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INA821: offset voltage, offset current, bias current evaluation between INA121

wang shu
Intellectual 280 points
Part Number: INA821
Other Parts Discussed in Thread: INA121

Tool/software:

Hi,

Our old product is using INA121. Now, I wish to use INA821 to replace it. but I found its offset voltage has one more output offset voltage and larger offset current and bias current.

Because our application is very sensitive about the change. So I wish to know how the evaluate the output error from offset voltage, bias current, and offset current for instrumentation amplifier. I found some reference, but all of them talk about regular operational amplifier but not differential amplifier or instrumentation amplifier. So could you please provide some reference how to evaluate. or do you have some training?

Best Regards,
Shu

  • 0
    TI__Mastermind 23965 points

    Hi Shu, 

    We have a training series that covers these topics that can be found here:

    https://www.ti.com/video/series/precision-labs/ti-precision-labs-instrumentation-amplifiers.html

    I hope this helps. 

    Best Regards, 

    Chris Featherstone

  • 0 in reply to Chris Featherstone
    Intellectual 280 points

    Hi Chris,

    The old application is below current generation using INA121. If I change the INA121 to INA821 and analyze the impact on the bias current. I assume only need to use Ios x R2 to get the Vos(current), am i right? The gain of my application is 1. Does it have to use Ib- and Ib+ to calculate. if yes, how to do it.

    I also wish simulate the circuit to analyze the impact. Does the TINA model include the offset voltage, bias current, offset current in the simulation? I saw the IN821 TINA reference design describe some bias current information, but not sure if the simulation can show the impact. and how about INA121 model?

  • 0 in reply to wang shu
    TI__Mastermind 23965 points

    Hi Wang, 

    The INA821 does not have an extra Vos term. It is just written differently in the product datasheet. You can see below that the INA121 also has a Voso term. It is written all in one line in the datasheet however. The INA821 has far better Vosi and Voso compared to the INA121.

    Yes our models contain Vos and Ib information. It would be far easier to simulate the conditions of your circuit instead of calculating the errors. I do not have a calculation to provide. Below you can see that the typical Vos is shown on the output in a gain of 1. This matches the typical offset provided in the datasheet of 60 uV which is a combination of Vosi and Voso. The typical bias current is also modeled as shown below. I also included the INA121 image showing Vos and Ib. 

    Best Regards, 

    Chris Featherstone

  • 0 in reply to Chris Featherstone
    Intellectual 280 points

    Hi Chris,

    I did similar simulation and get similar results as you shown abovethat INA821 bias current on positive pin is much larger than negative pin. INA121 bias current on each pin are close. why? device characteristics?

    So for INA821 i think the error come from bias current could be the difference between the 4.99k*bias current on positive and negative pin, 4.99k*(1.53pA-300fA), am i right? and INA121 is 4.99k*(11.32pA-10.32pA). So INA121 bias current error is smaller than INA821. 

    Of cause i also need to consider the offset voltage. but here i just wish to clear the bias current impact.

    Regards,
    Shu

  • +1 in reply to wang shu
    TI__Mastermind 23965 points

    Hi Shu, 

    The input bias currents are multiplied by the 4.99k resistors as you mentioned. My calculation below aligns with your simulation. 

    The INA devices are not very comparable because the INA121 is a FET input with lower Ib where the INA821 is a bipolar input with higher Ib and higher ios. In addition the INA821 is a current feedback topology as shown below. The INA821 has better CMRR. If the concern is in regards to noise I have an application note I recently wrote on replacing the front end of an INA with bipolar inputs with discrete JFETs. Here is a link to that article. 

    Ultra-Low-Noise JFET Preamplifier Design for High Impedance Sensors

    Best Regards, 

    Chris Featherstone

  • 0 in reply to Chris Featherstone
    Intellectual 280 points

    Thank you very much!