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Old 18th May 2012, 02:28 PM   #811
EUVL is offline EUVL  Europe
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Your schematics has a serious error with the basic SEN circuit.
Please see post #2.

The current source you posted has poor PSRR and is IMHO necessarily complicated.
I suggest you make up the one in post#784 for a trial first.
When it works, you can then still fiddle around with other CCS variants and see which one you like.
I already showed you what works for me, so it is up to you what you want to build.


Patrick
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Old 18th May 2012, 02:30 PM   #812
heb1001 is offline heb1001  China
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Otherside, if the 1792 is like the 1794 then you need to suck out the bias current from the DAC with a negative supply, not push it in with a positive one. You can check this in the data sheet by looking at how the example circuit wires up the op amps - from memory I think it's the same example as for 1794 which would mean you would need the negative supply like for 1794 but you should check for yourself.

Also I think you might need more JFETs in parallel for 1792 because the output current swing is quite a big fraction of the JFET bias current. I know that two quads (twice as many as you show) sounds good but I didn't try fewer so don't know for sure.

When you fix the bias circuit, you need to make sure its safe during the transient at switch on. I know from experience that it's possible to fry a PCM1794 by connecting the current output to -12V (slipped with the multimeter probe). I used a 1500R resistor to limit the current. Apart from my clumsiness, this has been OK but again, you should work it out for yourself or let someone with real experience tell you how to do it properly.

Last edited by heb1001; 18th May 2012 at 02:57 PM.
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Old 18th May 2012, 02:42 PM   #813
heb1001 is offline heb1001  China
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Will #784 work if you put a negative supply on the right hand side and connect the left hand side to the DAC?
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Old 18th May 2012, 03:36 PM   #814
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Patrick,
Thanks for the tip. I missed it when put together the schematic, my physical board is correct.
Yes like this it does have poor PSRR but that's easy to fix, whilst with the TL431 you can't and you're forced to have the Vref across the resistor and it has other disadvantages as well. I've used the ZR431 is many circuits so i'm very familiar with how it works.

Bottom line i was out of 431s and have loads of opamps so just for a first try will do.

Heb,

The 1792 is exactly the same as the 1794 only has an SPI bus for configuration.

I'm not quite sure where you got that about sinking/sourcing the current. It seems very odd... unless i'm missing something. I can't see why this wouldn't work.

The paralleling of the JFETs is a bit puzzling. As long as the current swing is smaller than the JFETs can handle, what exactly is the benefit of adding more?

Yeah startup condition is a bit of an issue. There are many ways to fix it in the long run, for now i'll just try it manually.

OPAMPs and noise... i'll leave that for another time...

Thanks for the feedback guys.
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Old 18th May 2012, 03:57 PM   #815
heb1001 is offline heb1001  China
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Quote:
Originally Posted by otherside View Post
I'm not quite sure where you got that about sinking/sourcing the current. It seems very odd... unless i'm missing something. I can't see why this wouldn't work.

The paralleling of the JFETs is a bit puzzling. As long as the current swing is smaller than the JFETs can handle, what exactly is the benefit of adding more?
I'm pretty sure, having made Sen work with 1794 and a negative bias supply voltage that the bias current is 6.2mA flowing out of the DAC chip.

The DAC output is supposed to be at 0V. If you want 6.2mA to flow out of it into something then the voltage on the other side of the something is going to have to be negative.

There is a trade-off in adding more JFETs in parallel between the added JFET gate capacitance which increases the distortion of the Sen circuit and reducing the input impedence of Sen which reduces the distortion from the DAC.

EUVL can correct me if I'm wrong here but I think the fraction of the bias current that the DAC swings translates into the fraction of the swing in the floating power supply. If you use a more significant fraction of the floating supply swing then I think that will also introduce more distortion.
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Old 18th May 2012, 04:08 PM   #816
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Hi Heb, sorry to be persistent on this, i'm just trying to understand.

Quote:
I'm pretty sure, having made Sen work with 1794 and a negative bias supply voltage that the bias current is 6.2mA flowing out of the DAC chip.
Besides your servo schematic, do you have a schematic that you've tried as in such configuration?

I was pretty sure that the PCM1792/4 was designed to sink current and not source it.
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Old 18th May 2012, 05:45 PM   #817
EUVL is offline EUVL  Europe
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> Will #784 work if you put a negative supply on the right hand side and connect the left hand side to the DAC

Yes.


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Old 18th May 2012, 05:57 PM   #818
heb1001 is offline heb1001  China
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Quote:
Originally Posted by otherside View Post
Hi Heb, sorry to be persistent on this, i'm just trying to understand.



Besides your servo schematic, do you have a schematic that you've tried as in such configuration?

I was pretty sure that the PCM1792/4 was designed to sink current and not source it.
If you look at the table on page 25 of the PCM1794 data sheet you'll see that VOUTN - the output of U1 in Figure 24 is -4.65V at the center of the DAC output when the current is 6.2mA.

The DAC output is connected to the inverting input of the op-amp U1. If the output of U1 is negative and the inverting input is close to ground then the current must be flowing through R1 out of the DAC from left to right.

Why do you think it was designed to sink current?

I only have schematics with servos. The servo below is the one I used for the results I posted most recently. I used it with 8 NiMH AA cells (9.8V?) for + and - supplies and I used a 1200R resistor. I have shown 1500R for +-12V supplies which I think will probably also work. It takes a few minutes to settle.

I don't know if the op-amp decoupling is as good as it could be and I don't know if the filter caps should be connected to ground as shown or to the negative supply for the best results.

The blue line at the top is the DAC output.

You can look at the measured results I posted and see some distortion that may be the result of this servo. If you improve it, please let me know.

I'm a complete amateur - you have been warned - so don't complain if it blows up your DAC.

I selected a JFET with about 10mA Idss.
Attached Images
File Type: png SEN-SERVO-1.png (37.2 KB, 397 views)

Last edited by heb1001; 18th May 2012 at 06:02 PM. Reason: Added detail about JFET.
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Old 18th May 2012, 07:39 PM   #819
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Quote:
The DAC output is connected to the inverting input of the op-amp U1. If the output of U1 is negative and the inverting input is close to ground then the current must be flowing through R1 out of the DAC from left to right.
You're quite right about this. A bit of a misunderstanding..

Out of curiosity why are you using servos and not a simple ccs?
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Old 18th May 2012, 08:03 PM   #820
heb1001 is offline heb1001  China
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Quote:
Originally Posted by otherside View Post
You're quite right about this. A bit of a misunderstanding..

Out of curiosity why are you using servos and not a simple ccs?
I want to try to get better measured results than my op amp IV and I thought that a servo would get closer than I could adjust manually. Also my listening room temperature is very variable and the earlier posts were talking about using thermistors to stabilize the example current sink circuits against thermal drift which put me off using them.
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