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My version of the G = 1000 low noise measurement amp (for Ikoflexer).
My version of the G = 1000 low noise measurement amp (for Ikoflexer).
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Old 11th January 2017, 02:45 PM   #161
EUVL is offline EUVL  Europe
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But you can also do the same to V1 ?


Patrick
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Old 12th January 2017, 11:43 AM   #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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Attached Files
File Type: asc Wurcer LNA for NJFET V3 PSRR.asc (3.3 KB, 20 views)
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Old 13th January 2017, 02:58 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,
Patrick
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Old 13th January 2017, 03:17 PM   #164
1audio is offline 1audio  United States
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My version of the G = 1000 low noise measurement amp (for Ikoflexer).
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, 04:37 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 04:47 PM.
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Old 13th January 2017, 05:33 PM   #166
EUVL is offline EUVL  Europe
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Thank for the hint.
Food for thought.......


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Old 24th January 2017, 03:07 PM   #167
AndrewT is offline AndrewT  Scotland
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Quote:
I had opted for smd 1206 0.1% 25ppm/C metal film for the 1k & 10k
and 0.5% for the 1r0
Quote:
Make sure you get the thin film version, NOT the thick film version.
Quote:
Originally Posted by jackinnj View Post
The Vishay resistors (R3,R10) -- CMF50 is 3.81mm

For a 1R in 3.81mm pitch, however, a bit difficult to find, perhaps BC Components or Yaego metal film, or a 1610 surface mount device.
I bought 1206 smd and they fitted well.
1k00 0.1% metal film 25ppm/C & 2k49 (+spare 2k7 if needed for tuning Id)
1r0 0.5% metal film 25ppm/C in 1206
56M 603 5% metal oxide
2M7 805 1% metal film 50ppm/C
10k 805 0.1% metal film 25ppm/C.
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Old 24th January 2017, 03:29 PM   #168
dimitri is offline dimitri  United States
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Quote:
I reduce the input bias resistor from 110 Meg to 4.7 Meg
some PAR/EG&G lock-in amplifiers have input ac/dc/gnd selector configured that way, that in gnd position input cap is charged from the signal source, but the input is shorted to gnd (this requires 0 dc on the input gate). I also noticed gate leakage with 100M resistor.
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Old 24th January 2017, 03:30 PM   #169
AndrewT is offline AndrewT  Scotland
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I will sumarise what I think you have been telling me is the set up procedure.
please tell me if any part is wrong.

Apply 12Vdc to power.
adjust VR2 to give an output offset of ~ 6Vdc (half supply voltage)
Check the current through R3. This is jFET Id.

What is the target Id? when the output reaches half supply voltage?

I found that Ir9 is over 8mA. Most coming through R10 from the output !
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Old 24th January 2017, 07:34 PM   #170
jackinnj is offline jackinnj  United States
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My version of the G = 1000 low noise measurement amp (for Ikoflexer).
Quote:
Originally Posted by dimitri View Post
some PAR/EG&G lock-in amplifiers have input ac/dc/gnd selector configured that way, that in gnd position input cap is charged from the signal source, but the input is shorted to gnd (this requires 0 dc on the input gate). I also noticed gate leakage with 100M resistor.
I believe that the Tektronix 7a22n and 5a22n differential preamplifiers do so similarly. On the 5A22N, there is a "precharge" button, don't have a 7A22N anymore.

The manual is available on BAMA: http://bama.edebris.com/download/tek...er-plug-in.pdf

Discussion of gate current compensation.
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