Re: [volt-nuts] HP3458 ADC integrator

2018-02-23 Thread 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 ___ volt-nuts mailing list -- volt-nuts@febo.com To

Re: [volt-nuts] HP3458 ADC integrator

2018-02-23 Thread 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

Re: [volt-nuts] HP3458 ADC integrator

2018-02-23 Thread 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

Re: [volt-nuts] HP3458 ADC integrator

2018-02-23 Thread 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

Re: [volt-nuts] HP3458 ADC integrator

2018-02-23 Thread 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

Re: [volt-nuts] HP3458 ADC integrator

2018-02-23 Thread 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'

Re: [volt-nuts] HP3458 ADC integrator

2018-02-23 Thread 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

Re: [volt-nuts] HP3458 ADC integrator

2018-02-23 Thread 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

Re: [volt-nuts] HP3458 ADC integrator

2018-02-23 Thread 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

[volt-nuts] HP3458 ADC integrator

2018-02-23 Thread Attila Kinali
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

Re: [volt-nuts] HP3458 ADC integrator

2018-02-23 Thread 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

Re: [volt-nuts] HP3458 ADC integrator

2018-02-23 Thread 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

Re: [volt-nuts] HP3458 ADC integrator

2018-02-23 Thread 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

Re: [volt-nuts] FW: HP3458 ADC integrator

2018-02-23 Thread Attila Kinali
Good evening! On Fri, 23 Feb 2018 19:02:42 - "David C. Partridge" 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