What is the purpose of this long capacitor/inductor chain on a DDS IC evaluation board?

Question

Image of the B output stage of the EVAL-CN0304 AD9834 DDS Evaluation Board

IOUT_B is a current source, not potential source, so the initial say half of the circuit I get. The resistor gives us a proportional voltage, where it is centred on the input range + put through a unity gain amplifier, so we do not load down the input + cause issues. What I don't get then is why it is put through this 51 ohm voltage divider and all these capacitors and inductors. It seems like this would create a bunch of nasty resonant peaks from the LC oscillators.

Answer 1

Your circuit is a low pass circuit with a very sharp cut-off at about 17.8 MHz. The sharp cut-off is due to the high order of the filter.

The output of the circuit probably goes to a 50\$\Omega\$ transmission line, which is itself terminated with a 50 \$\Omega\$ termination resistor. The 51 \$\Omega\$ resistors would then be for impedance matching, to prevent reflections back into the amplifier part of the circuit, or back into the transmission line.

^{simulate this circuit – Schematic created using CircuitLab}