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THS3215: I want to convert a differential 450 ohm signal to a 50 ohm single-ended using the THS3215.

yoshitsugu hirata
Prodigy 60 points
Part Number: THS3215

Tool/software:

I want to convert a differential 450 ohm signal to a 50 ohm single-ended using the THS3215.
(1) I need to know the resistance value when the differential input is 450 ohms.
(2) I want to put a filter designed for 50 ohms between pins 6 and 9, but I need to know the resistance value to match the impedance.
(3) I want to know how to calculate the gain in a single-ended 50 ohm output circuit, the resistance between pin 11 and pin 12, and between VMID_out and pin 12. is pin 11 output connected in series with 50 ohms?
(4)Is VMID_out connected to VREF and pin 12?

  • 0
    TI__Genius 16565 points

    Hi,

    If the goal is to impedance match with a differential output impedance of 450ohms, I do not see why you would not be able to set the input resistors to 225ohms to ground on each input. For the impedance looking into 9, this would be roughly 18.5kOhms. For the gain calculation, the resistance between pin 11 and 12 will be the feedback resistor. This will depend on the gain you require. Table 8-2 recommends the feedback resistor and gain resistor to use for different gains. The VMID_out node is ideally not gained by any stage. It is meant to bias the D2S stage and the OPS stage if doing something like single supply configuration. This voltage is seen directly at the output of the D2S stage as this is how you would bias the output of a difference amplifier circuit. If the goal is to impedance match following this device, we typically have a series 50ohm resistor like you mention. This would be at the output where pin 11 is. Figure 9-6 is a good example of where this would be placed. VMID_out is not connected to VREF or pin 12, it is a bias node that can then be tied to different parts of the circuit if needed. Figure 9-9 is a good reference for how this node is implemented. If in split supply, you would leave this node floating and tie everything to ground, like Figure 8-15 shows.

    Best Regards,

    Ignacio