Microvolts, how not to mess it up?...

[Piotr Wyderski]

Morning everyone,

It is for another high-bandwidth current measurement attempt, in
parallel to the Rogowski coil research. The bandwidth is assumed to be
2MHz; the shunt voltage is <= 15V relatively to the measuring circuit in
all possible arrangements. No other constraints at the moment.

For now, it is just for evaluation purposes. Sensing current is one
thing, but I would like to check how precise it can be nowadays as well.
There is a shunt that produces 30mV max., but I would like to retain as
much accuracy and resolution as possible. So I want to process the
uV-level signals as well. The pipeline is supposed to look as follows:

Low impedance shunt -> differential amplifier -> ADC driver ->
high-resolution SAR ADC.

Ideally, the amplification should be ~40 times to cover the full ADC
range. So:

1. There are four-terminal shunts widely available, so there should be
no issue. The connections may cause problems, e.g. due to the contact
voltage. Welding instead of soldering and some digital offset
cancellation might be necessary.

2. Then there will be two cascaded amplification stages based on the
OPA189 with a gain of 6.3 each. The specs are insane for that price:
400nV offset @ 14MHz BW. Unfortunately, I cannot find FDAs of similar
performance, so the signal path by necessity would be differential
input/single-ended output. I don\'t like that, but what can I do?

3. The common-mode voltage will be well within the opamp\'s capabilities
(it is rated up to 36V VCC), but not spoiling the accuracy would require
insanely precise resistors. 0.1% would give me only ~60dB of CMRR. So
floating the entire pipeline looks better, despite the added complexity.
What do you think?

4. What to do with that 1.25V signal to drive a differential ADC? Just
buffer the +end with another OPA189 and apply the common-mode voltage to
the -end or should I use a more fancy single-ended to differential
converter? If so, what would be the secret sauce?

What else can go wrong?

Best regards, Piotr

 

[Jan Panteltje]

On a sunny day (Sun, 6 Sep 2020 08:44:50 +0200) it happened Piotr Wyderski
<peter.pan@neverland.mil> wrote in <rj20h1$ilv3$1@portraits.wsisiz.edu.pl>:

Morning everyone,

It is for another high-bandwidth current measurement attempt, in
parallel to the Rogowski coil research. The bandwidth is assumed to be
2MHz; the shunt voltage is <= 15V relatively to the measuring circuit in
all possible arrangements. No other constraints at the moment.

For now, it is just for evaluation purposes. Sensing current is one
thing, but I would like to check how precise it can be nowadays as well.
There is a shunt that produces 30mV max., but I would like to retain as
much accuracy and resolution as possible. So I want to process the
uV-level signals as well. The pipeline is supposed to look as follows:

Low impedance shunt -> differential amplifier -> ADC driver -
high-resolution SAR ADC.

Ideally, the amplification should be ~40 times to cover the full ADC
range. So:

1. There are four-terminal shunts widely available, so there should be
no issue. The connections may cause problems, e.g. due to the contact
voltage. Welding instead of soldering and some digital offset
cancellation might be necessary.

2. Then there will be two cascaded amplification stages based on the
OPA189 with a gain of 6.3 each. The specs are insane for that price:
400nV offset @ 14MHz BW. Unfortunately, I cannot find FDAs of similar
performance, so the signal path by necessity would be differential
input/single-ended output. I don\'t like that, but what can I do?

3. The common-mode voltage will be well within the opamp\'s capabilities
(it is rated up to 36V VCC), but not spoiling the accuracy would require
insanely precise resistors. 0.1% would give me only ~60dB of CMRR. So
floating the entire pipeline looks better, despite the added complexity.
What do you think?

4. What to do with that 1.25V signal to drive a differential ADC? Just
buffer the +end with another OPA189 and apply the common-mode voltage to
the -end or should I use a more fancy single-ended to differential
converter? If so, what would be the secret sauce?

What else can go wrong?

Best regards, Piotr

Have not tried it, but to measre uV DC maybe use a chopper system
and AC amplifier, followed by rectification and an ADC?
Cannot be that hard to periodically switch between ground and the signal
from the shunt to get an AC square wave? CMOS switch?

 

[Piotr Wyderski]

Jan Panteltje wrote:

Have not tried it, but to measre uV DC maybe use a chopper system
and AC amplifier, followed by rectification and an ADC?

Yes, this is the well-known approach, now integrated into that auto-zero
opamps. But the BW is not even close to what is needed.

Cannot be that hard to periodically switch between ground and the signal
from the shunt to get an AC square wave? CMOS switch?

If a sub-microvolt offset part is available for less than 3 dollars,
then a brute-force straightforward solution starts looking applicable.

The question then becomes \"how do I not flush this available performance
down the toilet?\"

Best regards, Piotr

 

[Bill Sloman]

On Sunday, September 6, 2020 at 5:20:59 PM UTC+10, Piotr Wyderski wrote:

Jan Panteltje wrote:

Have not tried it, but to measure uV DC maybe use a chopper system
and AC amplifier, followed by rectification and an ADC?
Yes, this is the well-known approach, now integrated into that auto-zero
opamps. But the BW is not even close to what is needed.

The chopper is there to get rid of any DC offsets - thermocouple voltages and the like. If you want lots of AC bandwidth, you can have it to - at least within each chopped segment. You might end up with two parallel signal paths with alternating positve and negative gain, which you\'d fip between the ends of centre-tapped 1:1 ratio transformer (which happens to look very like a transmission-line transformer, since it is typically wound with twisted pair, and should end up being 1:1 to about one part per billion, if you do it right).

Cannot be that hard to periodically switch between ground and the signal
from the shunt to get an AC square wave? CMOS switch?
If a sub-microvolt offset part is available for less than 3 dollars,
then a brute-force straightforward solution starts looking applicable.

The question then becomes \"how do I not flush this available performance
down the toilet?\"

Always the tricky part of any design,

--
Bill Sloman, Sydney

 

[Jan Panteltje]

On a sunny day (Sun, 6 Sep 2020 09:20:53 +0200) it happened Piotr Wyderski
<peter.pan@neverland.mil> wrote in <rj22kl$iopb$1@portraits.wsisiz.edu.pl>:

Jan Panteltje wrote:

Have not tried it, but to measre uV DC maybe use a chopper system
and AC amplifier, followed by rectification and an ADC?

Yes, this is the well-known approach, now integrated into that auto-zero
opamps. But the BW is not even close to what is needed.

Cannot be that hard to periodically switch between ground and the signal
from the shunt to get an AC square wave? CMOS switch?

If a sub-microvolt offset part is available for less than 3 dollars,
then a brute-force straightforward solution starts looking applicable.

10 turn trimpots are a great thing to have.
If it is for lab testing only, where the temperature is sort of always
the same, why not.

 

[Jan Panteltje]

PS
as I usually pick what I have used in the past, and possibly is in the junkbox,
my first try would be a 74HC4053 for the switch, 10 turn trimpot for offset correction,
switch at a few MHz, and then use one of those video 5 MHz wide AC amplifier chips..
Diode rectifier + RC filter or maybe some diode opamp thing, video FLASH ADC (32 MHz sampling).

But maybe I overlooked something

 

[server]

On Sun, 6 Sep 2020 08:44:50 +0200, Piotr Wyderski
<peter.pan@neverland.mil> wrote:

Morning everyone,

It is for another high-bandwidth current measurement attempt, in
parallel to the Rogowski coil research. The bandwidth is assumed to be
2MHz; the shunt voltage is <= 15V relatively to the measuring circuit in
all possible arrangements. No other constraints at the moment.

For now, it is just for evaluation purposes. Sensing current is one
thing, but I would like to check how precise it can be nowadays as well.
There is a shunt that produces 30mV max., but I would like to retain as
much accuracy and resolution as possible. So I want to process the
uV-level signals as well. The pipeline is supposed to look as follows:

Low impedance shunt -> differential amplifier -> ADC driver -
high-resolution SAR ADC.

I like to use a single-ended amplifier first, riding on one side of
the shunt. That would probably need isolated power in your case. That
gives a lot of gain first without any common-mode error. Get your
signal from millivolts to volts before dealing with the common-mode.

Ideally, the amplification should be ~40 times to cover the full ADC
range. So:

1. There are four-terminal shunts widely available, so there should be
no issue. The connections may cause problems, e.g. due to the contact
voltage. Welding instead of soldering and some digital offset
cancellation might be necessary.

A proper solder joint shouldn\'t have thermal offsets. But by all means
measure the offset at known zero current, and subtract that out.

2. Then there will be two cascaded amplification stages based on the
OPA189 with a gain of 6.3 each. The specs are insane for that price:
400nV offset @ 14MHz BW. Unfortunately, I cannot find FDAs of similar
performance, so the signal path by necessity would be differential
input/single-ended output. I don\'t like that, but what can I do?

3. The common-mode voltage will be well within the opamp\'s capabilities
(it is rated up to 36V VCC), but not spoiling the accuracy would require
insanely precise resistors. 0.1% would give me only ~60dB of CMRR. So
floating the entire pipeline looks better, despite the added complexity.
What do you think?

Gain is easier to get if there\'s no common-mode voltage to take out.



--

John Larkin Highland Technology, Inc

Science teaches us to doubt.

Claude Bernard

 

[Piotr Wyderski]

jlarkin@highlandsniptechnology.com wrote:

I like to use a single-ended amplifier first, riding on one side of
the shunt. That would probably need isolated power in your case. That
gives a lot of gain first without any common-mode error. Get your
signal from millivolts to volts before dealing with the common-mode.

This is how I see it as well, but once you have isolated power and the
signal in the volts range, floating the ADC too looks like a natural
next step. Then the cheapest available digital isolator in SSOP to cross
the whopping 15V barrier and it\'s done.

A proper solder joint shouldn\'t have thermal offsets. But by all means
measure the offset at known zero current, and subtract that out.

Yes; I just plan to do the calibration fully digitally to keep the
analog path simple.

> Gain is easier to get if there\'s no common-mode voltage to take out.

I see it the same way. An array of ultra-high-precision resistors to
deal with the wide common mode voltage would cost more than the rest of
the circuit. So floating it instead of going into excessive precision
appears kind of logical.

Thank you for your contribution, it\'s very appreciated.

Best regards, Piotr

 

[Phil Hobbs]

On 2020-09-06 02:44, Piotr Wyderski wrote:

Morning everyone,

It is for another high-bandwidth current measurement attempt, in
parallel to the Rogowski coil research. The bandwidth is assumed to be
2MHz; the shunt voltage is <= 15V relatively to the measuring circuit in
all possible arrangements. No other constraints at the moment.

For now, it is just for evaluation purposes. Sensing current is one
thing, but I would like to check how precise it can be nowadays as well.
There is a shunt that produces 30mV max., but I would like to retain as
much accuracy and resolution as possible. So I want to process the
uV-level signals as well. The pipeline is supposed to look as follows:

Low impedance shunt -> differential amplifier -> ADC driver -
high-resolution SAR ADC.

Ideally, the amplification should be ~40 times to cover the full ADC
range. So:

1. There are four-terminal shunts widely available, so there should be
no issue. The connections may cause problems, e.g. due to the contact
voltage. Welding instead of soldering and some digital offset
cancellation might be necessary.

2. Then there will be two cascaded amplification stages based on the
OPA189 with a gain of 6.3 each. The specs are insane for that price:
400nV offset @ 14MHz BW. Unfortunately, I cannot find FDAs of similar
performance, so the signal path by necessity would be differential
input/single-ended output. I don\'t like that, but what can I do?

3. The common-mode voltage will be well within the opamp\'s capabilities
(it is rated up to 36V VCC), but not spoiling the accuracy would require
insanely precise resistors. 0.1% would give me only ~60dB of CMRR. So
floating the entire pipeline looks better, despite the added complexity.
What do you think?

4. What to do with that 1.25V signal to drive a differential ADC? Just
buffer the +end with another OPA189 and apply the common-mode voltage to
the -end or should I use a more fancy single-ended to differential
converter? If so, what would be the secret sauce?

If you attach two regular old noninverting amplifiers across the shunt,
you can get a differential output with a CM gain of 1.0000 and a
differential gain of whatever you like. (The input resistors to the
noninverting stages go to the opposite end of the shunt.) One good thing
about this measurement is that you don\'t care too much about high-Z
inputs--it\'ll just perturb the effective value of the shunt resistance a
little. This is a slight modification of the usual instrumentation amp
front end.

A composite amp with something like an LM6171 inside the feedback loop
of the OPA189 would allow you to get much higher GBW while maintaining
the low input error. (I\'m normally not a huge fan of composite amps
because their long-time settling behaviour tends to be a bit strange.)

You can use a chopamp to force the offset voltage of your fave
differential-output amp to zero. You\'ll need good resistors, but that\'s
all happening after the main amplifier, so you don\'t need such good
accuracy there.

Cheers

Phil Hobbs

--
Dr Philip C D Hobbs
Principal Consultant
ElectroOptical Innovations LLC / Hobbs ElectroOptics
Optics, Electro-optics, Photonics, Analog Electronics
Briarcliff Manor NY 10510

http://electrooptical.net
http://hobbs-eo.com

 

[Piotr Wyderski]

Phil Hobbs wrote:

A composite amp with something like an LM6171 inside the feedback loop
of the OPA189 would allow you to get much higher GBW while maintaining
the low input error.

I didn\'t know this composite opamp idea, but now, knowing the name, I
was able to find some good papers on this subject. This is extremely
interesting on its own, thank you!

You can use a chopamp to force the offset voltage of your fave
differential-output amp to zero.

How would that happen? Are we still considering the composite two opamp
loop?

Best regards, Piotr

 

[John Larkin]

On Sun, 6 Sep 2020 19:39:03 +0200, Piotr Wyderski
<peter.pan@neverland.mil> wrote:

jlarkin@highlandsniptechnology.com wrote:

I like to use a single-ended amplifier first, riding on one side of
the shunt. That would probably need isolated power in your case. That
gives a lot of gain first without any common-mode error. Get your
signal from millivolts to volts before dealing with the common-mode.

This is how I see it as well, but once you have isolated power and the
signal in the volts range, floating the ADC too looks like a natural
next step. Then the cheapest available digital isolator in SSOP to cross
the whopping 15V barrier and it\'s done.

A proper solder joint shouldn\'t have thermal offsets. But by all means
measure the offset at known zero current, and subtract that out.

Yes; I just plan to do the calibration fully digitally to keep the
analog path simple.

Gain is easier to get if there\'s no common-mode voltage to take out.

I see it the same way. An array of ultra-high-precision resistors to
deal with the wide common mode voltage would cost more than the rest of
the circuit. So floating it instead of going into excessive precision
appears kind of logical.

Thank you for your contribution, it\'s very appreciated.

Best regards, Piotr

Here\'s an isolated current shunt thing.

https://www.dropbox.com/s/v9qax6kxflrasae/23S902A_sh2.pdf?dl=0

https://www.dropbox.com/s/lulztcfar8o6sc8/P902_Shunts.jpg?raw=1

Probably too slow for you, about 160 KHz at 15 bits.





--

John Larkin Highland Technology, Inc trk

The cork popped merrily, and Lord Peter rose to his feet.
\"Bunter\", he said, \"I give you a toast. The triumph of Instinct over Reason\"

 

[Phil Hobbs]

On 2020-09-06 16:30, Piotr Wyderski wrote:

Phil Hobbs wrote:

A composite amp with something like an LM6171 inside the feedback loop
of the OPA189 would allow you to get much higher GBW while maintaining
the low input error.

I didn\'t know this composite opamp idea, but now, knowing the name, I
was able to find some good papers on this subject. This is extremely
interesting on its own, thank you!

You can use a chopamp to force the offset voltage of your fave
differential-output amp to zero.

How would that happen? Are we still considering the composite two opamp
loop?

    Best regards, Piotr

It\'s easier with a single-ended circuit. For an inverting amp, you
connect the chopamp as an (inverting) integrator and use its output to
drive the noninverting input of the main amp. (It\'s often useful to
drive the noninverting input via a voltage divider, so that the main
amp\'s summing junction dynamics don\'t mess things up.)

For a fully-differential amp (with external resistor feedback), you can
make the input error go to zero using false summing nodes. The FDA is
going to servo itself to null out its idea of its differential input
voltage, so you can\'t do much about that. However, if you put resistors
between its inputs and the summing nodes, you can use the chopamp to
dump some current into the FDA\'s input nodes such that the differential
voltage between the two summing nodes gets zeroed out. (Full
disclosure: I\'ve never actually needed to do that.)

Cheers

Phil Hobbs

--
Dr Philip C D Hobbs
Principal Consultant
ElectroOptical Innovations LLC / Hobbs ElectroOptics
Optics, Electro-optics, Photonics, Analog Electronics
Briarcliff Manor NY 10510

http://electrooptical.net
http://hobbs-eo.com

 

[Hul Tytus]

Piotr whats the low frequency limit? If it\'s fairly high,
a capacitor would serve well & reduce offset errors.

Hul

Piotr Wyderski <peter.pan@neverland.mil> wrote:

Morning everyone,

It is for another high-bandwidth current measurement attempt, in
parallel to the Rogowski coil research. The bandwidth is assumed to be
2MHz; the shunt voltage is <= 15V relatively to the measuring circuit in
all possible arrangements. No other constraints at the moment.

For now, it is just for evaluation purposes. Sensing current is one
thing, but I would like to check how precise it can be nowadays as well.
There is a shunt that produces 30mV max., but I would like to retain as
much accuracy and resolution as possible. So I want to process the
uV-level signals as well. The pipeline is supposed to look as follows:

Low impedance shunt -> differential amplifier -> ADC driver -
high-resolution SAR ADC.

Ideally, the amplification should be ~40 times to cover the full ADC
range. So:

1. There are four-terminal shunts widely available, so there should be
no issue. The connections may cause problems, e.g. due to the contact
voltage. Welding instead of soldering and some digital offset
cancellation might be necessary.

2. Then there will be two cascaded amplification stages based on the
OPA189 with a gain of 6.3 each. The specs are insane for that price:
400nV offset @ 14MHz BW. Unfortunately, I cannot find FDAs of similar
performance, so the signal path by necessity would be differential
input/single-ended output. I don\'t like that, but what can I do?

3. The common-mode voltage will be well within the opamp\'s capabilities
(it is rated up to 36V VCC), but not spoiling the accuracy would require
insanely precise resistors. 0.1% would give me only ~60dB of CMRR. So
floating the entire pipeline looks better, despite the added complexity.
What do you think?

4. What to do with that 1.25V signal to drive a differential ADC? Just
buffer the +end with another OPA189 and apply the common-mode voltage to
the -end or should I use a more fancy single-ended to differential
converter? If so, what would be the secret sauce?

What else can go wrong?

Best regards, Piotr