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Original question:

LMH32404: Output polarity of LMH32404 Tina model

LMH32404: The latest Tina simulation model still has errors

Amy Luo
Expert 6895 points

Part Number: LMH32404
Other Parts Discussed in Thread: TINA-TI

Hi Team,

A few days ago, when asked about the reverse polarity of the Tina model output of LMH32404, TI Sean Cashin replied that it was a model error and the model netlist needed to be changed to G_G83         TIA20K MID I_IN N1634840 1  and indicates that the model on ti.com will be updated as soon as possible. 

However, the customer has reported that the latest model has been re-downloaded and found that the output polarity is correct, but has introduced a new problem: the size and direction of the input current does not match Figure 6-21 in the datasheet, please help to confirm again.


In addition, the latest simulation circuitry, without setting the input capacitance to variable, does not directly produce a graph of multiple curve comparisons when performing noise and bandwidth analysis. Similar to the one shown in the following figure.

Best Regards,

Amy Luo

  • 0
    TI__Mastermind 23226 points

    Hi again Amy,

    Figure 6-21 describes that the output clamps for input current <-40uA and >80uA. That is what my model does. The input PNP does not support higher currents than these.

     

    In order to compare simulations of multiple capacitor values in TINA-TI, you can use this button:

    And then select the input capacitor C1. Then select the ellipses next to "Capacitance (F) ; 1pF [...]. Under Sweep type, select "List", and the "Set List".

    Best regards,

    Sean

  • 0 in reply to Sean Cashin
    TI__Expert 6895 points

    Hi Sean,

    I'm sorry, it's my fault, I didn't make the customer's question clear. I re-describe the customer's issue as follows:

    The main point of the problem is the current direction in Figure 6-21.
    Is the positive current direction in Figure 6-21 referring to the direction of the PD's current flowing into the TIA? Similarly, is the negative current direction in Figure 6-21 referring to the direction of the PD's current flowing out of the TIA? 

    If the answer to the above question is yes, then the TINA emulation results do not match Figure 6-21 in the dataset. The explanation is as follows: 

    The current direction of the IG1 in the TINA emulation circuit is out of the TIA, so the positive current for the IG1 setting should correspond to the negative current in Figure 6-21; similarly, the negative current for the IG1 setting should correspond to the positive current in Figure 6-21. But the TINA simulation results in the opposite. That is, the positive current for the IG1 setting corresponds to the positive current in Figure 6-21; the negative current for the IG1 setting corresponds to the negative current in Figure 6-21. 

    It's like a tongue twister, and I hope you understand the customer's puzzle. 

    Regards，

    Amy

  • 0 in reply to Amy Luo
    TI__Mastermind 32395 points

    Hello Amy,

      This does feel like a riddle. The current in a photodiode is from cathode to anode which is the correct current direction in the datasheet diagram. However since a photodiode is like a reversed LED, so I believe this creates a negative current from cathode to anode. This should result in a positive output out the amplifier. If the photodiode is reversed, then the amplifier output would be negative.

       Is that the correct understanding?

    Thank you,
    Sima 

  • +1 in reply to Amy Luo
    TI__Mastermind 23226 points

    Hi Amy, the answer to the above question was in fact "no". Positive current in Figure 6-21 is defined as current flowing out of the TIA.

    So the model is doing, I believe, what it is supposed to be doing.

  • 0 in reply to Sima Jalaleddine
    TI__Expert 6895 points

    Hi Sima,

    The customer already knows this, thank you for your response

  • 0 in reply to Sean Cashin
    TI__Expert 6895 points

    Hi Sean, 

    Thank you. I appreciated your support.