[volt-nuts] FW: HP3458 ADC integrator

[Attila Kinali]

Good evening!

On Fri, 23 Feb 2018 19:02:42 -0000
"David C. Partridge" <david.partridge at perdrix.co.uk> wrote:

> This is a bit time delayed as the attachment was too large, but posting anyway 

Ah.. I have the HP3458 schematics, that's where my circuit comes from.
Maybe I should have made that more clear.

 
> Take a look at Figure 8 at the bottom of page 12 in the HPJ article.
> 
> U110-U112 in the attached portion of the CLIP just operate (AFAICT) as a 
> single high gain opamp running open-loop which is shown in Figure 8 as a 
> single op-amp (the left hand one).  U140 to the right is the middle op-amp in 
> figure 8 and this feeds finally into the comparator (not shown in the 
> attached).

I got this so far. I understand that U110-U112 form an integrator
with U111 and U112 added to increase SR and GBW. U110 has been
choosen for its low I_bias and V_os TC and provides that to the
overall integrato (ie compensates V_os drifft of U111 and U112).
Q110 acts as a fast unity gain buffer to isolate U111/U112 from
the summing point. U140 acts as a simple amplifier. I included
it in my simulations just because it feeds back into the integrator
during reset.

But I don't understand why the integrator has been done that way. 
U111 seems redundant when adding U112. It feels like I am missing a key
point in what changes in SR and GBW when connecting two opamps.

				Attila Kinali
-- 
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