Author: Stephen Stebbing
AD9850 Direct Digital Synthesis Modules
The circuit is taken directly from the example in the AD9850 datasheet. and uses a 125MHz clock and a 40MHz output low-pass filter. Output impedance is around 100 Ohms and power is of the order tens of microwatts, so an amplifier will often be required. The boards come in two different form factors.
1. DIL Board Type
These boards are easy to mount on a motherboard and fit into a 20-pin IC socket.
Things to note:
- Beware that many of the modules have a large clock oscillator (as shown on the left) which requires a 3.3v supply. This is incorrect as the datasheet states that at 125MHz, the chip must be run at 5v. In operation at 5v, the oscillator gets very warm and after a few minutes and the waveform distorts severly.
- There have been reports that some of the 5v boards can be run at 3.3v, but has not been tested personally.
- All examples seen of the board with the small clock oscillator (as shown on the right) use the correct 5v version.
- The trimpot adjusts the duty cycle of the square wave output which is dependent on the frequency.
- The annoying, power wasting LED can be easily desoldered.
- Many boards do not have the pin functions marked, but are as shown below:
- SINB is the filtered sinewave output, SINA is unfiltered quadrature output.
- QP and QN are the square wave outputs.
2. Circuit
- The output low-pass filter has an input and output impedance of 200 ohms.
- Resistor R6 sets the output level. Decreasing its value will increase the output level. rudiswiki9 suggests that soldering a 10k resistor in parallel with R6 gives 2k2 which results in an output level of closer to 0dBm.
3. Odd Shaped Board Type
These modules have two rows of pins at one end, and one row at the other making them more difficult to mount on a mother board.
- For all examples seen, the clock module is the correct 5v type.
- The pin functions are marked.
- The circuit is here.
- The purpose of the jumpers is shown on the circuit.