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OPA862: OPA862 - A1 input bias current

John wysocki
Prodigy 10 points
Part Number: OPA862

2023-03-24 16:40:54 CDT - Mark Philip Felipe Customer
Response


Hello John,     Also you can call me on my cell phone:  847 890-0637

In the datasheet on page 5 it says that input Current(I) bias is 1u amp for A1 block input to chip, but on page 16 under "Overview" it says "The noninverting input of A1 offers high input impedance (325 MΩ typical)
for interfacing single-ended sensors that often have a non-zero output impedance to differential input analog", 325 M Ohms is not the inverse of 1 ua amp and why does it address non-inverting input only usually they are the same ??? for inverting and non-inverting inputs to OP-AMP?


Thank you for contacting Texas Instruments Customer Support.

After further evaluation of your inquiry, I believe that a discussion with one of our product/applications engineer would be a great help for your inquiry. With this, I would like to advise you to post your inquiry to our engineer-to-engineer (E2ETm) forum. E2E is where we have the Engineering Applications Specialist according to product line. The forum is supported by our experts, and you will be immediately interacting with them.

You can post your inquiry using this link: https://e2e.ti.com/p/addpost

Once you've posted your questions on E2E, we would appreciate if you can provide to us the link. Also, if you did not receive any updates in your E2E post within two business days, please inform us, we will be glad to assist you to accelerate its progress.

If you have further question, feel free to contact us.

Thank you very much.

Stay safe and healthy.

Regards,

Mark Philip Felipe
Texas Instruments | Customer Support



2023-03-24 13:42:57 CDT - System Customer
Response		 


Number: CS1653473
Contact: John wysocki
First name: John
Last name: wysocki
Short description: OPA862 inquiry
Email: wysockij@lunainc.com
Company: LUNA INNOVATIONS
Company phone: 
Language: English
State: Open
Provide case details or comments:

In the datasheet on page 5 it says that input Current(I) bias is 1u amp for A1 block input to chip, but on page 16 under "Overview" it says "The noninverting input of A1 offers high input impedance (325 MΩ typical)
for interfacing single-ended sensors that often have a non-zero output impedance to differential input analog", 325 M Ohms is not the inverse of 1 ua amp and why does it address non-inverting input only usually they are the same ??? for inverting and non-inverting inputs to OP-AMP?
  • +1
    TI__Genius 14141 points

    Hello John,

    I may have misunderstood the question, but here is my initial response:

    The OPA862 is intended to be used as a fully-differential amplifier (FDA) for driving an ADC; specifically the device is considered for receiving input signals from high-impedance sources/sensors.  As the block structure shows, A1 is an amplifier intended to be used solely in the non-inverting configuration.  The inverting node (VFB for feedback) is not intended to receive the signal input, rather VFB is used with Rf and Rg (feedback and gain-setting) resistors to set the gain of stage A1.  The 'high impedance' nature of the non-inverting input is further explained as a feature/benefit in Section 8 (specifically 8.2.2).  The datasheet specifies this high impedance node specifically as the non-inverting input is the preferred/selected input for using the OPA862.  I would not recommend (and neither does the datasheet) using the inverting input as the signal input for the OPA862.  

    Because of this one-sidedness of the OPA862 and it being a very specific device to drive ADCs with high-impedance signal sources, these specifications are correct.  The high impedance noninverting input node does have a common mode input impedance of 325 MegaOhms.  Likewise, the input bias current of the A1 block is 1uA; this corresponds to the BJT input transistors and the inverting node.  

    Unless you have a specific set of data or concerns regarding how the A1 input bias current affects overall circuit operation, I believe that is the best I can do to explain without further research.  

    In short: the OPA862 is a very application specific part, designed to work with the A1 block in a noninverting configuration, the A2 block in a fixed inverting -1 gain configuration, and with a high impedance input for use with non-zero output impedance sensors.

    If you require assistance selecting an ADC driver for your customer's project or need to debug or review a design, please let me know.  This part is certainly not a specified as a typical FDA or op-amp would be, thank you for your question.

    Best,

    Alec

  • 0
    Guru 140200 points

    Hi John,

    an input bias current of 1µA and an input resistance of 325M is no contradiction. Keep in mind that the input resistance is the differential value dU / dI. So, for the input resistance what counts is the change of input bias current with the common mode input voltage and from figure 6-14 of datasheet you can see that the input bias current is changing only very very little.

    Kai 

  • 0 in reply to kai klaas69
    TI__Genius 14141 points

    Hello Kai,

    Thank you for helping convey the message in clearer terms.

    Hi John,

    There is no need for concern, as Kai simply provided above.

    Best,

    Alec

  • 0 in reply to Alec Saebeler
    Prodigy 10 points

    Hi Alec and Kai,

    Thank you for the response and we are all set, question has been answered.

    I did not see Alec's response before - it helps as well.

    Incorrectly - I wanted to use this part for a transimpedance Amplifier where as you know the diode goes to the inverting input( kinda low input impedance for my application),       I see now that this is intended to be used with a sensor on the non-invertong input to A1.

    John W