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TIPD185: Why do we need 49.9 ohms in these places?

Waleed
Prodigy 90 points
Part Number: TIPD185
Other Parts Discussed in Thread: OPA376

Hello,

IN the reference design for AC coupled inverting/non-inverting amplifier, the board has R10 and R9 of 49.9 ohms. May I know why do we need these two resistors? Is it for impedance matching? thanks

  • 0
    TI__Guru* 93105 points
    Hello Waleed,

    Yes, they are for impedance matching. The inclusion of R10, the 49.9 ohm resistor, serves as an broadband near 50 ohm termination for an instrument having a specified 50 Ohm output impedance, or a cable having a 50 Ohm surge impedance. R9, also 50 Ohms, is a back termination resistor. The closed-loop output impedance (Zol) of the OPA376 is very low. When the 49.9 Ohms is added to Zol the impedance is close to 50 Ohms, which matches the surge impedance of 50 Ohm cables.

    Regards, Thomas
    Precision Amplifiers Applications Engineering
  • 0 in reply to Thomas Kuehl
    Prodigy 90 points

    Thanks Thomas for your response.

    I read OPA376 datasheet, I came by its output open loop impedance

    For R10 , isn't going to consume more current as input impedance is in parallel to capacitor? Would that affect HPF value since R10 is now in parallel with C?

    For R9, R0 is higher than 50 ohms. So why so we need to place 50 ohms in series?

    Thanks again Thomas

  • 0 in reply to Waleed
    TI__Guru* 93105 points

    Hi Waleed,

    Your questions:

    For R10 , isn't going to consume more current as input impedance is in parallel to capacitor? Would that affect HPF value since R10 is now in parallel with C?

    Since R10 is connected from the input connector to ground it will load the source connected to J1. That resistor will consume power when a signal is present. But if you need to avoid transmission line effects such as standing waves on a long cable, it is necessary to have R10 in place. If the frequency is low and you are not concerned about terminating a transmission line into its surge impedance, then you can likely leave R10 out of the circuit. For instance, if your input frequency is tens or hundreds of kilohertz in frequency you probably aren't going to need to terminate the line.

    R10 and one of the series input capacitors C3, or C6 form a high-pass function and with a roll-off below approximately 318 Hz. These dc blocking capacitors are needed to maintain the correct dc voltage at the op amp input.

    For R9, R0 is higher than 50 ohms. So why so we need to place 50 ohms in series?

    Ro, or Zo is the open-loop output impedance. As soon as the loop is closed the closed-loop output impedance (Zol) is presented. It is Zo divided by the very large loop gain, and results in a Zol of less than an ohm. Therefore, that very low Zol in series with 50 Ohms looks much closer to 50 Ohms when looking back into the output of the circuit.

    Regards, Thomas

    Precision Amplifiers Applications Engineering