SR620/PM66xx/CNT-90 input stages
Hi,
I had a look at the PM668x[1] and CNT-90[2] schematics yesterday and
noticed one thing: The input stages are strikingly similar to the
SR620[3] (down to the parts used) and all of them have a gain 1 amplifier
infront of the comparator. The PM668x service manual explicitly calls it
"impedance converter", even. Now, if the downstream circuit would be low
impedance, I could understand that, but the sink is a comparator with a
high impedance input (only a few µA input current). I am sure the engineers
had a good reason to add those amplifiers, but I cannot guess why. Would
someone be so kind and enlighten me?
Attila Kinali
[1] http://bee.mif.pg.gda.pl/ciasteczkowypotwor/Fluke/Fluke_PM6681_Service_Manual.pdf
[2] http://assets.fluke.com/manuals/6690____smeng0000.pdf
[3] http://www.ko4bb.com/getsimple/index.php?id=download&file=02_GPS_Timing/Stanford_Research_Systems/SR620_Universal_Time_Interval_Counter_Schematics.pdf
--
It is upon moral qualities that a society is ultimately founded. All
the prosperity and technological sophistication in the world is of no
use without that foundation.
-- Miss Matheson, The Diamond Age, Neil Stephenson
Its somewhat difficult to achieve a 1Megohm input with just a bipolar comparator input stage especially if AC coupling is required.
Bruce
On 20 June 2017 at 19:20 Attila Kinali <attila@kinali.ch> wrote:
Hi,
I had a look at the PM668x[1] and CNT-90[2] schematics yesterday and
noticed one thing: The input stages are strikingly similar to the
SR620[3] (down to the parts used) and all of them have a gain 1 amplifier
infront of the comparator. The PM668x service manual explicitly calls it
"impedance converter", even. Now, if the downstream circuit would be low
impedance, I could understand that, but the sink is a comparator with a
high impedance input (only a few µA input current). I am sure the engineers
had a good reason to add those amplifiers, but I cannot guess why. Would
someone be so kind and enlighten me?
Attila Kinali
[1] http://bee.mif.pg.gda.pl/ciasteczkowypotwor/Fluke/Fluke_PM6681_Service_Manual.pdf
[2] http://assets.fluke.com/manuals/6690____smeng0000.pdf
[3] http://www.ko4bb.com/getsimple/index.php?id=download&file=02_GPS_Timing/Stanford_Research_Systems/SR620_Universal_Time_Interval_Counter_Schematics.pdf http://www.ko4bb.com/getsimple/index.php?id=download&file=02_GPS_Timing/Stanford_Research_Systems/SR620_Universal_Time_Interval_Counter_Schematics.pdf
--
It is upon moral qualities that a society is ultimately founded. All
the prosperity and technological sophistication in the world is of no
use without that foundation.
-- Miss Matheson, The Diamond Age, Neil Stephenson
_______________________________________________
time-nuts mailing list -- time-nuts@febo.com
To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts
and follow the instructions there.
Hi
I think the purpose is to get high input impedance for high frequency signal. The input impedance of comparator is high at low frequency range only.
Li Ang / BI7LNQ
---Original---
From: "Attila Kinali"attila@kinali.ch
Date: 2017/6/20 15:20:44
To: "Discussion of precise time and frequency measurement"time-nuts@febo.com;
Subject: [time-nuts] SR620/PM66xx/CNT-90 input stages
Hi,
I had a look at the PM668x[1] and CNT-90[2] schematics yesterday and
noticed one thing: The input stages are strikingly similar to the
SR620[3] (down to the parts used) and all of them have a gain 1 amplifier
infront of the comparator. The PM668x service manual explicitly calls it
"impedance converter", even. Now, if the downstream circuit would be low
impedance, I could understand that, but the sink is a comparator with a
high impedance input (only a few µA input current). I am sure the engineers
had a good reason to add those amplifiers, but I cannot guess why. Would
someone be so kind and enlighten me?
Attila Kinali
[1] http://bee.mif.pg.gda.pl/ciasteczkowypotwor/Fluke/Fluke_PM6681_Service_Manual.pdf
[2] http://assets.fluke.com/manuals/6690____smeng0000.pdf
[3] http://www.ko4bb.com/getsimple/index.php?id=download&file=02_GPS_Timing/Stanford_Research_Systems/SR620_Universal_Time_Interval_Counter_Schematics.pdf
--
It is upon moral qualities that a society is ultimately founded. All
the prosperity and technological sophistication in the world is of no
use without that foundation.
-- Miss Matheson, The Diamond Age, Neil Stephenson
time-nuts mailing list -- time-nuts@febo.com
To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts
and follow the instructions there.
Attila wrote:
Now, if the downstream circuit would be low
impedance, I could understand that, but the sink is a comparator with a
high impedance input (only a few µA input current). I am sure the engineers
had a good reason to add those amplifiers, but I cannot guess why. Would
someone be so kind and enlighten me?
I was going tp post the Smith chart for the input of the AD96885/7 ECL
comparator they use, but was surprised to find that AD did not include
it on the datasheet. However, just from the spec table we can see why a
buffer is necessary:
Input Resistance: 200k ohms
Input Capacitance: 2pF
For starters, even at low frequencies the input impedance is only 200k
ohms, much less than the 1Mohm rated impedance. Further, the
capacitance seriously degrades bandwidth to as low as 400kHz (this
depends on the source impedance, but even with a 10kohm source the BW is
only 8MHz).
(Note that the input current spec is not necessarily a good proxy for
input impedance -- consider a FET-input amplifier with an input current
of 1uA and a 100k gate resistor.)
Best regards,
Charles
If the source impendance at dc is 10k at dc a bias current of a few microamp produces a dc offset of tens of millivolts which may be an issue if the signal amplitude is low and has a low slew rate.
Bruce
On 21 June 2017 at 09:11 Charles Steinmetz <csteinmetz@yandex.com> wrote:
Attila wrote:
Now, if the downstream circuit would be low
impedance, I could understand that, but the sink is a comparator with a
high impedance input (only a few µA input current). I am sure the engineers
had a good reason to add those amplifiers, but I cannot guess why. Would
someone be so kind and enlighten me?
I was going tp post the Smith chart for the input of the AD96885/7 ECL
comparator they use, but was surprised to find that AD did not include
it on the datasheet. However, just from the spec table we can see why a
buffer is necessary:
Input Resistance: 200k ohms
Input Capacitance: 2pF
For starters, even at low frequencies the input impedance is only 200k
ohms, much less than the 1Mohm rated impedance. Further, the
capacitance seriously degrades bandwidth to as low as 400kHz (this
depends on the source impedance, but even with a 10kohm source the BW is
only 8MHz).
(Note that the input current spec is not necessarily a good proxy for
input impedance -- consider a FET-input amplifier with an input current
of 1uA and a 100k gate resistor.)
Best regards,
Charles
_______________________________________________
time-nuts mailing list -- time-nuts@febo.com
To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts
and follow the instructions there.
Moin,
On Tue, 20 Jun 2017 17:11:24 -0400
Charles Steinmetz csteinmetz@yandex.com wrote:
I was going tp post the Smith chart for the input of the AD96885/7 ECL
comparator they use, but was surprised to find that AD did not include
it on the datasheet. However, just from the spec table we can see why a
buffer is necessary:
Input Resistance: 200k ohms
Input Capacitance: 2pF
Thanks everyone! I was caught again in my DC line of thinking.
Even though I saw the capacitance spec, it didn't registers.
At some point I need to learn how to design RF circuits....
Attila Kinali
--
It is upon moral qualities that a society is ultimately founded. All
the prosperity and technological sophistication in the world is of no
use without that foundation.
-- Miss Matheson, The Diamond Age, Neil Stephenson
Bruce wrote:
If the source impendance at dc is 10k at dc a bias current of a few
microamp produces a dc offset of tens of millivolts which may be an
issue if the signal amplitude is low and has a low slew rate.
Jeez, Bruce, it was nothing but a thought experiment to illustrate why
an amplifier's DC input current is not a reliable proxy for input
impedance It was not a suggestion for how to design an amplifier input.
And that should have been obvious to you -- I'm sure it was to
everyone else.
Get a life.
Charles
Your assumptions about my comment are unfounded. It was just intended as an additional reason as to why direct connection to a comparator input isnt a good idea with a high source impedance. The original post asked why a buffer was needed.
Bruce
On 21 June 2017 at 22:14 Charles Steinmetz <csteinmetz@yandex.com> wrote:
Bruce wrote:
If the source impendance at dc is 10k at dc a bias current of a few
microamp produces a dc offset of tens of millivolts which may be an
issue if the signal amplitude is low and has a low slew rate.
Jeez, Bruce, it was nothing but a thought experiment to illustrate why
an amplifier's DC input current is not a reliable proxy for input
impedance It was not a suggestion for how to design an amplifier input.
And that should have been obvious to you -- I'm sure it was to
everyone else.
Get a life.
Charles
_______________________________________________
time-nuts mailing list -- time-nuts@febo.com
To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts
and follow the instructions there.