ADC32RF45: Matching Input Impedance to 50 Ohm

Francois Tolmie

Part Number: ADC32RF45

Good day,

We are using the ADC32RF45 on one of our boards (sampling at 3 GSPS). We are using the input drive circuit of Figure 239 in the ADC32R45 datasheet (SBAS747C), except we do not have the optional 5 Ohm series resistors (causes too much attenuation) and we also do not have the resistors between the two transformers. However, we do use the TC1-1-43 as suggested and the coupling capacitors.

At high frequencies (1.42 GHz - 2.75 GHz), we are missing SFDR datasheet performance by more than 10 dB. According to Figure 63 in the datasheet, the input resistance is very low, 20 Ohm, at these high frequencies. We assume that we are getting bad performance due to impedance mismatch.

Does TI provide a method/advice of matching the low input impedance at these frequencies closer to 50 Ohm without introducing too much attenuation in the input drive circuit.

Kind regards,
Francois

over 4 years ago

0 Vijayendra Varma Siddamsetty over 4 years ago

TI__Genius 10505 points

Hi Francois,

There is an app note titled "S-Parameters for ADC32RF45: Modeling and Application" at http://www.ti.com/lit/an/sbaa219/sbaa219.pdf. This can used for matching circuit design. As the matching circuit uses only inductors and capacitors, it shouldn't introduce much attenuation.

However I don't expect impedance mismatch to cause 10 dB degradation SFDR. Are you using bypass mode for these tests? I see that frequency range you mentioned is across 1st and 2nd nyquist zones. Are you selecting the right config file for each nyquist zone? If you can tell which spurs are degrading SFDR at one particular frequency, I can provide further help in the debug.

Regards,

Vijay

0 Francois Tolmie over 4 years ago in reply to Vijayendra Varma Siddamsetty

Prodigy 130 points

Hi Vijay,

Thank you, I will follow the app note to design the matching circuit.

I have tested the performance with and without bypass mode.

It may be that I am not using the right config file for each Nyquist zone (or outdated config files). Can you please provide me a link to the latest version of these config files (the link in the datasheet is not working).

Kind regards,

Francois