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OPA361: Asking for the OPA361 design questions

SHH
Mastermind 30666 points
Part Number: OPA361
Other Parts Discussed in Thread: OPA360


Hi Team,
Customer originally uses the OPA360, and customer would like to swithc to OPA361.
OPA360 has the pin3 SAG function to reduce the output cap.

OPA361 do not has the SAF pin, OPA361 has the Rset pin on pin3. please help for the questions.
Q:
1. Why OPA 361 do not need the SAG funciton? Do OPA361 has the SAG function inside?
2. If customer want to change to OPA361, What is the suggested way for the pin 3 Rset pin?


Thanks,
SHH

  • 0
    TI__Guru* 93105 points

    Hello SHH,

    Q:
    1. Why OPA361 do not need the SAG function? Do OPA361 has the SAG function inside? 

    The OPA361 was developed after the OPA361, and more specifically for use with the OMAP242x; however, it may be used in other applications as well. The OPA361 does not provide the SAG reduction feature that the OPA360 does. The OPA361 will require larger value ac coupling capacitors to pass the lower frequencies when used in equivalent applications.

    2. If customer want to change to OPA361, What is the suggested way for the pin 3 Rset pin? 

    The 4-kohm RSET resistor was included internal to the OPA361 specifically for use by the video DAC internal to the OMAP242x. It was included in the OPA361 to reduce the number of discrete resistor count by one. For other applications PIN 3 may be left disconnected, shorted to ground, or used if a 4-kohm resistor to ground is of use.

    Regards, Thomas

    Precision Amplifiers Applications Engineering

  • 0 in reply to Thomas Kuehl
    TI__Mastermind 30666 points
    Hi Thomas,
    What is the larger value cap for OPA361?
    I do not see in datasheet.

    Thanks,
    SHH
  • 0 in reply to Thomas Kuehl
    TI__Guru* 93105 points

    Hello SHH,

    Your question: What is the larger value cap for OPA361?

    The OPA361 output is often direct coupled so its output response is down to dc. However, in some cases it may be desirable to block the OPA361 output's dc level and only couple the ac signal. In such cases a dc blocking capacitor, or ac coupling capacitor, can be added in series with the output. 

    The OPA361 is often applied where it drives a 150-ohm load, which is comprised on a series 75-ohm output resistor and and other 75-ohm resistor to ground at the input of whatever follows the OPA361. This is seen in datasheet Figures 1, 2, and 10. This circuit technique is often applied to correctly terminate 75-ohm coaxial transmission lines. The two 75-ohm resistors form a -6 dB attenuator. It is a dc coupled circuit so it will divide down any dc output level by 1/2.

    The ac coupling capacitor's value will depend on the circuit's low frequency cutoff requirement. For example, if the low frequency cutoff (-3 dB frequency) is 5 Hz, then with a 150-ohm output load the capacitor would have to be about 220 uF. If the cutoff frequency is set higher, then the capacitor value will decrease. Its value just comes down to what the application requires.

    If a response to dc is acceptable, then no ac coupling capacitor is used.

    Regards, Thomas

    Precision Amplifiers Applications Engineering