#148 AD9833 Basic Demo Cycle

Run a basic waveform demo with an AD9833 module

Notes

I bought an AD9833 module from a seller on aliexpress with the intention of using it to build a function generator to add to my collection of test gear.

This project is a first test to explore:

• the SPI protocol it supports
• different waveforms it can produce
• controlling the frequency

Conclusion:

• once you get the SPI configuration correct, it works extremely well, and is very easy to control.
• accuracy at frequencies under 1MHz or so is very good
• accuracy over 1MHz inconclusive .. I need to do some more testing

Module Interface

Here’s the module, with pins attached (with my imperfect soldering, so don’t blame the manufacturer!):

The labeling of the pins is a bit unusual. Here’s the pin definition, and how they connect to an Arduino Uno for hardware SPI:

NB: the “analog ground” is internally linked to “digital ground” according to the circuit diagram for the module.

SPI Specifics

The module operates in SPI_MODE2 with MSBFIRST:

• data shifted on falling edge
• FSYNC pin is active low,
• accepts the most significant bit (MSB) of each transfer first.
• The SCLK pin is also specified to be kept high when not in use

Waveform demo

The BasicDemoCycle.ino script is a simple non-interactive demo that runs through the different waveforms.

It uses the new SPI.beginTransaction/SPI.endTransaction syntax, although since the script only works with one SPI device, it is redundant to wrap each command sequence in a transaction. But it is a good test of the “right way to do it”. NB: many other scripts found on the net pre-date this syntax.

It is running at very low frequencies in order to produce a visual demo. It sends the measure waveform to the serial port. LEAP#090 PlotNValues (a simple Processing sketch) reads the data from the serial port and plots the output value over time, with some coloration effects thrown in for good measure.

Here’s a sample trace. It shows a few cycles of each waveform. From the left:

• sine wave
• square wave (normal cycle time)
• triangle wave
• square wave (halved cycle time i.e. twice as fast)
• output off

Note that square waves are rail-to-rail, since they bypass the DAC. Other waveforms peak at 0.65V per the datasheet.

Frequency Accuracy?

A few quick measurements with a frequency counter gave very good results under a few MHz. But when I started pushing into the 6-12.5MHz realm, accuracy appeared to drop off markedly.

This may simply be the influence of running the module on a breadboard. I should try it on a PCB.

For now, here are some results (AC-coupled scope traces) with a sinusoid waveform in the breadboard layout.

At 1MHz, still a decent output:

By 8MHz, the waveform is quite distorted and unstable but still on frequency:

By 12MHz, it has devolved to a heavily-modulated mess:

Construction

NB: I created a custom Fritzing Part available here for these diagrams.

Running at 300kHz, pretty accurate!

Credits and References

• Thanks to Michaël via email for the idea of an “off” step in the demo cycle
• Module from a seller on aliexpress
• AD9833 datasheet
• Arduino SPI Reference
• Sending out the right signals - informative round-up of various signal generator modules
• Julian’s Postbag: #41 - AD9833 Waveform Generator - my inspiration to try the unit!
• AD9833 Waveform Generator - good project write-up
• Waveform Generation with AD9833, and SPI - some more good details