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Old 18th August 2013, 03:46 PM   #1
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Default OPA861 for I/V

Noticed there was no specific thread for this ...



From TI's informative 2009 Current vs Voltage Feedback presentation (OPA861 pg. 8)

Features:

WideBandwidth OTA(>80Mhz)
900V/usecSlewRate
2.4nV/√HzInputNoiseVoltage
Very Flexible Circuit Building Block
Ideal Complementary Transistor Function
Tuneable Transconductance
Externally settable supply current

http://www.ti.com/lit/ds/symlink/opa861.pdf

http://www.ieee.li/pdf/viewgraphs/cu...amplifiers.pdf

Pedja's AYA II circuit:



http://www.audialonline.com/?wpdmact...d=MS5ob3RsaW5r

And some good threads:
(go back a few pages)
Best I/V for multiple tda1541a, no tube no transformer

Yet Another - I want To Make a DAC

Output level of TDA1545


DIYHiFi.org • View topic - Discrete I/V

Last edited by Mooly; 29th April 2014 at 07:37 AM. Reason: Copyright image removed.
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Old 18th August 2013, 07:25 PM   #2
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Good stuff from TI's 2009 Current vs Voltage Feedback presentation:

V vs C opamps.JPG

V vs C opamps2.JPG


V vs C opamps chart.JPG
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Old 19th August 2013, 12:46 AM   #3
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Here is something I found which could help those wishing to try the OPA860 or OPA861.

Cheers George
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File Type: jpg OPA860 IV.jpg (249.0 KB, 815 views)
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Old 19th August 2013, 09:18 AM   #4
Calvin is offline Calvin  Germany
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Hi,

I simmed the OPA861 for PCM179x DACs as sources quite a while ago with a cascoded NJFET-Buffer (LSK389C or BF862 + 4391) that is THD and Noise transparent.
pros:
- simplicity
- very constant input impedance into the 10s of MHzs
- very fast --> clean square response
- nice THD spectrum with clean decay of harmonics

cons (not really, but rather mediocre performance):
- THD-niveau (close to -60dB/0.1% at 0dBfs)
- Noise figures just ok, though depending to a large dgree on the value of the IV-resistor.

It should be noted that DACs with offset currents require a current source or - sink to cancel that offset.
A simple JFET ccs may suffice here as best solution (noisewise) but will inevitably introduce some noise.
In fact it might be the dominant noise source of the whole circuit!

jauu
Calvin
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Old 20th August 2013, 08:54 AM   #6
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1.
http://www.essex.ac.uk/csee/research...0amplifier.pdf

2.
AD811 as I/V with LM6172 as differential to single-end stage
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Old 21st August 2013, 01:44 AM   #7
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Has anyone seen the simplified schematic of what's inside a OPA861 or 860 like the AD844 data sheet gives attached?

Cheers George
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File Type: jpg AD844 internal schematic.JPG (73.5 KB, 681 views)
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Old 21st August 2013, 03:33 AM   #8
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opa861 ds pg12.JPG


v vs c pg 8.JPG


v vs c pg 9.JPG


v vs c pg 10.JPG


v vs c pg 11.JPG


Last edited by AudioLapDance; 21st August 2013 at 03:40 AM.
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Old 21st August 2013, 10:17 AM   #9
Ceglar is offline Ceglar  Australia
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I was using the OPA861 as per the above schematic as part of the AYA II DAC.

It's good, and to my ears better than passive I/V into WE396A (with a good CCS fed shunt reg B supply). This was with the 2mA current injection as per the scheme.

I was nulling for 0VDC at the buffers output and driving directly a rather decent TVC. I did find that for some reason it didn't like driving a purely inductive load, directly, so went back to nulling for 0VDC at the 1541a output and coupling to TVC with a high quality capacitor, after trying many, preferring actually BG Hi NX.. Which surprised me.

Although I didn't like the idea compared with direct connection, it actually sounded a bit better and it was stable. I had fried a few OPA driving the inductance, for no other reason that I could discern and those SMD parts are a PITA to replace.

Recently I tried another tube stage, 6072A biased by the current offset of the 1541a itself through a 33R resistor, same as what Thorsten proposed. Near 0V bias, 10R carbon in the cathode cct. Pimm CCS loaded with mu output of that device and parafeed into the TVC connected as AVC. Removed the CCS fed shunt reg and used tube regulated choke input filter. To my ears is a better stage and will probably stay that way.

I'd encourage a build using OPA, for sure. And if you have the space, try Thorstens tube stage, the recent one as hybrid mu follower.

Regards,
Shane
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Old 28th April 2014, 12:10 PM   #10
regal is offline regal  United States
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Quote:
Originally Posted by Ceglar View Post
I'd encourage a build using OPA, for sure. And if you have the space, try Thorstens tube stage, the recent one as hybrid mu follower.

Regards,
Shane
Do you have a link I missed that one?
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