[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 <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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