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Old 3rd January 2008, 06:19 PM   #991
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For current control I found the Howland current pump to be a good solution.

Cheers

Andrea
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Old 3rd January 2008, 06:39 PM   #992
EUVL is offline EUVL  Europe
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Thanks for the tipp Andrea.

I wonder whether you have any practical experience of using that iver 3 decades of current range.

I mean if you take a JFET, for example, it would be find from 2mA to 20mA, but below 2mA it runs into noise problems, and one would have to use another device type for lower current, etc.

I have yet to find an active device / circuit that would operate equally well over 3 decades.


Patrick
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Old 3rd January 2008, 09:09 PM   #993
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Default Currrent Drive

Hi Patrick,

My approach to current drive is using AD420 16 bit DACs. Ignoring inaccuracies for the moment you have a potential resolution of just over 3uA when using the 0 to 20mA range.

It is taking longer than I would like (!) due to other pressures and the desire to make a multi-channel PC based version from the outset.

Good luck

http://www.analog.com/UploadedFiles/...eets/AD420.pdf
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Old 3rd January 2008, 10:11 PM   #994
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Default Oops

Should have been 0.3uA....
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Old 4th January 2008, 06:41 AM   #995
EUVL is offline EUVL  Europe
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The datasheet did not show any information on output current noise.

While I can understand your motivation of using a DAC if you were to go multi-channel and digital remote, etc., I have my doubts as to whether this is a better alternative than a variable resistor (in whatever form) driven by constant voltage, in pure performance terms.

So I hope pietjers would still be kind enough to share some more details of his implementation.


Regards,
Patrick
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Old 4th January 2008, 06:52 AM   #996
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Hey guys, now that this thread has kicked again, how about putting in your vote for a Passive Pre-amp forum?

Vote here http://www.diyaudio.com/request/

Cheers George
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Old 4th January 2008, 07:46 AM   #997
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Quote:
Originally posted by EUVL
Thanks for the tipp Andrea.

I wonder whether you have any practical experience of using that iver 3 decades of current range.

I mean if you take a JFET, for example, it would be find from 2mA to 20mA, but below 2mA it runs into noise problems, and one would have to use another device type for lower current, etc.

I have yet to find an active device / circuit that would operate equally well over 3 decades.


Patrick
Hi,
I ran some simulations and it seemed to perform quite well, especially if you use precision opamps and buffer the -sense signal.

The AD420 is pretty interesting, having a Iout of the needed range, but it needs to be fed from a microcontroller.


Cheers

Andrea
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Old 4th January 2008, 08:38 AM   #998
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Default Icss for Lightspeed

If you are going to use remote controls with dac chips, ROMS, etc, voltage control via "normal" series/shunt stepped attenuators and CCS or perhaps fet gain stage systems, and maybe Maxim ICs to dial in the individual currents to save matching the LDRs, then why not add some i/p and o/p buffers to reduce the effects of impedance loading with a power supply from the "Blowtorch" to finish it off!

Sorry George, couldn't resist!
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Old 4th January 2008, 10:43 AM   #999
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Default Re: Icss for Lightspeed

Quote:
Originally posted by jameshillj
If you are going to use remote controls with dac chips, ROMS, etc, voltage control via "normal" series/shunt stepped attenuators and CCS or perhaps fet gain stage systems, and maybe Maxim ICs to dial in the individual currents to save matching the LDRs, then why not add some i/p and o/p buffers to reduce the effects of impedance loading with a power supply from the "Blowtorch" to finish it off!

Sorry George, couldn't resist!
Hope that was said tongue in cheek, that's all we need more active crap in the signal path, hey this is the 1000th post got to be an Omen. for what I don't know, maybe now we can have a Passive Pre-amp forum?
Vote here http://www.diyaudio.com/request/

Cheers George
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Old 4th January 2008, 11:18 AM   #1000
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This is how I did it!!

The current source is built round a voltage reference. In this case a TLV431, which has an internal reference voltage of 1.24V. But it is also possible to use other references such as TL431 (2.45V) or LM336-2.5 (2.5V) or others. To get an even more stable reference voltage, I used a LM317 current source to bias the TLV431.

The current range of this current source depends on the reference voltage of the chosen device, the potentiometer value and set resistor R2/R4 value. With the set resistor you can set the maximum value of the current source: I = Vref / R2. In this example 10.3 mA. And by turning the attenuator (I use a DACT stepped attenuator) fully clockwise this value drops to I = 1.24 / 100k = 12.4uA. This gives on de LDR side a range of about 48 Ohm to 20 kOhm. But you can of course choose other values for the reference or the attenuator to get the desired range. When you are using a 2 pin reference it is also possible to influence the range by using a darlington transistor instead of a normal transistor, because the Vbe is subtracted from the reference voltage.

The optional 2 Meg potentiometers are for calibration purposes. With these pots you can shunt a small amount of current to get perfectly matched channels.

The sound of this CCS volume control is really great and outperforms the original design with ease (see also my previous post).

Cheers,

Peter
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