Re: [volt-nuts] HP3458 ADC integrator (SPARES)

[Andre]

Can you make use of an LM3900? I have one here *somewhere*.
I think its a quad Norton, not sure how long its been there for.

also have a few canned op-amps etc
-Andre

From: volt-nuts <volt-nuts-boun...@febo.com> on behalf of Attila Kinali 
<att...@kinali.ch>
Sent: 27 February 2018 18:25
To: Discussion of precise voltage measurement
Subject: Re: [volt-nuts] HP3458 ADC integrator

On Sat, 24 Feb 2018 12:59:21 +1300 (NZDT)
Bruce Griffiths <bruce.griffi...@xtra.co.nz> wrote:

> 1) Reverse the supplies on U4 (in real circuit reversed supplies will fry the 
> chip.)
> 2) Add 10k in series with reset switch
> 3) Swap connections to U4 inputs.
> 4) Use series shunt switch for reset switch as used in an actual 3458A.
> Once done together with swapping in LT1220 for LT1225 rest works as expected 
> and output slews due to gate current of J1 and input bias current of U1

Ok, I played around a bit more and I think I now mostly understand what's
going on. The main problem was really the stability of the opamps chosen.
For the integrator to work, the added up gain and phase shift must still
fulfill the stability condition (obvious, yes, but.. well I am stupid at times)
I am not sure how the intermediate steps in a 3 opamp integrator work,
but I guess that should imediatly be obvious once a complete stability
analyis is done.

Thanks everyone who provided insight and feedback.

Attila Kinali

--
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the prosperity and technological sophistication in the world is of no
use without that foundation.
 -- Miss Matheson, The Diamond Age, Neil Stephenson
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Re: [volt-nuts] HP3458 ADC integrator

[Attila Kinali]

On Sat, 24 Feb 2018 12:59:21 +1300 (NZDT)
Bruce Griffiths  wrote:

> 1) Reverse the supplies on U4 (in real circuit reversed supplies will fry the 
> chip.)
> 2) Add 10k in series with reset switch
> 3) Swap connections to U4 inputs.
> 4) Use series shunt switch for reset switch as used in an actual 3458A.
> Once done together with swapping in LT1220 for LT1225 rest works as expected 
> and output slews due to gate current of J1 and input bias current of U1 

Ok, I played around a bit more and I think I now mostly understand what's
going on. The main problem was really the stability of the opamps chosen.
For the integrator to work, the added up gain and phase shift must still
fulfill the stability condition (obvious, yes, but.. well I am stupid at times)
I am not sure how the intermediate steps in a 3 opamp integrator work,
but I guess that should imediatly be obvious once a complete stability
analyis is done.

Thanks everyone who provided insight and feedback.

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
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Re: [volt-nuts] HP3458 ADC integrator

[David C. Partridge]

I eat humble pie - I re-checked the datasheets low limit of 550V/V it is (wow 
that's really low)

David

-Original Message-
From: volt-nuts [mailto:volt-nuts-boun...@febo.com] On Behalf Of David C. 
Partridge
Sent: 23 February 2018 22:06
To: 'Discussion of precise voltage measurement'
Subject: Re: [volt-nuts] HP3458 ADC integrator

> The datasheet I have found (Natsemi Sept 1995) lists 550 V/V = 0.55 V/mV.
> Which I think is a bit low, but then, the opamp is old.

Check other datasheets, that's definitely wrong

David

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Re: [volt-nuts] HP3458 ADC integrator

[Bruce Griffiths]

1) Reverse the supplies on U4 (in real circuit reversed supplies will fry the 
chip.)
2) Add 10k in series with reset switch
3) Swap connections to U4 inputs.
4) Use series shunt switch for reset switch as used in an actual 3458A.
Once done together with swapping in LT1220 for LT1225 rest works as expected 
and output slews due to gate current of J1 and input bias current of U1 

Bruce
> On 24 February 2018 at 10:23 Attila Kinali  wrote:
> 
> 
> On Fri, 23 Feb 2018 18:48:24 -
> "David C. Partridge"  wrote:
> 
> > Looking your schematic, I see that you are using LT1225 for both U2 (U111) 
> > and U3 (U112)
> > 
> > Why did you use those rather than LM6361 and AD848?  Was it just because 
> > LTSpice didn't have models for them?  It can make a difference ...
> 
> Yes. LTSpice doesn't have an LM6361 or AD848. Linear lists the LT1225
> as replacement for LM6361, so I thought this was safe. I deemed the AD848
> to be close enough.
> 
> But as Bruce mentioned, the LT1225 is not unity gain stable, while the
> LM6361 is. That might explain the oscillations.
> 
> > BTW LM6361 is 550V/mv not .55V (I assume that was a mistake in your post) 
> > ... 
> > Slew rates seem a bit different at +/- 15V supply as well.
> 
> The datasheet I have found (Natsemi Sept 1995) lists 550 V/V = 0.55 V/mV.
> Which I think is a bit low, but then, the opamp is old.
> 
>   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
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Re: [volt-nuts] HP3458 ADC integrator

[Bruce Griffiths]

Why do you think that the datasheet is wrong?

Its the same as the 1989 version.

A lot of the early wideband opamps didn't have high open loop gain.

Even the LT1220 only has about 7 - 15x more gain at rated load.

The LT1220 has a a pair of cascaded complementary symmetry emitter followers in 
its output stage compared to the single complementary symmetry emitter follower 
output stage of the LM6361.

Bruce

 

> 
> On 24 February 2018 at 11:05 "David C. Partridge" 
>  wrote:
> 
> > > 
> > The datasheet I have found (Natsemi Sept 1995) lists 550 V/V = 0.55 
> > V/mV.
> > Which I think is a bit low, but then, the opamp is old.
> > 
> > > 
> Check other datasheets, that's definitely wrong
> 
> David
> 
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Re: [volt-nuts] HP3458 ADC integrator

[David C. Partridge]

> The datasheet I have found (Natsemi Sept 1995) lists 550 V/V = 0.55 V/mV.
> Which I think is a bit low, but then, the opamp is old.

Check other datasheets, that's definitely wrong

David

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Re: [volt-nuts] HP3458 ADC integrator

[Bruce Griffiths]

Attila

Some insight can be gleaned by using a single pole model for an opamp 
configured as an integrator and deriving the expression for the output signal.
Look at the non linearity in the output.
Look at the voltage at the inverting input.
The effective impedance at the inverting input due to feedback action can be 
modelled as a simple serries network. What are the network parameters?
What happens to the non linearity if the positive input is driven in such a way 
that the inverting input is held at ground?
How would one do this?
What does the resultant signal at the inverting input look like with real 
opamps?
Whats the maximum GBW of the corrector amp so that the phase margin of the 
combination isn't reduced too much? 

~30 years ago when I first analysed the limitations of opamp analog integrators 
I didn't have access to Spice and had to resort to Laplace transforms and 
analytical models of opamps. 

Bruce
> On 24 February 2018 at 10:23 Attila Kinali  wrote:
> 
> 
> On Fri, 23 Feb 2018 18:48:24 -
> "David C. Partridge"  wrote:
> 
> > Looking your schematic, I see that you are using LT1225 for both U2 (U111) 
> > and U3 (U112)
> > 
> > Why did you use those rather than LM6361 and AD848?  Was it just because 
> > LTSpice didn't have models for them?  It can make a difference ...
> 
> Yes. LTSpice doesn't have an LM6361 or AD848. Linear lists the LT1225
> as replacement for LM6361, so I thought this was safe. I deemed the AD848
> to be close enough.
> 
> But as Bruce mentioned, the LT1225 is not unity gain stable, while the
> LM6361 is. That might explain the oscillations.
> 
> > BTW LM6361 is 550V/mv not .55V (I assume that was a mistake in your post) 
> > ... 
> > Slew rates seem a bit different at +/- 15V supply as well.
> 
> The datasheet I have found (Natsemi Sept 1995) lists 550 V/V = 0.55 V/mV.
> Which I think is a bit low, but then, the opamp is old.
> 
>   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
> ___
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Re: [volt-nuts] HP3458 ADC integrator

[Attila Kinali]

On Fri, 23 Feb 2018 18:48:24 -
"David C. Partridge"  wrote:

> Looking your schematic, I see that you are using LT1225 for both U2 (U111) 
> and U3 (U112)
> 
> Why did you use those rather than LM6361 and AD848?  Was it just because 
> LTSpice didn't have models for them?  It can make a difference ...

Yes. LTSpice doesn't have an LM6361 or AD848. Linear lists the LT1225
as replacement for LM6361, so I thought this was safe. I deemed the AD848
to be close enough.

But as Bruce mentioned, the LT1225 is not unity gain stable, while the
LM6361 is. That might explain the oscillations.

> BTW LM6361 is 550V/mv not .55V (I assume that was a mistake in your post) ... 
> Slew rates seem a bit different at +/- 15V supply as well.

The datasheet I have found (Natsemi Sept 1995) lists 550 V/V = 0.55 V/mV.
Which I think is a bit low, but then, the opamp is old.

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
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Re: [volt-nuts] HP3458 ADC integrator

[Bruce Griffiths]

Hint: start by looking at the summing junction voltage of a single opamp 
integrator with R = 50k, C = 330pF.

NB the LT1225 isnt unity gain stable so its use for any of U2, U3 and U4 is 
questionable.

They will all likely oscillate on their own. 

Bruce

> 
> On 24 February 2018 at 08:45 Bruce Griffiths  
> wrote:
> 
> Moin attila
> 
> I'd start with simulating and understanding the integrator in the 34401A, 
> its a little simpler but still has a coupe of opamps in the integrator. Then 
> analysing the GBW requirements of the various opamps used in the HP3458A 
> shoul become somewhat clearer.
> 
> Bruce
> 
> > > 
> > On 24 February 2018 at 04:51 Attila Kinali  wrote:
> > 
> > Moin,
> > 
> > I am trying to understand how the HP 3458 integrator of its
> > dual-slope ADC works, as it's a bit more complicated than
> > a simple, single opamp integrator. As you can see from the
> > attached picture, it consists of 3 opamps (U1-U3) for the
> > integrator itself, and another for the slope amplification (U4).
> > Unfortunately, I do not get what U2 and U3 are doing, much less
> > why they are needed.
> > 
> > So I tried to simulate it in LTspice. But, the circuit oscillates.
> > In both, the reset state (when the integration-C shorting switch is 
> > closed)
> > and the integration state (though, less wildly so). I guess, this
> > is because I replaced the LM6361 and AD848 by the LT1225 but do not
> > understand why, or respectively, which of the differing parameters 
> > of
> > the LT1225 causes this oscillations.
> > 
> > Would someone be so kind and give me some hints?
> > 
> > Thanks in advance
> > 
> > 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
> > 
> > ___
> > volt-nuts mailing list -- volt-nuts@febo.com
> > To unsubscribe, go to 
> > https://www.febo.com/cgi-bin/mailman/listinfo/volt-nuts
> > and follow the instructions there.
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> > 
> > > 
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Re: [volt-nuts] HP3458 ADC integrator

[Bruce Griffiths]

Moin attila

I'd start with simulating and understanding the integrator in the 34401A, its a 
little simpler but still has a coupe of opamps in the integrator. Then 
analysing the GBW requirements of the various opamps used in the HP3458A shoul 
become somewhat clearer.


Bruce

> 
> On 24 February 2018 at 04:51 Attila Kinali  wrote:
> 
> Moin,
> 
> I am trying to understand how the HP 3458 integrator of its
> dual-slope ADC works, as it's a bit more complicated than
> a simple, single opamp integrator. As you can see from the
> attached picture, it consists of 3 opamps (U1-U3) for the
> integrator itself, and another for the slope amplification (U4).
> Unfortunately, I do not get what U2 and U3 are doing, much less
> why they are needed.
> 
> So I tried to simulate it in LTspice. But, the circuit oscillates.
> In both, the reset state (when the integration-C shorting switch is 
> closed)
> and the integration state (though, less wildly so). I guess, this
> is because I replaced the LM6361 and AD848 by the LT1225 but do not
> understand why, or respectively, which of the differing parameters of
> the LT1225 causes this oscillations.
> 
> Would someone be so kind and give me some hints?
> 
> Thanks in advance
> 
> 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
> 
> ___
> volt-nuts mailing list -- volt-nuts@febo.com
> To unsubscribe, go to 
> https://www.febo.com/cgi-bin/mailman/listinfo/volt-nuts
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> 
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Re: [volt-nuts] HP3458 ADC integrator

[David C. Partridge]

Looking your schematic, I see that you are using LT1225 for both U2 (U111) and 
U3 (U112)

Why did you use those rather than LM6361 and AD848?  Was it just because 
LTSpice didn't have models for them?  It can make a difference ...

As for why they chose to use 3 op-amps rather than 2?  I don't know except that 
when dealing with low level signals maybe they needed the extra gain.

BTW LM6361 is 550V/mv not .55V (I assume that was a mistake in your post) ... 
Slew rates seem a bit different at +/- 15V supply as well.

David

-Original Message-
From: volt-nuts [mailto:volt-nuts-boun...@febo.com] On Behalf Of Attila Kinali
Sent: 23 February 2018 18:02
To: Discussion of precise voltage measurement
Subject: Re: [volt-nuts] HP3458 ADC integrator

On Fri, 23 Feb 2018 18:31:47 +0100
Attila Kinali <att...@kinali.ch> wrote:

> And why are U2 and U3 wired up the way they are?

Let me expand here a little bit to make it clear what puzzles me.
The HP3458 circuit uses 3 opamps: LT1001, LM6361 and AD848.
The LT1001 is there for its low input bias current of typ 1.2ns, but has a GBW 
of typ 0.8MHz, SR of 0.25Vµs and A_vol of 700 V/mV.
The LM6361 comes with GBW of 40MHz, SR of 200 V/µs and A_vol of 0.55 V/mV.
The AD848 has a GBW of 175MHz, SR of 200 V/µs and A_vol of 13 V/mV.

Just by judging from the slew rate, the LM6361 alone should be enough to get 
the ciruit up to speed, and the AD848 wouldnt be needed.
The AD848 has a much higher GBW and A_vol and I can see that both would be 
beneficial for the integrator. But why not just use a AD848 and leave the 
LM6361 out? Why use both?

The jfet is obviously there to decouple the high speed opamps with their high 
bias current inputs from the sensitive summing point.
C2 is probably to limit the slew rate of U2 in order to keep the integrator 
stable. But this means that U2+U3 together have some stability problem that 
could be solved by removing one (preferably
U2 as it's the slower of the two).

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 
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Re: [volt-nuts] HP3458 ADC integrator

[Attila Kinali]

On Fri, 23 Feb 2018 18:31:47 +0100
Attila Kinali  wrote:

> And why are U2 and U3 wired up the way they are?

Let me expand here a little bit to make it clear what puzzles me.
The HP3458 circuit uses 3 opamps: LT1001, LM6361 and AD848.
The LT1001 is there for its low input bias current of typ 1.2ns,
but has a GBW of typ 0.8MHz, SR of 0.25Vµs and A_vol of 700 V/mV.
The LM6361 comes with GBW of 40MHz, SR of 200 V/µs and A_vol of 0.55 V/mV.
The AD848 has a GBW of 175MHz, SR of 200 V/µs and A_vol of 13 V/mV.

Just by judging from the slew rate, the LM6361 alone should be enough
to get the ciruit up to speed, and the AD848 wouldnt be needed.
The AD848 has a much higher GBW and A_vol and I can see that
both would be beneficial for the integrator. But why not just use
a AD848 and leave the LM6361 out? Why use both?

The jfet is obviously there to decouple the high speed opamps with
their high bias current inputs from the sensitive summing point.
C2 is probably to limit the slew rate of U2 in order to keep the
integrator stable. But this means that U2+U3 together have some
stability problem that could be solved by removing one (preferably
U2 as it's the slower of the two).

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
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Re: [volt-nuts] HP3458 ADC integrator

[Attila Kinali]

On Fri, 23 Feb 2018 17:14:28 -
"David C. Partridge"  wrote:

> The April 1989 HP Journal is mostly dedicated to the 3458A and there's an
> article about the dual slope integrator starting on page 8.
>  
> There's also a nice discussion about it in Art of Electronics 3rd Edition
> starting on page 918

The HP journal does only mentions a single opamp integrator. The Art
of electronics only deals with the close relative used in the HP 34420,
which is similar, but not the same. It uses a two opamp structure,
which TAoE3 explains to be to increase amplification and slewrate.
I guess that can be said about the HP3458 as well, but that begs the
question why did they chose a 3 opamp structure? And why are U2 and U3
wired up the way they are?


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
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Re: [volt-nuts] HP3458 ADC integrator

[David C. Partridge]

The April 1989 HP Journal is mostly dedicated to the 3458A and there's an
article about the dual slope integrator starting on page 8.
 
There's also a nice discussion about it in Art of Electronics 3rd Edition
starting on page 918

HtH
David
-Original Message-
From: volt-nuts [mailto:volt-nuts-boun...@febo.com] On Behalf Of Attila
Kinali
Sent: 23 February 2018 15:51
To: Discussion of precise voltage measurement
Subject: [volt-nuts] HP3458 ADC integrator

Moin,

I am trying to understand how the HP 3458 integrator of its dual-slope ADC
works, as it's a bit more complicated than a simple, single opamp
integrator. As you can see from the attached picture, it consists of 3
opamps (U1-U3) for the integrator itself, and another for the slope
amplification (U4).
Unfortunately, I do not get what U2 and U3 are doing, much less why they are
needed.

So I tried to simulate it in LTspice. But, the circuit oscillates.
In both, the reset state (when the integration-C shorting switch is closed)
and the integration state (though, less wildly so). I guess, this is because
I replaced the LM6361 and AD848 by the LT1225 but do not understand why, or
respectively, which of the differing parameters of the LT1225 causes this
oscillations.

Would someone be so kind and give me some hints?

Thanks in advance

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

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