[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-bounces at 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 <attila at 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
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