• State
  • Locked Locked
  • Replies 3 replies
  • Subscribers 38 subscribers
  • Views 376 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.

Original question:

TIDA-060029: Detailed calculation method of test results

TIDA-060029: OPA IC changing and MUX add in Rf/Rg selecting

Sakura Lin
Prodigy 20 points
Part Number: TIDA-060029
Other Parts Discussed in Thread: OPA2810, THS4551, OPA301, OPA300, ADS127L11, TMUX1104, BUF634A, TINA-TI

Tool/software:

Hi,

In the design, can we use OPA300/301 replace OPA2810? I notice the OPA 300/301 have a 60dB AOL at 100KHz and input bias current is 1pa level, but it has a 5mv Vos, is this parameter affect a lot in the design? after the OPA301 is a THS4551 and ADS127L11, all the R are 0.1% and C are 2%.

And another thing is about the Rg and Rf changing, is it good to use a mux IC like TMUX1104 to change the Rf/Rg in the measuring to auto change the value? or is TI have a new design for this?

Thanks, lin

  • 0
    TI__Mastermind 28645 points

    Hello Lin,

       I do not see an issue using the OPA300/301 instead of the OPA2810. With the lower AOL compared to OPA2810, there is only a slight adjustment needed for a stable design which is adding a couple components to avoid oscillations such as isolation resistors and local feedback capacitor, which is shown in the schematic in the reference design as Figure 3.1.1, below is the highlighted components that are important for stability of your design, I highly recommend adding these components:

      With components: C11 and R9 are very critical additions.

       BUF634A is not required for the second stage since you are going to a FDA --> ADC, which does not require high driving capabilities. Having the BUF634A in the first stage is critical if you are driving heavy impedances for your LCR measurement. Using a TMUX1104 for switching rather than jumpers as done in the design is also fine. I checked the datasheet for the mux and it does have pretty low capacitance, and during calibration you can take account of the TMUX on-resistance. The main disadvantage is the mux's capacitance will cut-off your lower frequency measurements. 

    Thank you,
    Sima 

  • 0 in reply to Sima Jalaleddine
    Prodigy 20 points

    Hello sima,

    Thanks for your reply, it is very helpful.

    For R41, will it short the R10,R13,R21? and if I remove the second BUF634A, should I move the R17, R9, R18 to the opa in front of it?

    Thank you,

    Lin

  • 0 in reply to Sakura Lin
    TI__Mastermind 28645 points

    Hello Lin,

      I am glad it was helpful.

      Here is a rough schematic to show it better what is critical to add especially for tuning/debugging on PCB board when it comes to testing:

        And for the OPA30x, you need about 1uF feedback local feedback capacitor for proper stability:

        Adding R41 will not short R10, R13, R21. It will create a path for voltage measurement at Vx in order to do the calculation:

       In order to find Zx (unknown impedance).

       The first path at the first stage to U1B in schematic, is only for calibration purposes to determine the gain when there is no impedance selected and only measuring the exact value of the RG/RF selected. When calibration is done, R42 should be removed, and R41 will be replaced with a short, or I found 100Ohm resistor to be necessary for stability. 

       For second question, if you remove the BUF634A, I would suggest keeping R17 and R9. As shown in Tina-TI circuit of first image of this reply as R19 and R10. 

    LCR using OPA30x.TSC

    OPA300x LCR Stability.TSC

    Thank you,
    Sima