LM8262 – About input bias current (IB)

Hello,

Sorry about the late reply (I had missed your question).

The LM8262 input bias current should not be 150uA and it should be less than 5uA as you have already noted.

Is this the behavior of all devices you have tried, or only one? Is this a new design or have you just encountered this issue with an already working design? What is the output voltage (is it what you'd expect with Vin= 2.2V, or does the output voltage indicate some kind of malfunction)?

So, there is probably something wrong with this particular device.

Let me know and I'll try and help as much as I can.

Regards,

Hooman

Hi, Hooman

Thank you for the information.

My customer uses LM8262 at differential input.

Therefore, I think that current of 150uA flows into Non-inverting pin.

(I used "LM8272 datasheet P11 & 12" as reference.)

Could you please give me advice.

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<LM8272 datasheet citation>

The input stage is protected with the combination of R9-R10 and D1, D2, D3 and D4 against differential input

over-voltages. This fault condition could otherwise harm the differential pairs or cause offset voltage shift in case

of prolonged over voltage. As shown in Figure 38, if this voltage reaches approximately ±1.4V at 25°C, the

diodes turn on and current flow is limited by the internal series resistors (R9 and R10). The Absolute Maximum

Rating of ±10V differential on VIN still needs to be observed. With temperature variation, the point were the

diodes turn on will change at the rate of 5mV/°C

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< codicil>

I got advice from you in the following E2E.

“The simplified schematics of the LM8272 and the LM8262 are nearly identical.”

http://e2e.ti.com/support/amplifiers/etc_amplifiers__other_linear/f/18/p/278786/974092.aspx#974092

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Hello,

The protection diodes on the input (from the LM8272 or LM8261 Figure 51, Application Information) are also there for the LM8262. Under closed loop conditions (like the pdf schematic you uploaded, link below), these diodes are off and have no effect on input bias current:

http://e2e.ti.com/cfs-file.ashx/__key/telligent-evolution-components-attachments/00-18-00-00-00-27-87-58/LM8262-Non-inverting-amplifier-circuit.pdf

However, if the LM8261 is operated open loop or if there is enough differential input voltage (such as during transient conditions) to turn on these diodes, you will get current flow through them, as shown in the LM8261 datasheet Figure 52. If operation is closed loop and the static bias current of 150uA is observed, there is something wrong with the device (try another one?), or connections / schematic.

Hope this answers your question.

Regards,

Hooman