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Original question:

OPA134: Technical Clarification on OPA134 – PCN 20240902002.1

OPA134: Technical Clarification on OPA134 – PCN 20240902002.1

Ranjeet Patil
Prodigy 20 points
Part Number: OPA134

Tool/software:

As you mentioned earlier, the open-loop output impedance change might affect stability but both datasheet (SBOS058A and SBOS058B) show 10Ω at 10kHz, also their capacitive load graphs (Rev A Fig. 26 vs. Rev B Figs. 5.23, 5.24) look consistent.

However, Rev B includes results with 0Ω, 24Ω, and 51Ω output resistors. In our circuit with Rev A, we didn’t use any output resistor to control overshoot. We want to confirm: for Rout = 0Ω, can we expect the same output with both revisions, or is it now must to add a 24Ω or 51Ω resistor when using Rev B?

  • 0
    Prodigy 20 points

    Also want to get information on High open loop gain which earlier was 120 dB @ 600 Ω and now is 120 dB @ 2 kΩ. The new version has the same BW for higher load.

    Additionally, could you briefly elaborate on your earlier statement. "I see that an op amp's BW may be increased, the open loop output impedance is slightly different --> this may affect the loop stability somewhat and may require additional compensation, if it is driving capacitive load."

  • 0 in reply to Ranjeet Patil
    TI__Intellectual 1090 points

    Hi Ranjeet,

    Regarding using output resistors to control overshoot: this resistor value will vary depending on the circuit diagram. The configuration in the datasheet shows the opamp in a noninverting configuration, but the overshoot is also dependent on the feedback of the circuit, capacitive load, etc. 

    I can confirm that the AOL for the new version of OPA134 does have a gain of 120dB at both 2k and 10k.

    The opamp's typical BW has increased slightly from 8MHz to 11MHz. Since BW is directly correlated to loop stability, the zero crossing point of closed loop gain is likely different than before, and some analysis is needed to ensure the circuit is stable.

    For more information on stability, you can access our Precision Labs training on stability here.

    Please let me know if you have further questions. If you share your circuit, I can analyze it and determine if any changes are necessary to ensure the circuit is stable with the new version of OPA134.

    Thank you,
    Carrie

  • 0 in reply to Carrie
    Prodigy 20 points

    Hi Carrie,

    Appreciate your prompt reply.

    You mentioned that the typical bandwidth of the op-amp has increased from 8 MHz to 11 MHz. I reviewed the Rev. B datasheet of the OPA134 but couldn’t find any reference to 11 MHz. I can see that it is still 8 MHz in Electrical Specifications. Could you please point out where exactly this is mentioned in the latest datasheet—page number or section?

  • 0 in reply to Ranjeet Patil
    TI__Guru* 77261 points

    Hi Ranjeet,

    Ranjeet Patil said:
    I can see that it is still 8 MHz in Electrical Specifications.

    You are correct. The datasheet revision and update are fallen behind. Currently both pre-PCN and post-PCN op amps are being sold from the inventory. The datasheet should be revised after the pre-PCN product will be depleted. 

    When OPA134's old FAB is changed and updated to a new FAB process, one of the side effect increases the BW of the previous design (smaller transistors, better matching, smaller packaging, lower parasitics etc.).  These side effects are very common due to the FAB change that it can impact on the BW of an op amp and open loop output impedance somewhat. 

    If we are able to review your existing schematic, we may be able to compensate the circuit and improve its loop stability and improve phase margin. 

    If you prefer to send your schematic privately, please post "friendship" request over E2E forum; the contact information via E2E private messaging forum can be established. We can discuss the matter privately via E2E private messaging system or via TI's internal email.  

    If you have other questions, please let us know. 

    Best,

    Raymond