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Old 30th October 2012, 07:35 PM   #541
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Quote:
Originally Posted by regal View Post
So let's say we have minimal DC offset. It may be a good idea to put some filtering after this and then a buffer. I'm thinking the DCB1.
A buffer could be useful with a DAC which outputs only around 1mA peak, such as the PCM1704. For DACs which output significant current, such as does the PCM1794A which outputs around 4mA peak, the resulting output impedance should likely be sufficiently low for almost any line level application without a buffer stage.
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Old 30th October 2012, 08:36 PM   #542
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Originally Posted by MiiB View Post
the TL431 has high impedance but starts loosing it after only 1K, but is stiller better at 100KHz than the two other... Can you make one cascoded Jfet... ??
I'll give it a try tomorrow.
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Old 30th October 2012, 09:14 PM   #543
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Thanks very much for the sims Avro. I was thinking in stabilize in temperature the first Ccs that you post with a transistor connected as a diode in series with the led. The tl431 sims nice but I am afraid that it will be the noisier. And difficult to do a complement.
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Old 30th October 2012, 09:19 PM   #544
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Originally Posted by Joachim Gerhard View Post
Have you tried to shunt R15 ?
No difference in doing that. I think that the only thing that we need is to chose the current sources type. Next week I want to have the PCB ready.
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Old 30th October 2012, 09:27 PM   #545
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Originally Posted by MiiB View Post
You may be right..., however my LT simulator says different.... I wonder if the models aren't sufficient even to calculate CCS impedance.... hmmm So much for for believing that..
In my experience the ltspice do very good simulations with bipolar transistors, very close to reality. But is not very good with jfet, it gives to much gate current leak in every models that I have use, but I don't have investigate the problem further.
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Old 30th October 2012, 09:38 PM   #546
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Quote:
Originally Posted by Ken Newton View Post
George, whether a 2.35k output impedance is too high or not will mostly depend on the total amount of shunt capacitance presented by the interconnect cable (typically, about 50pF per foot) plus that of the input of the next gain stage. The output impedance forms a low-pass filter in conjunction with that shunt capacitance.
Ken, I think George are thinking that the load for the dac will be 2.35k.

George, the dac will see a 3.3 ohms impedance. that is the iv converter input impedance ( in the version post here) . the 2.35k is the output impedance of iv converter .
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Old 30th October 2012, 10:58 PM   #547
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Quote:
Originally Posted by smms73 View Post
Ken, I think George are thinking that the load for the dac will be 2.35k.

George, the dac will see a 3.3 ohms impedance. that is the iv converter input impedance ( in the version post here) . the 2.35k is the output impedance of iv converter .
Yes, thanks for clearing that up, the way Ken worded it it (post 517) which I misconscrewed sounded like the input impedance of your I/V was whatever the output shunt resistor was, which left me scratching my head.
I am watching and your progress with this, hoping it will end up in a kit form or ready made to insert into cdp's and dacs.

Cheers George
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Old 30th October 2012, 11:20 PM   #548
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Sorry, for any confusion, George.
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Old 31st October 2012, 08:40 AM   #549
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CCS and impedance... or inverse of Impedance..
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File Type: jpg CCS's.JPG (68.5 KB, 250 views)
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Old 31st October 2012, 10:06 AM   #550
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I do not get too much gate currents in MicroCap. These two models for ex, give me pA of gate current at Vds = 20VDC:

.MODEL 2SK170V_10M NJF (AF=499.843M BETA=29.5953M CGD=19.8997P CGS=24P IS=10F
+ KF=.001681153F LAMBDA=1M RS=7.19294 VTO=-0.65)

.MODEL 2SJ74V_16M3 PJF (AF=500.034M BETA=35.5713M CGD=106.132P CGS=73P IS=10F
+ KF=.00163621F LAMBDA=1M RS=4.75307 VTO=-0.75)


LTspice cannot be doing something wrong with JFETs?



/S

Quote:
Originally Posted by smms73 View Post
In my experience the ltspice do very good simulations with bipolar transistors, very close to reality. But is not very good with jfet, it gives to much gate current leak in every models that I have use, but I don't have investigate the problem further.
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