TRF370417EVM: Base band Common Mode Bias

Steve Anthony

Part Number: TRF370417EVM
Other Parts Discussed in Thread: TRF370417

Are the base band inputs for this modulator internal biased to the 1.7V common mode? That is if these are AC coupled or transformer coupled to a differential DAC is an external bias voltage needed?

Thanks!

over 3 years ago

0 Kang Hsia over 3 years ago

TI__Guru* 80395 points

Hi Steve,

You will need to externally bias the TRF370417 input. If you are ac coupling, you will need a resistor ladder or transformer with center tap bias to bias the TRF370417.

The following app note will help

https://www.ti.com/lit/ug/slua647a/slua647a.pdf?ts=1646765543416&ref_url=https%253A%252F%252Fwww.ti.com%252Fproduct%252FDAC3484

0 Steve Anthony over 3 years ago in reply to Kang Hsia

Prodigy 50 points

It doesn't show the biasing network in the app note. I assume I'll need a resistor divider in each leg of the I and Q differential signals. It's odd that there is no mention of this in the data sheet. Thanks for your help.

0 Kang Hsia over 3 years ago in reply to Steve Anthony

TI__Guru* 80395 points

Hi Steve,

Yes, I agree you have to dig through the datasheet to find this. The following section talks about this

0 Steve Anthony over 3 years ago in reply to Kang Hsia

Prodigy 50 points

It sort of covers it. In my case I'm doing a low pass filter between the differential outputs and the IQ inputs. The DAC is buffered and the buffer needs an impedance of 200 to 1000 ohms while the IQ modulator wants 50ohms. The filter is a 7-pole LC differential network built with the 200ohm impedance in mind. A simple resistor divider won't work that's why I needed to be sure that the VCM needed to be applied externally before proceeding.

Thanks for your help.

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TRF370417EVM: Base band Common Mode Bias