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THS3120EVM: EVM circuit questions

Roy Chou1
Genius 16495 points
Part Number: THS3120EVM
Other Parts Discussed in Thread: THS3120

Tool/software:

Hi Team,

1. May you let me know what is the purpose of the 100 Ω resistor across the OP input terminals? Thank you.

2. From the data sheet SLOS420E, the schematic of THS3120EVM is shown as

At the output of the amplifier, R7A, R7B, R8B, Z2 are left open, but R8A is populated with a 49.9 Ω resistor. What is the purpose of this resistor?

Regards,

Roy

  • 0
    TI__Genius 17675 points

    Hi Roy,

    For the first circuit I cannot definitively say how the LDO team is capturing there results however with the configuration there using it looks like there setting the noise gain of the THS3120 with the 100ohms across the input which would effectively be a noise gain of 11V/V. The DC signal coming from DC out does not look like it will be gained up, but the noise gain of the device will. For the second circuit that is in the THS3120EVM, the R8A resistor is used to impedance match with the 50ohm characteristic impedance coax cable that will likely be used following the EVM as well as match the termination resistor that will be in place in the following stage. Impedance matching is a standard and recommended practice with higher speed signals to compensate for transmission line effects such as reflections. 

    Best regards,

    Ignacio