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Old 1st July 2010, 04:25 PM   #1
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Default I/V opamps: selecting/tweaking feedback caps + resistors

What's the component-selection and tweaking process for using the ubiquitious feedback cap + R seen in opamp-based I/Vs (see schematic below -- taken from PCM1794 datasheet). So, for given schematic topology, how did TI/BB come up with the values for C1/C2 and R1/R2?
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Old 1st July 2010, 04:50 PM   #2
Mooly is online now Mooly  United Kingdom
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I imagine R1/2 selected to be low value but within the drive capability of the 5534 to minimise noise contribution, caps C1/2 to absorb transients produced by the DAC output on each clock cycle.
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Old 1st July 2010, 05:14 PM   #3
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It says there - they choose 217kHz as corner frequency. Why? Don't know - maybe ti be safe for a 192kHz SR signal?
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Old 1st July 2010, 05:24 PM   #4
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I would make C1-C4 non-inductive, like silver mica's.
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Old 1st July 2010, 11:28 PM   #5
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The R's in the feedback path converts the current (see datasheet for value, approx 4 to 10mA) to voltage
-> U=IxR.

Keep in mind, that you have a swing around a single value,
e.g. voltage=0 -> +4mA,
voltage=+max -> +8mA,
voltage=-max -> 0mA.

The resulting offset is removed by the difference amplifier (3rd OPAMP).

The C can be calculated by fg=1/(2xPIxRxC) -> C=1/(2xPIxRxfg)
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Old 2nd July 2010, 01:14 AM   #6
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Voegelchen: Thx for the tips + formula. As far as the NE5534...In myriad CDPs and/or outboard D/A processors that use this opamp, I can pretty much roll-in opamps** (w/o any other component change, e.g., C1 or R1) with results that other opamp rollers note for specific opamp brands/models. A further question is just how sensitive opamp performance i to the values for C and R are in the feedback locations. E.g., for R, I've seen this value vary from 500 - 1200 ohms.

**Note: I realize a lot of so-called "experts" DIYers shun opamp rolling. But if you are looking for opamp suggestions, and don't have a lot of time to 'scope each and every opamp, simply look at the sheer number of opamps rolled (e.g., at Tangentsoft.net and head-fi.org -- look for topical threads/articles there). The statistical significance of all these rollers is not insignificant.

Last edited by hollowman; 2nd July 2010 at 01:23 AM.
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Old 2nd July 2010, 01:57 AM   #7
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Quote:
Originally Posted by Mooly View Post
I imagine R1/2 selected to be low value but within the drive capability of the 5534 to minimise noise contribution, caps C1/2 to absorb transients produced by the DAC output on each clock cycle.
Have you looked at the output waveforms of DACs like these? I have, although with ADI's equivalent part the AD1955, not the TI/BB one. The edges are very fast - a few nS - so fast that the output of an NE5534 (which looks inductive due to the falling OL gain) cannot really control them. So I think the transients get more 'smeared out' than controlled. I conclude that TI chose this large capacitor because it gives a low enough measured noise for the datasheet.

<edit> notice also that the textbook decouplers are shown in this application, about the worst possible thing for the sound. That's because the fast edges from the DAC go through C1, via the output stage of the 5534, straight through C11/12 and thereby into pin3. duh!
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Old 2nd July 2010, 02:06 AM   #8
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Originally Posted by hollowman View Post
A further question is just how sensitive opamp performance i to the values for C and R are in the feedback locations. E.g., for R, I've seen this value vary from 500 - 1200 ohms.
My Asus sound card uses this DAC (or one of the same family) and I found a marked improvement in the sound by decreasing the C1/C2 values. But the NE5534 whilst being an excellent audio opamp, is way out of its league in this particular application where the bandwidth of the incoming signal is huge. This particular application does demand a much wider bandwidth opamp - I'm at present using LM6172 (dual) with my AD1955 and am happy as a pig in clover with the sound when compared to a 5532.

In substituting a wider bandwidth opamp, the C1 and C2 values do need to be reduced considerably - I found that a wider bandwidth opamp when used with the stock values sounds worse than a 5532. Only with much lower valued caps do the video opamps come into their own.
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Old 2nd July 2010, 04:07 AM   #9
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I think cascading single poles is poor practice - I'd try for Butterworth maximally flat - even the phase/group delay isn't too bad at 1/2 fc - which would be 20KHz if you set the filter corner at 40KHz - and as long as phase is reasonably smooth, monotonic it really does't mater much as long as it matches between channels

few commercial recordings will have been done with higher fc mics

some other DAC I/V app circuits use 2nd order multiple feedback differential-to-single ended summer/filter which allows for designing a 3rd order response with the I/V pole

NP0/C0G ceramic smt caps will have good characteristics including very low esl to pass the fast edges - I'd still prefer polystyrene for audio frequency - it is possible to use both where they excel but it requires extra parts to "crossover" from the poly to the NP0 as frequency rises

several of this decade's better op amps likely could improve on the 5534 - I like the specs of the ADA4898 for DAC I/V
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Old 2nd July 2010, 04:22 AM   #10
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Quote:
Originally Posted by abraxalito View Post
My Asus sound card uses this DAC (or one of the same family) and I found a marked improvement in the sound by decreasing the C1/C2 values. But the NE5534 whilst being an excellent audio opamp, is way out of its league in this particular application where the bandwidth of the incoming signal is huge. This particular application does demand a much wider bandwidth opamp - I'm at present using LM6172 (dual) with my AD1955 and am happy as a pig in clover with the sound when compared to a 5532.

In substituting a wider bandwidth opamp, the C1 and C2 values do need to be reduced considerably - I found that a wider bandwidth opamp when used with the stock values sounds worse than a 5532. Only with much lower valued caps do the video opamps come into their own.
Okay, cool. What values of C (and R for that matter ) are you using with your AD1955/LM6172 combo?
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