Under unity buffer

[throbscottle]

I need a buffer which attenuates the signal. I want to use the + input of quarter of an OPA4140 for the input. Do I just need to string the output across a potential divider, or is there a better way to do it?

Tia

 

[ericgibbs]

hi throb,
How much attenuation are you requiring.??

A OPA's , non inverting input can tolerate a high resistance value divider, or for an inverted OPA output, you use a OPA as an attenuator.
For example... Rin= 10k, Rfb =5k would give a Gain of 0.5....

E

 

[throbscottle]

At the moment, no greater than V/10, however could go as high as V/30. Can't tolerate any reduction in input resistance since the input is from the DUT on an ohms converter, which could be 100M or more, hence it has to be non-inverting. This is just my route to being able to measure at significant voltage (ie, as high as this particular opamp can stand) across the DUT.

I figured out how to make a fully differential ohms converter but then discovered it needs a single sided mode as well, which is what this is for... <sigh>

 

[ericgibbs]

hi,
Look at this clip note the input impedance.
E

 

[throbscottle]

Thanks Eric, but the OA I am looking at has a typical bias current of 0.5pA typical (10pA max) so this isn't my issue. It really is just a case of identifying if there's a better method of doing the attenuation than just sticking a couple of resistors across the output of a non-inverting buffer.

 

[ericgibbs]

Thanks Eric, but the OA I am looking at has a typical bias current of 0.5pA typical (10pA max) so this isn't my issue. It really is just a case of identifying if there's a better method of doing the attenuation than just sticking a couple of resistors across the output of a non-inverting buffer.

ok,
Look at the LMC660 .
E

https://uk.farnell.com/texas-instruments/lmc660cm/op-amp-quad-cmos-smd-soic14-660/dp/9487115

 

[throbscottle]

I was hoping to find a magic active attenuator in there, but I guess you just wanted me to look at an opamp with lower bias current. As it happens, the offset voltage is a little high when a low value DUT is inserted - in simulation at least the opa4140 works better.

So anyway I think I may have found a magic bullet in all this if I change some other things...

Thanks for looking though