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dac I/V convertion with very low distortion
dac I/V convertion with very low distortion
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Old 19th March 2019, 11:19 PM   #881
smms73 is offline smms73  Europe
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No, I'd never seen that.
But he is using too much gain in the Sziklai pair, the circuit will have problems with parasitic oscillations.
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Old 26th March 2019, 02:31 PM   #882
dreamth is offline dreamth  Romania
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What's the noise of this circuit?
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Old 26th March 2019, 02:37 PM   #883
dreamth is offline dreamth  Romania
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Could somebody tell a discrete op-amp that is fet input, very high slew rate,very high gain in open loop, unity gain stable , very low en noise and high output power? The only op-amp that i saw like this would be ths4631 .Do you know something even better in term of voltage noise but similar slew rate, current noise, etc...?
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Old 26th March 2019, 03:54 PM   #884
smms73 is offline smms73  Europe
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Quote:
Originally Posted by dreamth View Post
What's the noise of this circuit?
What circuit are you referring to?

Quote:
Originally Posted by dreamth View Post
Could somebody tell a discrete op-amp that is fet input, very high slew rate,very high gain in open loop, unity gain stable , very low en noise and high output power? The only op-amp that i saw like this would be ths4631 .Do you know something even better in term of voltage noise but similar slew rate, current noise, etc...?
Ths4631 seems to be a good opamp to use in an IV converter, but you will have to be very careful when drawing the printed circuit board to avoid parasitic oscillations because this amplifier is very fast
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Old 26th March 2019, 04:14 PM   #885
smms73 is offline smms73  Europe
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Audio opamp are not the better choice for a IV converter and one of the problems is the input impedance which is not constant and increases greatly with increasing frequency.
For example with an opa134 the impedance is only 0.002 ohms at 20hz and increase to almost 2 ohms at 20khz an increase of 1000 times, and then continues to increase being 6 ohms at 100kz and 110ohms
at 10mhz
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File Type: png opamp_impedance.png (72.7 KB, 190 views)
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Old 26th March 2019, 04:40 PM   #886
smms73 is offline smms73  Europe
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this weekend I have been modifying my current conveyor to use a complementary compound input stage as Michael Smedegaard proposes on paper posted by jan.didden in # 880 and the input impedance is very low and constant, being 0.023 ohms at 20hz and only 0.026 at 20khz, at 100khz is only 0.079, and it only reaches 2ohms at 2Mhz, which opa134 had at only 20Khz.

I am designing this current conveyor to use with an ES9018, because I need 8 channels I ended up choosing this DAC, this week I hope to test the circuit in the breadboard, then post here if anyone is interested in seeing.
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File Type: png circ_1_impedance.png (42.0 KB, 189 views)
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Old 26th March 2019, 05:53 PM   #887
dreamth is offline dreamth  Romania
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Quote:
Originally Posted by smms73 View Post
What circuit are you referring to?
The one presented by Ian Didden, or yours...as it looks similar.do they show lower noise than the classical opamp in i/v? I want exceptionally high slew rates and exceptionally low noise too in such a circuit to make the i/v of some old pcm dac that deliver up to 1ma at full scale output.

Last edited by dreamth; 26th March 2019 at 05:56 PM.
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Old 26th March 2019, 06:21 PM   #888
dreamth is offline dreamth  Romania
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Quote:
Originally Posted by smms73 View Post
Audio opamp are not the better choice for a IV converter and one of the problems is the input impedance which is not constant and increases greatly with increasing frequency.
For example with an opa134 the impedance is only 0.002 ohms at 20hz and increase to almost 2 ohms at 20khz an increase of 1000 times, and then continues to increase being 6 ohms at 100kz and 110ohms
at 10mhz
I think it would be even better if that impedance would be 1 million times higher over 20khz...
I think that the op-amps are very good if their slew rate , noise and output power are adequate.I'm sure that a discrete circuit can be better, but i don't believe that any discrete circuit would be better than any op-amp.
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Old 26th March 2019, 06:56 PM   #889
chris719 is offline chris719  United States
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dac I/V convertion with very low distortion
Quote:
Originally Posted by smms73 View Post
Audio opamp are not the better choice for a IV converter and one of the problems is the input impedance which is not constant and increases greatly with increasing frequency.
For example with an opa134 the impedance is only 0.002 ohms at 20hz and increase to almost 2 ohms at 20khz an increase of 1000 times, and then continues to increase being 6 ohms at 100kz and 110ohms
at 10mhz
OPA134 is old and slow, though.
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Old 26th March 2019, 07:25 PM   #890
smms73 is offline smms73  Europe
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I donīt have nothing against op-amp or any other circuit that works with a global feedback loop (quite the opposite), but in this particular case of converting current to voltage, current conveyor have some advantages, they have ultra low noise , low and constant input impedance and are very fast, they can have more parts if you want more stability and better performance , but I use very inexpensive parts, all the parts for one of my current conveyor costs less than 1 euro.
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