to
make a dB-linear attenuator with 0.5dB or bigger steps, the approach will be
much easier software wise. Also I do not like Figure8 of the dac 8811, if
you want more than 60dB attenuation. Depends all on your application.
 
J

John Larkin

Jan 1, 1970
0
What do you think of TI's dac8811 as a low-distortion gain control
(less than 0.0025% at 35kHz)? What opamp would you use with it?
http://focus.ti.com/docs/prod/folders/print/dac8811.html

Funny, we're doing something very similar right now: a 16-bit bipolar
dac, feeding a 10-bit 2q mdac to scale the range of the whole thing,
steps from +-10v to +-2mv. We're probably going to use a DAC8831
feeding an AD5432, both serial. We'll use slow cmos chopamps for high
analog precision.

We looked at using digital pots as the gain scalers, but they often
seem to have scairy linearity behavior near the bottom tap. The r-2r
dacs don't guarantee the absolute division accuracy when the code is,
say, 0000000001 (lsb only, divide by 1024) either, but they still look
like a safer bet.

John
 
W

Winfield Hill

Jan 1, 1970
0
Ban wrote...
I wonder because the graphs show at 20kHz less distortion than the 8811.

Aha! You failed to see they take the measurements with a 20kHz filter,
which of course reduces the distortion harmonics for fundamentals over
10kHz. See the plot going down? Very fishy, until you see the note.
If you want to use a fine external amp the Wolfson WM8816 is similar to
the TI device, but requires external opamps. The big question if you want
to make a dB-linear attenuator with 0.5dB or bigger steps, the approach
will be much easier software wise. Also I do not like Figure8 of the
dac 8811, if you want more than 60dB attenuation. Depends all on your
application.

Indeed. Perhaps I shouldn't use the word "gain control" because I'm
just setting the operating level for a precision capacitance sensor.
 
B

Ban

Jan 1, 1970
0
Winfield said:
Aha! You failed to see they take the measurements with a 20kHz
filter, which of course reduces the distortion harmonics for
fundamentals over 10kHz. See the plot going down? Very fishy, until
you see the note.

Yeah, I had another look, that filter must cut in quite strongly. The
built-in opamp is not optimized for BW, but noise. And has only 3MHz GBW.

Indeed. Perhaps I shouldn't use the word "gain control" because I'm
just setting the operating level for a precision capacitance sensor.

Ahh, quite different from audio, where also very low frequencies matter.
With 35kHz you are well above the noise corner of cmos-opamps, which are
avoided for audio.
The Wolfson is IMHO the best digital attenuator for Audio use, especially in
balanced operation. You can achieve >120dB dynamic range.
 
M

martin griffith

Jan 1, 1970
0
Winfield Hill wrote:
replace good stuff with a said:
Ahh, quite different from audio, where also very low frequencies matter.
With 35kHz you are well above the noise corner of cmos-opamps, which are
avoided for audio.
The Wolfson is IMHO the best digital attenuator for Audio use, especially in
balanced operation. You can achieve >120dB dynamic range.

I looked at the WM8816 a couple of years ago with Barry Porter, for an
audio project, but ISTR that there was a latch up problem,mentioned in
athe data sheet. any way Baz had laid out the board wrong, and we had
to hand wire with kynar wire the chips to the board. This destroyed
them. They seemed very static sensitive

Have they improved the ic at all in this respect, I'd love to try them
again. Not having a internal opamp is most excellent.


martin
 
S

Stephan Goldstein

Jan 1, 1970
0
Try the LT1028's data sheet. The LT1115 and the LT1028 are really the
same part.

Yes, but beware of both when comparing to the AD797. Note that the LT data
sheets don't show wideband noise (as in above the audio band). They actually
have a rather large noise peak around 5-6MHz, you need to be careful about
this. The AD797 does not have this "feature", it's well-behaved noisewise
everywhere. BTW, I learned about this from the AD797 designer, and as a
disclaimer I also work at ADI by day.

Steve
 
P

Pooh Bear

Jan 1, 1970
0
martin said:
I think I saw that someone had designed an audio compressor around
the 2310, possibly on the prodigi forum.
But long time ago, I was trying to use parallel DACS in Eq's, the
biggest problem was charge injection zipper noise, especially when the
MSB flips. I wonder how TI do it?

This is where the internal design of the part really counts.

I'll be doing level control in DSP so I don't have that particular problem.

Graham
 
K

Ken Smith

Jan 1, 1970
0
Stephan Goldstein said:
Yes, but beware of both when comparing to the AD797. Note that the LT data
sheets don't show wideband noise (as in above the audio band). They actually
have a rather large noise peak around 5-6MHz, you need to be careful about
this. The AD797 does not have this "feature", it's well-behaved noisewise
everywhere. BTW, I learned about this from the AD797 designer, and as a
disclaimer I also work at ADI by day.

Yes, the noise peak can be well below that point depending on the exact
unit you pick. At about 100KHz, they show about 3nV/sqrt(Hz). The noise
increase is due to the fall of gain in the first stage letting the second
stage's noise show up.

I does strike me as odd that no-one has brought out op-amps much below
1nV/sqrt(Hz). It seems that there would be enough applications that
wouldn't mind the higher current etc that would be needed to make it
happen. It is after all, mostly a question of input transistor size and
current.
 
J

John Larkin

Jan 1, 1970
0
Try the LT1028's data sheet. The LT1115 and the LT1028 are really the
same part.

LT1028 has low voltage noise and huge current noise, so works well in
very low-impedance circuits, probably not a good choice to back-end a
current-mode dec.

John
 
B

Ban

Jan 1, 1970
0
John said:
LT1028 has low voltage noise and huge current noise, so works well in
very low-impedance circuits, probably not a good choice to back-end a
current-mode dec.

John

I wrote already that I like the OPA627, but I got the prefix wrong. This amp
will probably do better in this application, the distortion is maybe 0.001%
at 35kHz in unity gain, but it is quite costly. At least the datasheet shows
a filter only at 80kHz, so it is possible to predict the performance.
 
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