Circuit in question is the diff amp current to voltage U2 upper right hand
corner.

Jim

--
"It is the mark of an educated mind to be able to entertain a thought
without accepting it."
--Aristotle

"RST Engineering \(jw\)" wrote:

Circuit in question is the diff amp current to voltage U2 upper right
hand corner.

Jim

It's a bit blurred, but it looks like a LM358. If so, the input voltage
range is 0 to (Vcc - 1.5V), so it won't work at the positive rail.

Also, I wonder if placing the circuit just after the LM317 would be a bit
better. It would give more headroom for the LM358, and the 0.25 ohm current
sense resistor shouldn't bother the LM317.

Regards,

Mike Monett

Loading the output of A or B with a resistive load (1.0K) makes not a whit
of difference.

Jim

--
"It is the mark of an educated mind to be able to entertain a thought
without accepting it."
--Aristotle


"BobW" wrote in message
...

"RST Engineering (jw)" wrote in message
...
Circuit in question is the diff amp current to voltage U2 upper right
hand corner.

Jim


Is the meter connected to the U101A or U101B?

If it's connected to U101B, try moving it to U101A to see if it helps
pulling the output closer to your minus supply when the load current is
zero.

Bob
--
== All google group posts are automatically deleted due to spam ==

It is a TL082. This thread started over in SEdesign and was ported over
here strictly to be able to upload a schematic. THe 358 we knew wouldn't
work and we were looking for a drop-in to replace it.

Jim

--
"It is the mark of an educated mind to be able to entertain a thought
without accepting it."
--Aristotle


"Mike Monett" wrote in message
...
"RST Engineering \(jw\)" wrote:

Circuit in question is the diff amp current to voltage U2 upper right
hand corner.

Jim

It's a bit blurred, but it looks like a LM358. If so, the input voltage
range is 0 to (Vcc - 1.5V), so it won't work at the positive rail.

Also, I wonder if placing the circuit just after the LM317 would be a bit
better. It would give more headroom for the LM358, and the 0.25 ohm
current
sense resistor shouldn't bother the LM317.

Regards,

Mike Monett

It is a TL082. This thread started over in SEdesign and was ported over
here strictly to be able to upload a schematic. THe 358 we knew wouldn't
work and we were looking for a drop-in to replace it.

Jim

--
"It is the mark of an educated mind to be able to entertain a thought
without accepting it."
--Aristotle


"Mike Monett" wrote in message
...
"RST Engineering \(jw\)" wrote:

Circuit in question is the diff amp current to voltage U2 upper right
hand corner.

Jim

It's a bit blurred, but it looks like a LM358. If so, the input voltage
range is 0 to (Vcc - 1.5V), so it won't work at the positive rail.

Also, I wonder if placing the circuit just after the LM317 would be a bit
better. It would give more headroom for the LM358, and the 0.25 ohm
current
sense resistor shouldn't bother the LM317.

Regards,

Mike Monett

"RST Engineering \(jw\)" wrote:

It is a TL082. This thread started over in SEdesign and was ported
over here strictly to be able to upload a schematic. THe 358 we knew
wouldn't work and we were looking for a drop-in to replace it.

Jim

The TL082 has a wierd common mode spec. The minimum spec is +/- 11V for a
+/- 15V supply, but the numbers in the typical column are +15V, -12V, so
the range depends on the bias voltage on the input JFETs, J1 and J2.

I'd move the current sense circuit to the output of the LM317, and make
sure there is enough headroom to meet the minimum value of +11V at the
TL082 input pins. That would take some rework which might not be desirable.

An alternative might be to reduce the input common mode voltage in your
circuit by dropping the value of R102 and R109 to between 1k and 10k,
depending on your requirements. The goal is to bring the TL082 input to
something less than 11V, but not attenuate the desired signal by a large
amount. You could make up any gain loss in the second TL082.

Another method of extending the common mode range is shown in the article
"Remove Large Common-Mode Signals Using Standard Op Amps", at

http://electronicdesign.com/Articles...4832/4832.html

I hope this helps!

Best Regards,

Mike Monett

"RST Engineering \(jw\)" wrote:

It is a TL082. This thread started over in SEdesign and was ported
over here strictly to be able to upload a schematic. THe 358 we knew
wouldn't work and we were looking for a drop-in to replace it.

Jim

The TL082 has a wierd common mode spec. The minimum spec is +/- 11V for a
+/- 15V supply, but the numbers in the typical column are +15V, -12V, so
the range depends on the bias voltage on the input JFETs, J1 and J2.

I'd move the current sense circuit to the output of the LM317, and make
sure there is enough headroom to meet the minimum value of +11V at the
TL082 input pins. That would take some rework which might not be desirable.

An alternative might be to reduce the input common mode voltage in your
circuit by dropping the value of R102 and R109 to between 1k and 10k,
depending on your requirements. The goal is to bring the TL082 input to
something less than 11V, but not attenuate the desired signal by a large
amount. You could make up any gain loss in the second TL082.

Another method of extending the common mode range is shown in the article
"Remove Large Common-Mode Signals Using Standard Op Amps", at

http://electronicdesign.com/Articles...4832/4832.html

I hope this helps!

Best Regards,

Mike Monett

On Fri, 05 Dec 2008 01:24:53 +0000, Mike Monett wrote:

"RST Engineering \(jw\)" wrote:

It is a TL082. This thread started over in SEdesign and was ported
over here strictly to be able to upload a schematic. THe 358 we knew
wouldn't work and we were looking for a drop-in to replace it.

Jim

The TL082 has a wierd common mode spec. The minimum spec is +/- 11V for a
+/- 15V supply, but the numbers in the typical column are +15V, -12V, so
the range depends on the bias voltage on the input JFETs, J1 and J2.

I'd move the current sense circuit to the output of the LM317, and make
sure there is enough headroom to meet the minimum value of +11V at the
TL082 input pins. That would take some rework which might not be desirable.

An alternative might be to reduce the input common mode voltage in your
circuit by dropping the value of R102 and R109 to between 1k and 10k,
depending on your requirements. The goal is to bring the TL082 input to
something less than 11V, but not attenuate the desired signal by a large
amount. You could make up any gain loss in the second TL082.

Another method of extending the common mode range is shown in the article
"Remove Large Common-Mode Signals Using Standard Op Amps", at

http://electronicdesign.com/Articles...4832/4832.html

I hope this helps!

Best Regards,

Mike Monett

Common mode range of TL08x extends to the rails.

...Jim Thompson
--
| James E.Thompson, P.E. | mens |
| Analog Innovations, Inc. | et |
| Analog/Mixed-Signal ASIC's and Discrete Systems | manus |
| Phoenix, Arizona 85048 Skype: Contacts Only | |
| Voice480)460-2350 Fax: Available upon request | Brass Rat |
| E-mail Icon at http://www.analog-innovations.com | 1962 |

I love to cook with wine Sometimes I even put it in the food

Mike Monett wrote:

[...]

An alternative might be to reduce the input common mode voltage in
your circuit by dropping the value of R102 and R109 to between 1k and
10k, depending on your requirements. The goal is to bring the TL082
input to something less than 11V, but not attenuate the desired signal
by a large amount.

Thinking more on this circuit in my head, you would need one more resistor
from the negative input to ground. It would have the same value as the R109
and make a bridge. Matched resistors would probably eliminate the need for
a trim pot, and the gain loss could be made up by increasing R102.

This is probably the minimum change approach and easiest to implement.

Regards,

Mike Monett

Mike Monett wrote:

[...]

An alternative might be to reduce the input common mode voltage in
your circuit by dropping the value of R102 and R109 to between 1k and
10k, depending on your requirements. The goal is to bring the TL082
input to something less than 11V, but not attenuate the desired signal
by a large amount.

Thinking more on this circuit in my head, you would need one more resistor
from the negative input to ground. It would have the same value as the R109
and make a bridge. Matched resistors would probably eliminate the need for
a trim pot, and the gain loss could be made up by increasing R102.

This is probably the minimum change approach and easiest to implement.

Regards,

Mike Monett

On Thu, 04 Dec 2008 18:33:54 -0700, Jim Thompson
wrote:


On Fri, 05 Dec 2008 01:24:53 +0000, Mike Monett wrote:

"RST Engineering \(jw\)" wrote:

It is a TL082. This thread started over in SEdesign and was ported
over here strictly to be able to upload a schematic. THe 358 we knew
wouldn't work and we were looking for a drop-in to replace it.

Jim

The TL082 has a wierd common mode spec. The minimum spec is +/- 11V for a
+/- 15V supply, but the numbers in the typical column are +15V, -12V, so
the range depends on the bias voltage on the input JFETs, J1 and J2.

I'd move the current sense circuit to the output of the LM317, and make
sure there is enough headroom to meet the minimum value of +11V at the
TL082 input pins. That would take some rework which might not be desirable.

An alternative might be to reduce the input common mode voltage in your
circuit by dropping the value of R102 and R109 to between 1k and 10k,
depending on your requirements. The goal is to bring the TL082 input to
something less than 11V, but not attenuate the desired signal by a large
amount. You could make up any gain loss in the second TL082.

Another method of extending the common mode range is shown in the article
"Remove Large Common-Mode Signals Using Standard Op Amps", at

http://electronicdesign.com/Articles...4832/4832.html

I hope this helps!

Best Regards,

Mike Monett

Common mode range of TL08x extends to the rails.

...Jim Thompson

You could show your students an analysis of how small errors in the
classic "differential amplifier" are disastrous.

The real world ain't AofE... the real world is nasty.

I penciled out a real world (nearly trivial) solution just now over
nachos... will post tomorrow ;-)

...Jim Thompson
--
| James E.Thompson, P.E. | mens |
| Analog Innovations, Inc. | et |
| Analog/Mixed-Signal ASIC's and Discrete Systems | manus |
| Phoenix, Arizona 85048 Skype: Contacts Only | |
| Voice480)460-2350 Fax: Available upon request | Brass Rat |
| E-mail Icon at http://www.analog-innovations.com | 1962 |

It's what you learn, after you know it all, that counts.

On Thu, 04 Dec 2008 19:45:44 -0700, Jim Thompson
wrote:


On Thu, 04 Dec 2008 18:33:54 -0700, Jim Thompson
wrote:


On Fri, 05 Dec 2008 01:24:53 +0000, Mike Monett wrote:

"RST Engineering \(jw\)" wrote:

It is a TL082. This thread started over in SEdesign and was ported
over here strictly to be able to upload a schematic. THe 358 we knew
wouldn't work and we were looking for a drop-in to replace it.

Jim

The TL082 has a wierd common mode spec. The minimum spec is +/- 11V for a
+/- 15V supply, but the numbers in the typical column are +15V, -12V, so
the range depends on the bias voltage on the input JFETs, J1 and J2.

I'd move the current sense circuit to the output of the LM317, and make
sure there is enough headroom to meet the minimum value of +11V at the
TL082 input pins. That would take some rework which might not be desirable.

An alternative might be to reduce the input common mode voltage in your
circuit by dropping the value of R102 and R109 to between 1k and 10k,
depending on your requirements. The goal is to bring the TL082 input to
something less than 11V, but not attenuate the desired signal by a large
amount. You could make up any gain loss in the second TL082.

Another method of extending the common mode range is shown in the article
"Remove Large Common-Mode Signals Using Standard Op Amps", at

http://electronicdesign.com/Articles...4832/4832.html

I hope this helps!

Best Regards,

Mike Monett

Common mode range of TL08x extends to the rails.

...Jim Thompson

You could show your students an analysis of how small errors in the
classic "differential amplifier" are disastrous.

The real world ain't AofE... the real world is nasty.

I penciled out a real world (nearly trivial) solution just now over
nachos... will post tomorrow ;-)

...Jim Thompson


If you've got a r-r amp, one thing to do is get some gain still
referenced to the V+ rail, then diff-amp that. That makes the cmrr of
the diffamp matter a lot less. That's what I do with current shunts...
get some good gain *before* dealing with the common-mode.

Even better, add a transistor or two. Teach the kids about discretes!

Or use a dpdt ssr as a flying-cap voltage mover.

Hey, this is weird...

John

Jim Thompson wrote:

Common mode range of TL08x extends to the rails.

...Jim Thompson

Typical, not guaranteed, and only to the positive rail.

From the National spec,

http://www.national.com/ds/TL/TL082.pdf

VCM Input Common-Mode Voltage Range with Vs = ±15V is ±11V min.

Mike Monett

John Larkin wrote:

If you've got a r-r amp, one thing to do is get some gain still
referenced to the V+ rail, then diff-amp that. That makes the cmrr of
the diffamp matter a lot less. That's what I do with current shunts...
get some good gain *before* dealing with the common-mode.

Why should it make any difference? If you are trying to get gain at the V+
rail, you are still dealing with cmrr. The opamp you are using for gain is
still taking a small difference between two large numbers.

Also, the circuit you drew requires base current for the NPN. That will add
aother undefined and variable error to the output.

John

Mike

On Fri, 05 Dec 2008 04:13:50 +0000, Mike Monett wrote:

John Larkin wrote:

If you've got a r-r amp, one thing to do is get some gain still
referenced to the V+ rail, then diff-amp that. That makes the cmrr of
the diffamp matter a lot less. That's what I do with current shunts...
get some good gain *before* dealing with the common-mode.

Why should it make any difference? If you are trying to get gain at the V+
rail, you are still dealing with cmrr. The opamp you are using for gain is
still taking a small difference between two large numbers.

I can't express it any simpler than I already have. A possible
schematic is attached.


Also, the circuit you drew requires base current for the NPN. That will add
aother undefined and variable error to the output.

Use a high-beta transistor, like a BCX71. The error will be far below
1%, and that will be swamped by the resistor tolerances. Or use a fet
if you have to. If you really want to whine about something, consider
stabilizing this loop.

John

Nachos be good.

Margaritas be unacceptable.

Real world is what we give to our students out-the-door.

{;0(

Jim


You could show your students an analysis of how small errors in the
classic "differential amplifier" are disastrous.

The real world ain't AofE... the real world is nasty.

I penciled out a real world (nearly trivial) solution just now over
nachos... will post tomorrow ;-)

...Jim Thompson
--
| James E.Thompson, P.E. | mens |
| Analog Innovations, Inc. | et |
| Analog/Mixed-Signal ASIC's and Discrete Systems | manus |
| Phoenix, Arizona 85048 Skype: Contacts Only | |
| Voice480)460-2350 Fax: Available upon request | Brass Rat |
| E-mail Icon at http://www.analog-innovations.com | 1962 |

It's what you learn, after you know it all, that counts.

John Larkin wrote:

On Fri, 05 Dec 2008 04:13:50 +0000, Mike Monett wrote:

John Larkin wrote:

If you've got a r-r amp, one thing to do is get some gain still
referenced to the V+ rail, then diff-amp that. That makes the cmrr
of the diffamp matter a lot less. That's what I do with current
shunts... get some good gain *before* dealing with the common-mode.

Why should it make any difference? If you are trying to get gain at
the V+ rail, you are still dealing with cmrr. The opamp you are using
for gain is still taking a small difference between two large numbers.

I can't express it any simpler than I already have. A possible
schematic is attached.

The bottom op amp is still connected to the +Vcc rail. How does that
eliminate cmrr error?

Also, the circuit you drew requires base current for the NPN. That
will add aother undefined and variable error to the output.

Use a high-beta transistor, like a BCX71. The error will be far below
1%, and that will be swamped by the resistor tolerances. Or use a fet
if you have to. If you really want to whine about something, consider
stabilizing this loop.

John


begin 644 DSCF1815.JPG

Attachment saved: C:\0DNLOAD\DSCF1815.JPG
`
end

Neither of your proposals reduce cmrr errors. Tossing more parts at it
doesn't help.

Mike

"John Larkin" wrote in message
...
On Fri, 05 Dec 2008 04:13:50 +0000, Mike Monett wrote:

John Larkin wrote:

If you've got a r-r amp, one thing to do is get some gain still
referenced to the V+ rail, then diff-amp that. That makes the cmrr of
the diffamp matter a lot less. That's what I do with current shunts...
get some good gain *before* dealing with the common-mode.

Why should it make any difference? If you are trying to get gain at the V+
rail, you are still dealing with cmrr. The opamp you are using for gain is
still taking a small difference between two large numbers.

I can't express it any simpler than I already have. A possible
schematic is attached.


Also, the circuit you drew requires base current for the NPN. That will
add
aother undefined and variable error to the output.

Use a high-beta transistor, like a BCX71. The error will be far below
1%, and that will be swamped by the resistor tolerances. Or use a fet
if you have to. If you really want to whine about something, consider
stabilizing this loop.


It's easy to stabilize if you:
1 swap the + and - inputs of the op-amp,
2 use an high-Beta PNP (or better, P-FET),
3 and swap it's E and C in your drawing.

That also leaves all 'high voltages' outside the opamp circuit. It's easy to
do accurate current measurements on a +120V rail that way. The opamp can be
fed from a R + Zener circuit.

Regards,
Arie de Muynck

On Fri, 05 Dec 2008 06:51:11 +0000, Mike Monett wrote:

John Larkin wrote:

On Fri, 05 Dec 2008 04:13:50 +0000, Mike Monett wrote:

John Larkin wrote:

If you've got a r-r amp, one thing to do is get some gain still
referenced to the V+ rail, then diff-amp that. That makes the cmrr
of the diffamp matter a lot less. That's what I do with current
shunts... get some good gain *before* dealing with the common-mode.

Why should it make any difference? If you are trying to get gain at
the V+ rail, you are still dealing with cmrr. The opamp you are using
for gain is still taking a small difference between two large numbers.

I can't express it any simpler than I already have. A possible
schematic is attached.

The bottom op amp is still connected to the +Vcc rail. How does that
eliminate cmrr error?

Nothing *eliminates* cmrr error. But in a case like this, the big
error isn't usually from the opamp, it's from mismatch in the four
diffamp resistors. You can buy a superb opamp for a fraction of the
price of a single 0.01% resistor. This configuration improves the cmrr
of the second amp, again dominated by resistor mismatch, by the
single-ended gain of the first amp.


Also, the circuit you drew requires base current for the NPN. That
will add aother undefined and variable error to the output.

Use a high-beta transistor, like a BCX71. The error will be far below
1%, and that will be swamped by the resistor tolerances. Or use a fet
if you have to. If you really want to whine about something, consider
stabilizing this loop.

John


begin 644 DSCF1815.JPG

Attachment saved: C:\0DNLOAD\DSCF1815.JPG
`
end

Neither of your proposals reduce cmrr errors. Tossing more parts at it
doesn't help.

Stop thinking about the cmrr spec of the opamp and start thinking
about the overall circuit performance. If you weren't being so
p(r)issy, I'd show you a few more tricks.

John

On Fri, 5 Dec 2008 07:54:42 +0100, "Arie de Muynck"
wrote:


"John Larkin" wrote in message
.. .
On Fri, 05 Dec 2008 04:13:50 +0000, Mike Monett wrote:

John Larkin wrote:

If you've got a r-r amp, one thing to do is get some gain still
referenced to the V+ rail, then diff-amp that. That makes the cmrr of
the diffamp matter a lot less. That's what I do with current shunts...
get some good gain *before* dealing with the common-mode.

Why should it make any difference? If you are trying to get gain at the V+
rail, you are still dealing with cmrr. The opamp you are using for gain is
still taking a small difference between two large numbers.

I can't express it any simpler than I already have. A possible
schematic is attached.


Also, the circuit you drew requires base current for the NPN. That will
add
aother undefined and variable error to the output.

Use a high-beta transistor, like a BCX71. The error will be far below
1%, and that will be swamped by the resistor tolerances. Or use a fet
if you have to. If you really want to whine about something, consider
stabilizing this loop.


It's easy to stabilize if you:
1 swap the + and - inputs of the op-amp,
2 use an high-Beta PNP (or better, P-FET),
3 and swap it's E and C in your drawing.


Absolutely. But then it's not as charmingly weird a circuit. It
becomes a standard current mirror.

My circuit has some really bizarre features. The output is the emitter
of an NPN that's inside the loop of an opamp, but the final output
impedance is *not* low. And any external load complicates an already
grossly complex loop dynamic.

Either circuit has a potential windup problem at near-zero load
currents. That may not matter if you're just driving a meter or
something.

John

On Thu, 4 Dec 2008 22:39:37 -0800, "RST Engineering \(jw\)"
wrote:

Nachos be good.

Margaritas be unacceptable.

Real world is what we give to our students out-the-door.

{;0(

Jim


You could show your students an analysis of how small errors in the
classic "differential amplifier" are disastrous.

The real world ain't AofE... the real world is nasty.

I penciled out a real world (nearly trivial) solution just now over
nachos... will post tomorrow ;-)

...Jim Thompson
[snip]

You don't like Margaritas ?:-)

See attachment for how I'd avoid the extraordinary matching required
to accomplish the "academic" differential OpAmp.

...Jim Thompson
--
| James E.Thompson, P.E. | mens |
| Analog Innovations, Inc. | et |
| Analog/Mixed-Signal ASIC's and Discrete Systems | manus |
| Phoenix, Arizona 85048 Skype: Contacts Only | |
| Voice480)460-2350 Fax: Available upon request | Brass Rat |
| E-mail Icon at http://www.analog-innovations.com | 1962 |

I love to cook with wine Sometimes I even put it in the food

On Thu, 4 Dec 2008 17:47:57 -0800, "BobW"
wrote:



It'll probably work to the rails but it's not guaranteed. Per the National
datasheet, the min common mode voltage range is +-11V when the supply is
+-15V.

http://www.national.com/ds/TL/TL082.pdf

On the other hand, per the Linear Tech LT1366 data sheet:

"Input Common Mode Range Includes Both Rails"
"Output Swings Rail-to-Rail"

Bob

For the price of a LT1366 Currently $6.52 Canadian at Newark,he might
as well just buy a LTC6102 for $3.72 and be done with it.

LTC6102 Data sheet

http://www.linear.com/pc/downloadDoc... P38188,D25174

On Fri, 05 Dec 2008 02:01:38 +0000, Mike Monett wrote:

Mike Monett wrote:

[...]

An alternative might be to reduce the input common mode voltage in
your circuit by dropping the value of R102 and R109 to between 1k and
10k, depending on your requirements. The goal is to bring the TL082
input to something less than 11V, but not attenuate the desired signal
by a large amount.

Thinking more on this circuit in my head, you would need one more resistor
from the negative input to ground. It would have the same value as the R109
and make a bridge. Matched resistors would probably eliminate the need for
a trim pot, and the gain loss could be made up by increasing R102.

This is probably the minimum change approach and easiest to implement.

Regards,

Mike Monett

Use U101a to level-shift with reduced gain and reintroduce gain at
U101B.

Alternately, the OPA2137 common mode input range includes +Vcc. There
must be others.

RL