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Old 15th December 2004, 11:28 PM   #1
fallow is offline fallow  Finland
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Default PGA2310 noise

Hi,

I have been building an preamplifier. Volume controller is PGA2310, but it has rather large
output noise, which is irratating(just hearable within 0.5m from speaker) at low volumes. Chip is between two operational amplifiers(OPA 227)

Then I got a "brigth" idea, to lower the noise of chip. Before the chip the voltage levels are
raised to four times original, and after the chip the voltage levels are lowered to line level signal(divided by four), so the noise should be also divided by four, am I correct?

At least the output noise is lowered to point which it is hearable about 10 cm from speaker.
Now was thinking maybe the output level could be divided futhermore, at least 3db more. But there is an possibility that the volume levels aren't suffient anymore, if I want someday use my system with less sensitive speakers(now sensitivity is 89db/1W) the volume levels aren't suffient anymore.

Is there any other way to reduse the noise on chip?
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Old 16th December 2004, 12:03 AM   #2
gmarsh is offline gmarsh  Canada
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sure it's not something other than the PGA2310?

I'm using the chip in a product at work, and i've measured an unweighted, 22hz-30khz, >100dB SNR through the entire product...
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Old 16th December 2004, 06:25 PM   #3
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Hi fallow ,
Are you sure ? The noise from PGA2310 chip .
These chip PGA2310 , CS3310 is good in volume control .
This site ( http://smartlee.idv.tw/pic/cs3310report.htm) have A.P.
test report use cs3310 chip .
Check your layout or power supply (Va & Vd is isolate) ?
good luck !
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Old 16th December 2004, 07:29 PM   #4
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I agree, it is unlikely that it is the chip. Is it the same on both channels? How are the opamps connected, do you have a diagram? How's the supply system configured?
You may have some hf oscillation, that very often manifests itself as noise.

Jan Didden
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Old 17th December 2004, 12:00 AM   #5
Alien8 is offline Alien8  Belgium
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Default Weird...

The only way I could have the pga's produce noise is to give them a setting above +15dB. Otherwise they are extremely quiet.

The only difficulty I have is some HF (25mV @ 10MHz) PSU noise on 'all' supplies which leaks through to the audio ouputs as well. But then again I seem to have trouble hearing that

You did connect the groundplanes for analog and digital grounds together ? Not that it matters a lot for the noise.

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Old 21st December 2004, 11:52 AM   #6
fallow is offline fallow  Finland
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Hi,
Here is a picture of shcemtic of amplifier.

addition to shematic:
The operational amplifiers are opa2227.
Attached Images
File Type: png amplifier_schematic.png (50.4 KB, 1336 views)
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Old 21st December 2004, 11:53 AM   #7
fallow is offline fallow  Finland
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and here are picuture of board
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File Type: png amplifier_board.png (30.6 KB, 1124 views)
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Old 21st December 2004, 12:11 PM   #8
fallow is offline fallow  Finland
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For the power supply, there are no picture available now,
but it is done by single 15VA 2x15V toroidial transformer. The voltage regulation to +5 V and +15 V is done by LM 317 and for -15V by LM337. Before each regulators there are two 1000uF capacitors(Panasonic FC) and one 100nF, after the regulator there are 3 220 nF and one 100 nF. The ground planes of digital and analog supply is compined rigth after diode bridge.(trying to find them somewhere if you wish to see them, and think they would be helpfull)

The output level of preamplifier, when no signal is feeded, is <20 micro voltages, don't know for sure, because the base noise on spectrum analyzer were around 20 micro voltages. (and when the amplifier is on or off the noise level did not rise.)
And when feeding a signal the signal peek is showing around amplification -92dB to -93dB.

Before dividing the output level by four(increasing before PGA2310 by four) when turning amplifier on rised the noise level in analyzer.

In both channel the behavioral is similar.
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Old 21st December 2004, 01:07 PM   #9
fallow is offline fallow  Finland
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could't find edit button...

Founded schematic of power supply.

and there were slight differences with which I've described in last message....
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File Type: png power_schematic.png (29.4 KB, 1057 views)
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Old 21st December 2004, 01:32 PM   #10
Alien8 is offline Alien8  Belgium
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Fallow

Did you try to connect the groundplanes together right next to (or under) the pga chip ?
This is stated in the product sheet.
Right now your 2 PSU's only share a common ground way back with the big caps, seems like a pretty long way for noise which can be picked up. I've found the 2310 to be quite sensitive to potential differences (and noise) between the digital and analog grounds. Problems went away when I connected them together next to the chip - minimal distance possible.

HTH

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