• State
  • Locked Locked
  • Replies 2 replies
  • Subscribers 48 subscribers
  • Views 428 views
  • Users 0 members are here
Support feedback
Options
Options
Related

This thread has been locked.

If you have a related question, please click the "Ask a related question" button in the top right corner. The newly created question will be automatically linked to this question.

INA333 issue with Multiplexer

Trevor Lee
Prodigy 10 points
Other Parts Discussed in Thread: INA333

Hi everyone,

I have recently designed an 8:1 differential analog input circuit making use of an analog differential multiplexer and the INA333.

The schematic is as shown in the attached picture

The INA333 is configured to have a gain of 100.

The differential input signals range from 0 to 50mV.

The interesting problem I have is, if my differential signal goes through the multiplexer(example 30mV), I can measure 30mV on VIN+ & VIN- of INA333. However, the output from my op amp is 60mV instead of the 3V.

When I removed the multiplexer and had 30mV differential voltage source applied directly on the VIN+ & VIN-, I can get 3V.

There seems to be nothing wrong with the multiplexer nor the INA333. Could anyone advise me on this issue? Is there some impedence issue that I am missing out on somewhere?

Hope to have your adivse soon. Thank you.

 

 

  • 0
    TI__Guru**** 189401 points

    Hi, Trevor,

    I'm going to move your post to the Precision Amplifier forum.

    -d2

  • 0
    TI__Mastermind 25865 points

    Hello Trevor,

    Thanks for you interest in the INA333.  After looking at your schematic here are a few observations:

    a)  We typically see resistors in series with the inputs (and in conjunction with C7, C32, and C35) that create common-mode and differential mode filters.
    b)  It is important to ensure that you have an input bias current return path.  Perhaps this happens in your MUX, but I'm not entirely sure.  You may want to look at that section of the datasheet to ensure each of your input sensors meet the criteria.
    c)  Ensure that the common-mode input voltage allows the output to swing to 3V.  Given a 5V single supply with Vref connected to ground, the input common-mode voltage should be in the range 1.75V to 3.25V to allow the output to swing to 3V.  This information was obtained from Figure 21 in the datasheet.

    I hope this helps!