[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 <attila at kinali.ch> wrote:
> 
> 
> On Fri, 23 Feb 2018 18:48:24 -0000
> "David C. Partridge" <david.partridge at perdrix.co.uk> 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
> -- 
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