Opamp offset question

onsdag den 1. december 2021 kl. 00.12.39 UTC+1 skrev Klaus Kragelund:
> On 30/11/2021 01.35, Lasse Langwadt Christensen wrote: 
> > tirsdag den 30. november 2021 kl. 01.20.00 UTC+1 skrev Klaus Kragelund: 
> >> On 29/11/2021 08.12, Jasen Betts wrote: 
> >>> On 2021-11-28, Klaus Kragelund <klau...@hotmail.com> wrote: 
> >>>> Hi 
> >>>> 
> >>>> I have a PT1000 circuit where a LMV358 is used in a differential coupling to feed it to an ADC 
> >>>> 
> >>>> PT1000 is pull up with a resistor 
> >>>> 
> >>>> I would NOT do it like that, this is a design I have inhireted 
> >>>> 
> >>>> Problem is the large offset voltage of the opamp is amplified producing large errors 
> >>> 
> >>> Can you drive it with AC instead? 
> >>> 
> >>> 
> >> That is not possible I am affraid 
> > 
> > could you toggle between two currents ? 
> >
> Yes, could be. So you are thinking like syncronous detection? Shifting 
> the signal to a higher frequency avoiding DC errors

something along the time lines of

two currents I1 and I2

I1 * R + offset = V1 , I2 * R + offset = V2

(I2 * R + offset)-(I1 * R + offset) = I2*R-I1*R + offset- offset => V2-V1 = (I2-I1)*R

R = (V2-V1)/(I2-I1)

On 01/12/2021 17.16, Lasse Langwadt Christensen wrote:
> onsdag den 1. december 2021 kl. 00.12.39 UTC+1 skrev Klaus Kragelund:
>> On 30/11/2021 01.35, Lasse Langwadt Christensen wrote:
>>> tirsdag den 30. november 2021 kl. 01.20.00 UTC+1 skrev Klaus Kragelund:
>>>> On 29/11/2021 08.12, Jasen Betts wrote:
>>>>> On 2021-11-28, Klaus Kragelund <klau...@hotmail.com> wrote:
>>>>>> Hi
>>>>>>
>>>>>> I have a PT1000 circuit where a LMV358 is used in a differential coupling to feed it to an ADC
>>>>>>
>>>>>> PT1000 is pull up with a resistor
>>>>>>
>>>>>> I would NOT do it like that, this is a design I have inhireted
>>>>>>
>>>>>> Problem is the large offset voltage of the opamp is amplified producing large errors
>>>>>
>>>>> Can you drive it with AC instead?
>>>>>
>>>>>
>>>> That is not possible I am affraid
>>>
>>> could you toggle between two currents ?
>>>
>> Yes, could be. So you are thinking like syncronous detection? Shifting
>> the signal to a higher frequency avoiding DC errors
> 
> something along the time lines of
> 
> two currents I1 and I2
> 
> I1 * R + offset = V1 , I2 * R + offset = V2
> 
> (I2 * R + offset)-(I1 * R + offset) = I2*R-I1*R + offset- offset => V2-V1 = (I2-I1)*R
> 
> R = (V2-V1)/(I2-I1)
> 

That's nice, so in effect the dynamic resistance is found between two 
load lines :-)