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Analog Line Level Preamplifiers , Passive Pre-amps, Crossovers, etc.

SE to BAL conversion with active filter
SE to BAL conversion with active filter
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Old 9th August 2019, 02:55 PM   #11
MagicBus is offline MagicBus  Greece
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Thanks Ken! Indeed, how to refer the circuit to Vcom is what bothers me at the moment. OPA1632 seems a very good option. In the suggested schematic they seem to spot the problem and solve it with a buffer. I'll keep it in mind. Meanwhile, I found this paper https://d3uzseaevmutz1.cloudfront.ne...te/an241-1.pdf where at page #5 it gives a schematic with opamps with a very nice explanation. They avoid Vcom - I guess they know better about their chips- and use a voltage divider, ordinary application for single rail supply. And with anti-aliasing filter... and for CS4272! But, as I said I need more voltage swing than 5V, so I built a prototype based on the attached schematic with isolated grounds for the DC booster. Most likely to be ready for test tonight. I'll be back.
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Old 9th August 2019, 06:24 PM   #12
Ken Newton is offline Ken Newton  United States
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Quote:
Originally Posted by MagicBus View Post
Thanks Ken! Indeed, how to refer the circuit to Vcom is what bothers me at the moment. OPA1632 seems a very good option. In the suggested schematic they seem to spot the problem and solve it with a buffer...
I saw that, and thought it odd that VCOM pin would require buffering. The OPA1632 data sheet specifies all input pins as having an input impedance of 34 Meg, which I assume includes the VCOM pin.
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Old 9th August 2019, 07:20 PM   #13
00940 is offline 00940  Belgium
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Another option is to use low voltage opamps such as the opa836 in inverting configuration. I once sketched something gathered from various datasheets and app notes here: USB powered ADC (pcm4202-cs8406)

One more project that didn't go anywhere...
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Old 9th August 2019, 09:24 PM   #14
MagicBus is offline MagicBus  Greece
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I am confused... I can't see 5,64Vpp here. How could it be so with single rail 5V and Vcom=2,5V? Is it only me who sees 2,82Vpp?


SE to BAL conversion with active filter-fs-jpg

But then it says full scale input voltage = 1,13*VA. What does this mean? For VA=5V it will be 5,65Vpp and for VA=4,5V 5,085Vpp? It always split VA in half to get Vcom and in real life I have VA=4,6V, Vcom=2,3V and it doesn't go to 0dB with full 4,6V rail swing and even more.


SE to BAL conversion with active filter-va-jpg

So, it needs 5,65Vpp input no matter what supply voltage and it simply cannot reach 0dBFS?
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Old 9th August 2019, 09:24 PM   #15
PRR is offline PRR  United States
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Originally Posted by Ken Newton View Post
I saw that, and thought it odd that VCOM pin would require buffering. The OPA1632 data sheet specifies all input pins as having an input impedance of 34 Meg, which I assume includes the VCOM pin.
Data sheet details clearly show a pair of 30k resistors off Vocm and speak of a "low impedance source" to force it off half-supply.
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Old 9th August 2019, 09:57 PM   #16
00940 is offline 00940  Belgium
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Quote:
Originally Posted by MagicBus View Post
I am confused... I can't see 5,64Vpp here. How could it be so with single rail 5V and Vcom=2,5V? Is it only me who sees 2,82Vpp?
2.82Vpp but it's differential ... so twice that as explained in the datasheet
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Old 9th August 2019, 10:02 PM   #17
MagicBus is offline MagicBus  Greece
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So, 5V supply is enough for this? And what would be the meaning for any higher. With Vcom=2,5V the lower part of the voltage swing is restricted to that. What do I miss?
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Old 9th August 2019, 10:23 PM   #18
00940 is offline 00940  Belgium
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Yes, 5V is enough, provided the circuit can swing down to 1.09V and up to 3.91V.

Think about it. In 6Vpp single ended signal (I'm rounding up), the signal is up to 3V away from the reference, once up, once down. In a differential signal, this same 3V difference can be achieved by moving both + and - 1.5V away from Vcom.
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Old 10th August 2019, 07:47 PM   #19
Ken Newton is offline Ken Newton  United States
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Data sheet details clearly show a pair of 30k resistors off Vocm and speak of a "low impedance source" to force it off half-supply.
Yup, you're correct. VCOM's input impedance is no higher than 15K. I should have thought to see the data sheet's simplified internal schematic diagram.
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Old 11th August 2019, 03:05 PM   #20
MagicBus is offline MagicBus  Greece
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It was a good thing to bring my queries in this thread! Your replies helped to find my way. I mistakenly, thought that the ADC couldn't be driven to full scale because of psu rail headroom. VA*1,13 tested from 4,7V to 5,5V is able to achieve that. The culprit was else. You can read more about this here Behringer UMC 202HD for measurements


One word about the active filter in the analog domain. It is recommended but at least for this converter is kind of academical. I couldn't see any difference with and without it. The built-in digital antialiasing filter seems to do all the job.
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