OPA891: Is the input Z of this device really just 6k?

Jim Pitman

Part Number: OPA891

Tool/software:

The spec sheet says the Input Z in the differential mode is 6K.

I downloaded the TINA simulation example which in a non inverting buffer, (gain of 1) and inserted a 100k resistor in series with the plus input. Sure enough, a transient simulation shows the output at about half of the input,.

Really?

1 month ago

0 Ignacio Vazquez Lam1 1 month ago

TI__Genius 9850 points

Hi Jim,

Can you share the TINA file you are using.

Best Regards,

Ignacio

0 Jim Pitman 1 month ago in reply to Ignacio Vazquez Lam1

Intellectual 390 points

OPA891_TINA_Ref_Design_V1p1.TSC

0 Ignacio Vazquez Lam1 1 month ago in reply to Jim Pitman

TI__Genius 9850 points

Hi Jim,

Thank you for sharing the tina file. The problem with the 100k series resistor is an offset established with the bias current going into the noninverting input. This shifts the output lower, but the peak-to-peak output signal is still 2Vpp. If you separate the signals, you will be able to see the output signal still being at the expected voltage.

Best Regards,

Ignacio

0 Jim Pitman 1 month ago in reply to Ignacio Vazquez Lam1

Intellectual 390 points

Ok. I see offset changes the display. Should have looked closer. It appears if I put a 100k in the feedback to the minus the offset goes away (predictably).

0 Ignacio Vazquez Lam1 1 month ago in reply to Jim Pitman

TI__Genius 9850 points

Hi Jim,

No worries and yes technically that will work as it will cancel out the effect of Ib however in your real application we advise against having large values for Rf as this can result in other ill-effects such as stability issues.

Best Regards,

Ignacio

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OPA891: Is the input Z of this device really just 6k?