LT1012 vs LT1001 input resistance

Hi gpablo,

Unfortunately, there's no available data regarding the input resistance differential mode for LT1012 yet. The only available data is the input resistance for two input pins. I am trying to check the collaterals that could give us an initial data for Rin,diff. I will let you know once I find something. 

Regards,

Mae

Thanks Mae for looking into this. Would you be able to gauge it with input bias current or input offset current? LT1001 Typical Ib typ 0.5nA and Ios of 0.3nA while that of LT1012 is 25pA and 15pA respectively. By simple ratio, both will give Rin diff for LT1012 a factor of 20, putting it in the range of 2Gohms...

Hoping you'd find something more reliable information from measurements if possible.

Hi gpablo:  yes and no.  The bias current is usually cancelled so is NOT the true base current.   In a bipolar diff pair, the differential input resistance is 2*beta*re.  re is 26mV/Ie.  Unfortunately LT1012 data sheet does not give the running current, but LT1097 does: 15uA tail.  That gives 7.5uA each transistor, so re=3.47k.  Beta we do not know... but I would guess it's about 3000 (superbeta).  So 2*beta*re gives about 20Meg Ohms for LT1012.

For the LT1001, the d.s. does not give a number.  The opamp runs 1.5mA total, and i would guess 1/4 of that is allocated to the diff pair.  That's about 380uA so re=.026/380u = 68 Ohms.  Beta is about 200 so 2*beta*re = 27k Ohms for LT1001.

Thanks Glen for sharing above information. For LT1001, the d.s does give a number typical Rin 80-100Meg so the assumption/guess above might be missing a term.

I found on this link (https://wiki.analog.com/university/courses/electronics/text/chapter-12) a formula for diff input resistance which accounts for the tail resistance. I believe the simplest formula for the tail resistance would be the early voltage Va divided by the tail current?

For LT1012 it could be in the Gohms range.