Output impedance of the LTC6957-3

Hello,

I'm trying to match the output impedance of the LTC6957-3 to a 50 Ohm transmission line. In the datasheet of the LTC6957 I didn't found any information regarding the output impedance of the -3/4 variant of the chip. In the datasheet of the evaluation board for the LTC6957-3 I found the following statement:

"NOTE: The CMOS outputs of the DC1766A are AC coupled and attenuated through two 100Ω resistors as shown in the DC1766A schematic diagram to present a 50Ω source impedance appropriate to connect to a 50Ω input impedance instrument." Source: https://www.analog.com/media/en/technical-documentation/user-guides/dc1766af.pdf (Page 2)

Through this impedance matching the output voltage level gets halved, which results in a Vp-p of approximately 1.65 V. This output swing is to small for the following circuit on my PCB. Therefore I'm looking for an alternative impedance matching. Do you have any suggestions?

I have seen other CMOS output drivers which have a output impedance of approx. 7 Ohms. In that case a 43 Ohm series resistor would be enough to match the output impedance to the transmission line and the output voltage swing would stay at 3.3 Vp-p. Is this approach suitable in the case of the LTC6957-3?

Thank you in advance.

Kind regards,

Lukas

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  • Lukas,

    Based on these plots, it looks like the LTC6957 CMOS output has ~30-40 ohms resistance.  Some variation with temperature.

    On the eval board we had to plan for the CMOS signal going into a 50 ohm termination to ground.  However, on your board you can plan for other termination schemes.  The LTC6954 has similar CMOS outputs to the LTC6957.  The LTC6954 datasheet has the following recommendations.

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  • Lukas,

    Based on these plots, it looks like the LTC6957 CMOS output has ~30-40 ohms resistance.  Some variation with temperature.

    On the eval board we had to plan for the CMOS signal going into a 50 ohm termination to ground.  However, on your board you can plan for other termination schemes.  The LTC6954 has similar CMOS outputs to the LTC6957.  The LTC6954 datasheet has the following recommendations.

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