PGA2310 output buffer? - Page 2

Freddie · 25th August 2003, 02:48 PM

Another Buffer...

Freddie · 25th August 2003, 03:05 PM

Oh, yes the Borbely buffer

capslock · 25th August 2003, 04:22 PM

The problem with the PGA is that you have no access to the feedback loop. So if you use any of these buffers which have no overall feedback loop, you have the worst of both worlds, i.e. a mediocre opamp with low open loop bandwidth and lots of feedback and a "no-feedback" buffer which has considerable residual distortion. This is going to satify neither school of thought...

Freddie · 25th August 2003, 04:35 PM

Quote:

The problem with the PGA is that you have no access to the feedback loop. So if you use any of these buffers which have no overall feedback loop, you have the worst of both worlds, i.e. a mediocre opamp with low open loop bandwidth and lots of feedback and a "no-feedback" buffer which has considerable residual distortion. This is going to satify neither school of thought...

Well.. One alternative is to forget about the PGA2310 and get an LM1972 instead. The LM1972 have no internal opamp, and should have a buffer at the output. Or run the PGA2310 without external buffer.

However, why can't I place an as you call "no-feedback" buffer after the PGA2310 if the buffer itself has low distorsion without feedback? I have seen some designs that have the outputstage (buffer) outside the feedback loop.

/Freddie

Freddie · 26th August 2003, 11:45 AM

In case someone is interested this is the buffer that I'm going to try. Thanks Sonny A.

I will probably try the borbely buffer too.

sonnya · 27th August 2003, 07:03 AM

Quote:

Originally posted by capslock
The problem with the PGA is that you have no access to the feedback loop. So if you use any of these buffers which have no overall feedback loop, you have the worst of both worlds, i.e. a mediocre opamp with low open loop bandwidth and lots of feedback and a "no-feedback" buffer which has considerable residual distortion. This is going to satify neither school of thought...

I do not think that the PGA is that bad a performer. i would estimate thd @ 20k to around .004% given from their datasheet and the bandwidth filter they use. and this is 3Vrms into 600R load.

If it should be better then there is PGA2311 (+/-5V suppply) in a-grade.

at 2Vrms (limited by supply voltage)

.0002% distortion at 1KHz
.001% distortion at 10kHz
and i would estimate .003% at 20KHz

is this "mediocre opamp" and mediocre "analog switch" performance?

capslock · 27th August 2003, 10:41 AM

Yeah, I wondered passingly about the PGA2311, how can it be better if it is only a +/- 5 V design?

Well the specs of the OPA2134 (that's essentially what is inside the PGA) are certainly good, but there are some golden ears that say that these are not enough. I would assume one needs to look at high order distortion products (mainly from crossover switching, probably hidden in the noise) or intermodulation distortion. There are certainly better op amps (AD797, OPA627), and you can always go discrete.

My experience with Op amps in my DAC is that there seem to be subtle differences between op amps with nominally adequate specs (such as the 2134 or the 2604) and some of the really good ones.

The no-feedback school is going to cry for another reason: what use is a unity gain, discrete, no global feedback buffer if the signal has already passed through one of those nasty op amps using lots of global feedback?

Freddie · 27th August 2003, 11:07 AM

capslock,

Quote:

The no-feedback school is going to cry for another reason: what use is a unity gain, discrete, no global feedback buffer if the signal has already passed through one of those nasty op amps using lots of global feedback?

One reason for placing a buffer on the output of PGA2310 is that hard loads for integrated circuits will introduce thermal distorsion (D. Self).

But what would you suggest. An opamp as buffer? No buffer at all? A discrete opamp?

/Freddie

sonnya · 27th August 2003, 09:09 PM

Quote:

Originally posted by capslock
Yeah, I wondered passingly about the PGA2311, how can it be better if it is only a +/- 5 V design?

its all about the analog switches. It looks like when they go down in supply voltage, the Ron and Delta Ron get up to ten times lower (+/-2.5V or +5V) than with +/-15V supply voltage ... Something to do with the process they are using. And geometry of the mosfets used in the swithes.

And +/- 2.5V supply is more than adequate if you use the switches right... In virtual ground mode....

Quote:

Originally posted by capslock

Well the specs of the OPA2134 (that's essentially what is inside the PGA) are certainly good, but there are some golden ears that say that these are not enough. I would assume one needs to look at high order distortion products (mainly from crossover switching, probably hidden in the noise) or intermodulation distortion. There are certainly better op amps (AD797, OPA627), and you can always go discrete.

I agree perfectly with you... run an IMD test with 18KHz+19KHz tones and check for harmonics ... Wonder if ... only the test equipment is good enough to reveal the differences of the really good opamp designs....

Maybe a settling test would be an good indicator too.....???

Quote:

Originally posted by capslock

My experience with Op amps in my DAC is that there seem to be subtle differences between op amps with nominally adequate specs (such as the 2134 or the 2604) and some of the really good ones.

I agree with you.

Quote:

Originally posted by capslock

The no-feedback school is going to cry for another reason: what use is a unity gain, discrete, no global feedback buffer if the signal has already passed through one of those nasty op amps using lots of global feedback?

Thats another story...

Only .. distortion introduced thermally could be minimized by adding a buffer....

But it would be very simple to make an active attenuator with high perfomance swithes and a discrete I/V converter with no feedback and a discrete buffer.... should be very easy to get sub -80dB THD.... When done right...

the sims shows it... just have to find some time to do real life tests....

sonnya · 27th August 2003, 10:22 PM

just wanted to post some FFT but even in zip-file the size is 170K

so i am only going mention the results

at 1KHz :

350mVrms - 2nd : -105dB - 3rd : -150dB
700mVrms - 2nd : -100dB - 3rd : -135dB
2.000Vrms - 2nd : -90dB - 3rd : -120dB - 4th : -150dB

IMD 19k+20k 350mVrms each :

all other components is 100dB below 19k+20k.

so total IMD is like ~ -92dB..

This is a discrete I/V without feedback.... and without the buffer...

so if you can live with that you can use a discrete No global feedback I/V or add a buffer and make the VAS node high impedance add a 33pF cap to the node and add a feedback resistor... and there you go...

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25th August 2003, 04:22 PM	#13
capslock diyAudio Member Join Date: Feb 2002 Location: Germany	The problem with the PGA is that you have no access to the feedback loop. So if you use any of these buffers which have no overall feedback loop, you have the worst of both worlds, i.e. a mediocre opamp with low open loop bandwidth and lots of feedback and a "no-feedback" buffer which has considerable residual distortion. This is going to satify neither school of thought...

27th August 2003, 10:41 AM	#17
capslock diyAudio Member Join Date: Feb 2002 Location: Germany	Yeah, I wondered passingly about the PGA2311, how can it be better if it is only a +/- 5 V design? Well the specs of the OPA2134 (that's essentially what is inside the PGA) are certainly good, but there are some golden ears that say that these are not enough. I would assume one needs to look at high order distortion products (mainly from crossover switching, probably hidden in the noise) or intermodulation distortion. There are certainly better op amps (AD797, OPA627), and you can always go discrete. My experience with Op amps in my DAC is that there seem to be subtle differences between op amps with nominally adequate specs (such as the 2134 or the 2604) and some of the really good ones. The no-feedback school is going to cry for another reason: what use is a unity gain, discrete, no global feedback buffer if the signal has already passed through one of those nasty op amps using lots of global feedback?

27th August 2003, 10:22 PM	#20
sonnya diyAudio Member Join Date: Oct 2001 Location: Denmark	just wanted to post some FFT but even in zip-file the size is 170K so i am only going mention the results at 1KHz : 350mVrms - 2nd : -105dB - 3rd : -150dB 700mVrms - 2nd : -100dB - 3rd : -135dB 2.000Vrms - 2nd : -90dB - 3rd : -120dB - 4th : -150dB IMD 19k+20k 350mVrms each : all other components is 100dB below 19k+20k. so total IMD is like ~ -92dB.. This is a discrete I/V without feedback.... and without the buffer... so if you can live with that you can use a discrete No global feedback I/V or add a buffer and make the VAS node high impedance add a 33pF cap to the node and add a feedback resistor... and there you go...