My version of the G = 1000 low noise measurement amp (for Ikoflexer). - Page 17 - diyAudio
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Old 11th January 2017, 03:45 PM   #161
EUVL is offline EUVL  Europe
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But you can also do the same to V1 ?


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Old 12th January 2017, 12:43 PM   #162
EUVL is offline EUVL  Europe
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I find that if one ties C2 to the top rail for the N-JFET version instead of to Gnd,
there is also quite some improvement in PSRR.


Patrick

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File Type: asc Wurcer LNA for NJFET V3 PSRR.asc (3.3 KB, 10 views)
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Old 13th January 2017, 03:58 PM   #163
EUVL is offline EUVL  Europe
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I have another silly question.

How to do AC signal coupling at 0.01Hz ?
I buillt a LT6018 1000x LNA to measure noise at 0.1Hz to 10Hz.
And then I realised (stupid me) that any mV offset from the signal source is going to cause a large DC.

Any ideas how to block that, bearing in mind an input current of 60nA for the LT6018 ?


Thanks in advance,
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Old 13th January 2017, 04:17 PM   #164
1audio is offline 1audio  United States
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How are you intending to use the LNA? You could use a servo for the LF corner. With a cutoff of 10 seconds or so it will be like watching wallpaper dry to use it.

Usually that range is something you plot with a digital scope (or plotter in the old days). What you will look for is P-P amplitude and things like popcorn noise, where there are sudden jumps. Except for the popcorn noise its usually not an issue for audio.

I built a phono preamp with flat 80 dB gain to DC and found it works great. Seemed less sensitive to acoustic feedback. Possibly from removing the high pass filter it removed one source of phase shift. Not easy to get DC stability.
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Old 13th January 2017, 05:37 PM   #165
gerhard is offline gerhard  Germany
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Quote:
Originally Posted by EUVL View Post
I have another silly question.

How to do AC signal coupling at 0.01Hz ?
I buillt a LT6018 1000x LNA to measure noise at 0.1Hz to 10Hz.
And then I realised (stupid me) that any mV offset from the signal source is going to cause a large DC.
Any ideas how to block that, bearing in mind an input current of 60nA for the LT6018 ?
Using a first stage with with small gain just to get the input current right , then
AC couple and do the lion's share of the gain? That should allow at least for
a few 100 mV of offset.

I have a similar problem with the measurement amplifier I'm currently working on.
I want to measure tiny noise voltages riding on potentially large DC ( phase
noise after a phase detector, sensitive supplies etc..).
Lower corner f is 0.1 Hz also and the C should be larger than required for this
for noise reasons.

Therefore I must AC couple right at the input, currently 10u foil + 110 Meg
+ many FETs. After connecting a DUT it takes an eternity to settle after the
FETs, cascode, transimp. stage that I need for bandwidth & noise.

I put a window comparator at the node cascode <-> active load
and when I'm not close to the chosen operating point I reduce the
input bias resistor from 110 Meg to 4.7 Meg, so things speed up by
a factor of > 20.

I use a MAX393 as a switch, that seems to work nicely. Does not
induce any transients during switching and the leakage is OK.

But my 25 BF862 seem to have some leakage, the 110 Meg
sometimes drop 1.5V DC from the bias servo.

Currently I play with bootstrapping the base of the cascode
to get rid of the FET's Cgd. Cgs is already taken care of by the
source feedback.

Gerhard
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Last edited by gerhard; 13th January 2017 at 05:47 PM.
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Old 13th January 2017, 06:33 PM   #166
EUVL is offline EUVL  Europe
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Thank for the hint.
Food for thought.......


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