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OPA2835 input bias current and common mode input impedance: the max input bias current spec for the OPA835/OPA2835 is 400 nA measured at mid supply. The common mode input impedance is given as 200k in parallel with 1.2 pF.

Other Parts Discussed in Thread: OPA835

The input bias current is specified for mid supply only, at 50 to 400 nA. There are no graphs of bias current vs. input common mode voltage.The input impedance is specified as 200k in parallel with 1.2 pF for common mode impedance, and 200k in parallel with 1pF for the differential input impedance. The 200k differential impedance would not be of much concern since in closed loop operation differential input voltage small. For common mode impedance though it is a different story, 200k input impedance change bias currents by 5 micro-amp/V and impact the accuracy of most non-inverting circuits. For instance, Figure 79 of the data sheet shows a Sallen Key low Pass filter with 6.7 k DC resistance in the non-inverting input signal path; a 200 k common mode impedance would generate over 3% gain error.

Section 9.1.3 describes a high impedance instrumentation amp configuration. No mention of 200k loading effects.

The typical current noise spec is 0.45 pA/rt-Hz, which corresponds to about 633 nA using the shot noise equation. Perhaps this means that the 400 nA max bias current consists of two opposite polarity currents. Anyway, does this current noise spec only apply at mid supply? If bias current really increased by 4 micro-amp  per volt the current noise would only apply over a very narrow signal range.

While I am on the subject of current noise and typo's, on page 10 the current noise 1/F corner frequency is given as 14.7 Hz, lower than the voltage noise corner frequency of 147 Hz. The current noise curve on Figure 42 looks more consistent with a 14.7 kHz 1/F corner.

Is the 200k common mode impedance a typo?  Is there any data on bias current vs. input common mode voltage available for these parts? Does current noise change drastically over the input common mode range? Obviously, I picked this part for its speed/power ratio, not its DC precision, but the spec as written does not give me good information to design with.

• Very good questions John,

I suspect the datasheet is in error and we are pursuing that with the original designer. I do know the current OPA835 TINA models do not have any input impedance modelled. That is also on the to do list to correct.

In the meantime, I am also defining new very power efficient devices and would love to hear (privately) about your detailed requirements. If you would be willing to pursue that, contact me at m-steffes@ti.com

• Hello John,
The designer was able to run some extensive simulations on the input impedance of the OPA835 and here are the numbers that came out:

Common-Mode Impedance (on each pin): 100MOhm || 2.2pF

Differential Impedance: 400kOhm || 0.1pF

I will make some measurements on the bench tomorrow to try and confirm the common-mode resistance as well.

Thank you for bringing this to our attention. I will be working on updating the TINA model and the datasheet to reflect these numbers.

-Samir