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Old 12th January 2013, 04:14 PM   #41
Calvin is offline Calvin  Germany
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Hi,

or the OPA861 which is the 860 wo Buffer.
The sim-model for the 861 I have says itīd be not reliable regarding THD.
Still though with 4mAp-p Input current, Riq=100Ohm and an RL of 2kOhm it sims at 0.07% THD@1kHz.
It sims a low and very constant input impedance and allows for Iq current setting.

Seem to be of similar or equal internal structure as the AD844, but looks more promising on paper than the AD844.

jauu
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Old 12th January 2013, 04:33 PM   #42
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Originally Posted by abraxalito View Post
Out of that ex-National range the only part I'd recommend for I/V duty would be the LME49713 because its CFB therefore no LTP to suffer the assault of all that RF.
Maybe so, but it does not sim very well in Tina, TI's version of spice.
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Old 12th January 2013, 04:36 PM   #43
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I've never played with Tina, only LTSpice - what goes wrong? I've yet to see a manufacturer provided opamp macromodel I've been impressed with, if I'm doing serious work simulating opamps I prefer to develop my own.
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Old 12th January 2013, 04:40 PM   #44
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Originally Posted by abraxalito View Post
I've never played with Tina, only LTSpice - what goes wrong? I've yet to see a manufacturer provided opamp macromodel I've been impressed with, if I'm doing serious work simulating opamps I prefer to develop my own.
I'm not capable of designing my own spice model, sorry.

I remember I had previously tried that opamp and got worse THD. I just tried it again and got good results, so who knows. I'd have to redesign my circuit board for this opamp though since it is not available as a DIP.
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Old 12th January 2013, 04:45 PM   #45
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Oh THD is one of the least reliable things to get out of a Spice sim of any flavour - take distortion figures with a huge pinch of salt. Particularly ignore THD in an I/V sim if you're not testing with a staircase waveform (as comes from a DAC) rather a sine (as many do on DIYA).

There's a great paper on how to design Spice opamp macromodels by Bowers and Alexander - both of whom were at ADI when they wrote it. Oddly though ADI doesn't appear to adopt their approach in their own models, go figure
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Old 12th January 2013, 05:04 PM   #46
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I think they and the app note were acquired along with PMI - I'd guess they got new responsibilities under ADI, weren't in positions to champion good modeling
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Old 12th January 2013, 05:12 PM   #47
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Ah, my oversight - the appnote pre-dated PMI's acquisition by ADI?
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Old 12th January 2013, 09:32 PM   #48
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Quote:
Originally Posted by abraxalito View Post
Oh THD is one of the least reliable things to get out of a Spice sim of any flavour - take distortion figures with a huge pinch of salt. Particularly ignore THD in an I/V sim if you're not testing with a staircase waveform (as comes from a DAC) rather a sine (as many do on DIYA).
I just threw THD out there as part of an overall picture. Obviously I don't expect the simulation to exactly reflect reality. It's just a guide in my opinion. I don't have a source or a way to create a staircase waveform.

Current feedback opamps typically can only have some high value of feedback resistor, like 1.2kohms in the case of the LME49713. So, a buffer in the feedback loop is required since a DAC like the PCM1794a puts out a lot of current and thus requires a lower feedback resistor value in the typical IV opamp circuit.
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Old 12th January 2013, 10:18 PM   #49
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Just thought I post this PDF by Pedja Rogic if nobody has seen it, about the AD844 being feed from a TDA5141 and compared to a discrete I/V he did, very interesting.

http://www.google.com/url?sa=t&rct=j...57700187,d.aGc

Cheers George
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Old 12th January 2013, 10:49 PM   #50
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Quote:
Originally Posted by abraxalito View Post
Oh THD is one of the least reliable things to get out of a Spice sim of any flavour - take distortion figures with a huge pinch of salt. Particularly ignore THD in an I/V sim if you're not testing with a staircase waveform (as comes from a DAC) rather a sine (as many do on DIYA).
Richard, that's an excellent point. For those of us who may not know how to set up the type of simulated signal source Ricahard is talking about, you can rather quickly gain a first order sense of the potential problem simply by applying high-frequency sinewave test signals to the simulated circuit. Even a 10kHz sine test signal will often reveal dramatic increases in higher order distortion products versus those produced by a 1kHz sine test signal, in particular with feedback based I/V circuits. Ultimately one would need to build, measure, and listen to a promising circuit.
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