ilovephysics
Newbie level 6
Hi,
I am working on a project that interfaces a Virtex 5 with a high speed DAC (AD9781). I will be controlling this DAC through SPI (serial port interface). At first I naively thought that I could just straight up connect my FPGA pins to the SPI pins on the DAC. Then after some research, I realized that I need to be more careful.
So assume that I set my FPGA'S output pins to 50 ohm output impedance through DCI and launch them to 50 ohm transmission lines, I will not need any series resistor termination at the source, correct? I cannot find any information on this SPI's input or output impedance. Since the digital pins of the SPI use 3.3 V, my educated guess is that the inputs use CMOS circuitry and thus the input impedance is quite high. Therefore near the SPI's input pins, I can use parallel termination with a 50 ohm resistor to ground, am I right? At the SPI's output pin, assuming a relatively low output impedance, I can use a series resistor of 30 to 50 ohm to terminate the source?
I am working on a project that interfaces a Virtex 5 with a high speed DAC (AD9781). I will be controlling this DAC through SPI (serial port interface). At first I naively thought that I could just straight up connect my FPGA pins to the SPI pins on the DAC. Then after some research, I realized that I need to be more careful.
So assume that I set my FPGA'S output pins to 50 ohm output impedance through DCI and launch them to 50 ohm transmission lines, I will not need any series resistor termination at the source, correct? I cannot find any information on this SPI's input or output impedance. Since the digital pins of the SPI use 3.3 V, my educated guess is that the inputs use CMOS circuitry and thus the input impedance is quite high. Therefore near the SPI's input pins, I can use parallel termination with a 50 ohm resistor to ground, am I right? At the SPI's output pin, assuming a relatively low output impedance, I can use a series resistor of 30 to 50 ohm to terminate the source?
