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Using the AD844 as an I/V
Using the AD844 as an I/V
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Old 25th June 2019, 12:22 PM   #2311
TimA is offline TimA
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PetrL
From your post my impression is that the direct output from pin 5 was from stacked AD844s. Have you tried it from a single?
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Old 25th June 2019, 12:54 PM   #2312
PetrL is offline PetrL  Czech Republic
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Yes, from stacked pins 5. I didn't try single pin 5. I'm not sure about necessity of stacking, but if there is any reason for TDA1541, then the current mirror should be stacked.
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Old 25th June 2019, 12:57 PM   #2313
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Stacking reduces the impedance seen by the TDA1541a and therefore should reduce errors in its output.
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Old 25th June 2019, 09:03 PM   #2314
georgehifi is offline georgehifi  Australia
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Quote:
Originally Posted by Terry Demol View Post
George,

This doesn't surprise me at all.

The open loop OP resistance of AD844 is listed as 15 ohms, so this will =
approx 30 ohms per OP device. There are no OP resistors shown on simplified
schematic, even if they did exist, everything points to the OP stage working at
very low bias current.

IME for an open loop follower to sound decent, there needs to be significant
bias (idle) current.

T
Hi Terry

The 844's output buffer was very ordinary, I even tried stacking them with isolation resistors, better but still ordinary.
What about what I asked before with no takers, is there a way of using feedback just around the 844's buffer to give it some better qualities?

Cheers George
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Old 26th June 2019, 01:36 AM   #2315
abraxalito is offline abraxalito  United Kingdom
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I can't think of a way of using feedback to improve the buffer. What I'd do is load the output with a current source and run balanced. That's about the best I could make of a classAB output stage.
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Old 26th June 2019, 04:39 PM   #2316
Hierfi is offline Hierfi  Canada
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Quote:
Originally Posted by georgehifi View Post
Hi Terry

The 844's output buffer was very ordinary, I even tried stacking them with isolation resistors, better but still ordinary.
What about what I asked before with no takers, is there a way of using feedback just around the 844's buffer to give it some better qualities?

Cheers George
Sure, only this requires another device comparing the source signal input to the output and making corrections to the input of the AD844 output buffer. This is normally done by incorporating the whole of the AD844 device. Since the input stage of the AD844 already has a reputation for being very good it seems reasonable to use it.

The AD844 permits placing a resistor at the Tz junction (pin 5) to ground. This permits the open loop gain of the complete AD844 to be reduced by paralleling the intrinsic 2MegOhm resistance at the Tz node junction. As an example a 2KiloOhm resistor can reduce open loop gain by 60dB. This also increases the bandwidth because this lower resistor is in parallel with the intrinsic 5pF of Tz node junction.

By controlling the Tz node resistance this permits an ability to tune the output buffer as a function of open to closed loop feedback. In conducting experiments it has proven useful to add something in the order of a miniature 1 Ohm 15uHenry or so coil in series with the output as to limit parasitic capacitances from the feedback path.

Cheers,
Gerrit
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Old 27th June 2019, 01:38 AM   #2317
Joe Rasmussen is online now Joe Rasmussen  Australia
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Quote:
Originally Posted by georgehifi View Post
I can understand OPA627's being faked with say TLO71's.

But an AD844?? what has the same pin 5 TZ output that it can be faked with it??

Cheers George
Hi George

I was thinking the same thing about Pin 5 Tz and I have been using these ADs on and off for many years, even got some from a Chinese source lately - and if you use it open loop and set Tz resistor to ground and it works, then I can't see it being a fake - like OPA627 faked as TL071 is just relabelling. What do you think, and others too? Because if that is true, then I haven't encountered any fakes yet. So if Tz gain works in open loop, then it should be OK, right?

Cheers, Joe
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Old 27th June 2019, 01:48 AM   #2318
georgehifi is offline georgehifi  Australia
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Hi Joe, yes that's should be real, but they'er only $10 with (free delivery for any price purchase) from RS Components, and we know they are authorised AD resellers.

I not playing around at the moment with the AD844's but it would be nice for someone who is, to try going from pin 6 back to pin 5 with a feedback loop to see if this then can equal a OPA627 in sonic performance.

Cheers George
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Old 27th June 2019, 02:30 AM   #2319
Joe Rasmussen is online now Joe Rasmussen  Australia
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Quote:
Originally Posted by georgehifi View Post
Hi Joe, yes that's should be real, but they'er only $10 with (free delivery for any price purchase) from RS Components, and we know they are authorised AD resellers.

I not playing around at the moment with the AD844's but it would be nice for someone who is, to try going from pin 6 back to pin 5 with a feedback loop to see if this then can equal a OPA627 in sonic performance.

Cheers George
Thanks George. Re Pin 6 back to 5, the AC signal should be the same and in phase through unity gain buffer, so I don't think that will achieve anything. I would have thought stacking 844s and using its own buffer x 3 and triple the current, that should work better than a 627 with 100% feedback. But then again I am by nature inclined against using opamps as buffer. My admission.

Cheers, Joe
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Old 27th June 2019, 02:40 AM   #2320
Hierfi is offline Hierfi  Canada
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Quote:
Originally Posted by Joe Rasmussen View Post
Hi George

I was thinking the same thing about Pin 5 Tz and I have been using these ADs on and off for many years, even got some from a Chinese source lately - and if you use it open loop and set Tz resistor to ground and it works, then I can't see it being a fake - like OPA627 faked as TL071 is just relabelling. What do you think, and others too? Because if that is true, then I haven't encountered any fakes yet. So if Tz gain works in open loop, then it should be OK, right?

Cheers, Joe
Agreed. A legitimate AD844 also has laser trimmed input offset in the 150 uVolt region. The input buffer has a characteristic 50 Ohm output impedance at the inverting terminal, hence with inputs grounded the error current is 150uVolt/ 50 Ohm. The error current is mirrored into the Tz node. Placing a 50K Ohm resistor from the Tz node to ground will magnify the input offset voltage at the output by about 1000 (50K Ohm/50 Ohm). if the output is in the order of 150 mVolt this indicates that the device under test has an active pin 5 Tz node and that the device has low input offset as likely laser trimmed.

I can't say that all the AD844's purchased are legitimate, only that they behave as reasonably expected.

Cheers,
Gerrit
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