FMC-BB-4DA User Guide: Analog Outputs

The FMC-BB-4DA has four analog outputs, labeled DAC A/B/C/D on the PCB. Each output is tied to one DAC channel in an AD9116. The AD9116 labels its DACs I and Q. The mapping of FMC-BB-4DA analog outputs (A/B/C/D) and AD9116 DACs is listed in the table below.

Output AD9116 DAC

Output Circuits

When driving a high impedance load the output voltage range is [0, 1]v, where 0v corresponds to the minimum digital value (-1 for Fix12_11 data).

The FMC-BB-4DA output circuits are designed to drive only high impedance loads, like the 1 Mohm input of an oscilloscope. Lower impedance loads will reduce the voltage swing observed at the MCX jack.

The FMC-BB-4DA output circuits are intentionally very simple. Each DAC outputs a current proportional to the digital sample provided by the user design. This current is converted to a voltage by a 50 ohm resistor on the board. This voltage is tied directly to the MCX jack. The module implements no reconstruction filter between the DAC and MCX jack. This design allows the analog signal to transition very quickly, ideal for observing waveforms like constellations driven by a real-time PHY. If your application requires a reconstruction filter at the DAC output, you must implement this filter externally.

The images below show the raw DAC output and a filtered version of the same signal. In both figures the FMC-BB-4DA DAC is producing a 5MHz sinusoid sampled at 80MHz (i.e. 16 samples per cycle). The left figure shows the raw output of the DAC, as viewed on an oscilloscope with a 1Mohm input and 100MHz input bandwidth. The second figure shows the same signal with an external 20MHz low-pass filter applied (the 20MHz bandwidth option of the scope input, in this case).

DAC output - No external filter DAC output through External 20MHz LPF


The FMC-BB-4DA has four analog outputs routed to coaxial connectors. Each connector is a 50 ohm, standard-polarity, standard-gender MCX jack. Mating cables should be 50 ohm coax with standard MCX plugs. The board does not ship with any cables; users must supply cables appropriate to their application.

Last modified 11 years ago Last modified on Feb 19, 2013, 11:41:09 AM