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THS4303 application

Pier Bortot
Prodigy 30 points
Other Parts Discussed in Thread: THS4303

I am currently contemplating of using the THS4303 device in my design, but I did notice a discrepancy in the TI information. The data sheet of this device it clearly states that pin-13 is a NC pin, but when I look at the TI evaluation board, pin-13 is still shown as a NC pin, however the PCB and the evaluation board are connecting this pin-13 to GROUND. My question is which piece of information is wrong?

My second question is: The evaluation board PCB shows that the ground plane is removed from under the input and output areas of the PCB, can someone tell me the reasons for doing this alteration?

I would like to know this information very quickly because I am holding up production of my PCB design at the moment.

  • 0
    TI__Genius 17905 points

    Pier,

    Pin 13 is not connected to any internal circuitry, so it is OK to either leave it floating or connect it to GND. Connecting Pin 13 to GND makes the layout simpler:  in most cases, Vin- (Pin 14) will be connected to GND, so the corner with pin 13 and 14 can be part of a GND fill that extends to the PowerPAD below the chip as shown on the EVM.

    Parasitic capacitances can lead to stability issues when using high-speed amplifiers such as the THS4303. A large parasitic capacitance directly at the output of the amplifier adds an additional pole in the open loop gain response of the amplifier that can lead to instability. The parasitic capacitance can be reduced by cutting out the copper fills/planes below the output pins.

    Cutting out the ground plane below the non-inverting input as shown in the datasheet is not critical but recommended. A parasitic capacitance (e.g. 1-3pF) at the non-inverting pin can unintentionally band limit your signal if the input signal source has a large enough output impedance that creates a single RC pole with this capacitance. For example, a 50-ohm source and 3pF parasitic capacitance creates a single pole at ~1GHz, and the THS4303 has  bandwidth of 1.8GHz.