Choosing between the TRF3722 vs. TRF372017

The TRF3720 and the TRF3722 operate with different modulator cores.  The '22 provides improved linearity performance and better phase noise performance over the '20.  One of the other primary differences between the TRF3720 and the TRF3722 is the input common mode voltage.  The '20 operates at 1.7V and the '22 operates at 0.25V.  The TX DAC on the AFE7225 has a common mode voltage of 3.0 +/- 0.5 V.  The AFE will not interface with either the '20 or '22 directly.  There are a few options to consider with regard to the interface.

For the '20, there is an option to AC couple and supply the modulator Vcm internally.  The limitation here is that signals very near DC will be attenuated.  This is likely the recommended approach.

An interface network can be used to translate the common mode voltage to a suitable value for the modulator input.  Check out the app note that discusses this option: http://www.ti.com/lit/pdf/slaa399.  The translation network will introduce a series resistor that will introduce insertion loss and bandwidth limitations (due to R + input C of device).  The larger the voltage translation the larger the resistor must be and hence more insertion loss and band limiting.  These issues can be mitigated by placing an capacitor across the series element to introduce an AC short across the resistor.  The only drawback there is that information near DC is attenuated.  The app note discusses these options.

Alternatively, you can select the TRF3703-33 modulator device that has a 3.3V common mode input voltage that will interface with the AFE7225 DAC seamlessly.  This is the set-up provided on the AFE7225 EVM.  The TRF3765 can be used as the LO source in this application.  This will have no near-DC attenuation issues.

--RJH

OK, thanks.
I like the suggestion of using the TRF3703-33 to interface directly to the AFE7225 DAC.

The EVM schematic gets a little busy with all the different options...
I will run the AFE7225 off AVDD3 = 3.3V, this way VCM at the DAC outputs will be 3.3V. At that point, do I just need pull-ups to 3.3V at the DAC outputs? what value is recommended? I will then follow with the reconstruction LP filter and right into the modulator. I may need to provide a termination for the filter at the modulator input.

There was a recommendation in the data sheet to use 5V on the pullups, but is this really necessary? I'd like to keep the interface as simple as possible - that's why I'm gravitating toward the TRF3703-33 part.

John

John:

Check out the AFE7225 EVM design package that includes the schematics :http://www.ti.com/lit/zip/slrr001.  This board includes the TRF3703-33 device and includes the recommended connection circuitry between the DAC and modulator with a placeholder for LPF.  Operation at 5V rail is needed to have some headroom to keep common mode voltage at 3.3 V.

--RJH

Got it, Thanks!

Do you know it TI provides the Orcad library symbols for any of these parts: (AFE7225, TRF3703-33, TRF371125, LMX2581)?