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Old 10th June 2011, 10:46 AM   #1
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Default Chip amps as I/V converters

Hi,

I was wondering if anyone evertried using a chipamp as the IV converter of a Sigma delta DAC (for example AD1853). Then use the differential output of the IV converter to drive the speakers directly. It would be sort of a bridge amplifiers.

Volume control would be implemented digitally. of course somewhere along the way somebody has to upsample the 16 bit to 24 bit.

Of course you would need a much higher resistor in the I/V to implement the gain stage. Any thoughts on that?

PowerDaC of sorts.

Oon
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Old 10th June 2011, 11:07 AM   #2
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Quote:
Originally Posted by oon_the_kid View Post
Hi,

I was wondering if anyone evertried using a chipamp as the IV converter of a Sigma delta DAC (for example AD1853). Then use the differential output of the IV converter to drive the speakers directly. It would be sort of a bridge amplifiers.

Volume control would be implemented digitally. of course somewhere along the way somebody has to upsample the 16 bit to 24 bit.

Of course you would need a much higher resistor in the I/V to implement the gain stage. Any thoughts on that?

PowerDaC of sorts.

Oon
I think you have it worked out. Don't see any reason why it shouldn't work.
Not sure about stability, with the LM3668 (for example) not being unity gain stable, but I don't know how that works out in an I/V config.
Maybe just try or sim?

jan didden
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Old 10th June 2011, 02:26 PM   #3
qusp is offline qusp  Australia
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I do the same thing, but with a transportable headphone amp, i only need about 1.5x differential gain though, so the IV resistor can stay pretty small for lower noise. i use opa1632dgn on the output of a sabre 9018 and use the dacs digital volume. it works really very well. of course you would want an additional current gain stage.
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Old 10th June 2011, 02:51 PM   #4
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Quote:
Originally Posted by oon_the_kid View Post
I was wondering if anyone evertried using a chipamp as the IV converter of a Sigma delta DAC (for example AD1853).
Haven't actually tried this myself but have played with the sibling part to the AD1853 into various I/V opamps. Given the minimum gain of the chip amp (26dB is it?) you'll find yourself rather quickly running out of slew rate. But don't let that put you off....

Quote:
Volume control would be implemented digitally. of course somewhere along the way somebody has to upsample the 16 bit to 24 bit.
AD1853 has a rather neat volume control capability which doesn't look to be digital - you can vary the reference current. Think its good for around 40dB range.
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Old 10th June 2011, 03:38 PM   #5
Ipanema is offline Ipanema  Malaysia
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Opamp I/V converter works as unity gain and most power opamps are not stable at unity gain. Won't work except for OPA549 etc.

Power opamp tends to be noisier than small-signal opamp.

A scaled up version of Pass D1 or Zen will work well as power I/V having low noise and without stability issue.
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Old 11th June 2011, 04:36 PM   #6
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As far as I know, the input pin of chipamps is the non-inverting one. However, an IV converter uses the inverting input. I see some problems with input impedance.
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Old 11th June 2011, 04:48 PM   #7
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Chipamps can be configured as inverting or non-inverting. Both + and - pins work fine as inputs. The main problem, as Ipanema mentions is the lack of unity gain stability. A 'gain stealing' network could be used but this would probably compromise settling performance rather badly.
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Old 13th June 2011, 02:08 AM   #8
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Originally Posted by abraxalito View Post
Chipamps can be configured as inverting or non-inverting. Both + and - pins work fine as inputs. The main problem, as Ipanema mentions is the lack of unity gain stability. A 'gain stealing' network could be used but this would probably compromise settling performance rather badly.
Technically, the I/V is a openloop inverting amplifier rather than unity gain voltage amp. Not sure how stable a chipamp in a that configuration, but I believe most op-amps used for I-V conversion are probably not unity gain stable either. However there is the DC issue that must be taken care off too I believe... since most DACs are run off single supply hence a DC output will probably exist...

Oon
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Old 13th June 2011, 02:09 AM   #9
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Quote:
Originally Posted by Ipanema View Post
Opamp I/V converter works as unity gain and most power opamps are not stable at unity gain. Won't work except for OPA549 etc.

Power opamp tends to be noisier than small-signal opamp.

A scaled up version of Pass D1 or Zen will work well as power I/V having low noise and without stability issue.
Having a thread on the Pass side

Oon
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Old 13th June 2011, 02:12 AM   #10
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Originally Posted by abraxalito View Post
Haven't actually tried this myself but have played with the sibling part to the AD1853 into various I/V opamps. Given the minimum gain of the chip amp (26dB is it?) you'll find yourself rather quickly running out of slew rate. But don't let that put you off....



AD1853 has a rather neat volume control capability which doesn't look to be digital - you can vary the reference current. Think its good for around 40dB range.
Did you manage to try the volume control? Can I just put a variable resistor in series with the the reference resistor and that will do the trick?

Oon
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