Hi all,
In my application, I use LMH 6554 as Single Ended Input To Differential Output Operation.
In my next stage, the ADC input impedance is already 100 Ohm differential input.
Do I still need the output resistor RO in the Figure 2 of LMH 6554 datasheet to do the impedance matching?
Thank you all in advance!
Best regards,
Nicholas
Nicholas,
Strickly speaking, impeadance matching is not need if the op amp and ADC are electrically close, because the op amp will absorb any reflections and control the output voltage. Since most all ADCs are voltage driven devices it is ok.
But the Ro resistors in figure 2 should be used to isolate the amplifier's outputs from capacitive load. Refer to the "Frequency Response vs. Capacitive Load" and "Suggested ROUT vs. Capacitive Load" graphs on page 6 of the data sheet. The ADC will have input capacitance and for best stability the Ro resistors should be used to isolate the FDA's outputs from the ADC's input capacitance and parasitic board capacitance. 10 ohm to 50 ohms in each output are typically useful values to use, the higher the value, the more loss of signal amplitude and different values should be tested to find the optimum.
Also note that figure 2 is very simplified and most applications will want to use a filter between amplifier and ADC to limit the bandwidth. The filter design will need to account for and use the input impedance of the ADC and the Ro of the LMH6554.
Regards,
Jim Karki
TI High Speed Amplifiers