Waveform generator with frequency, amplitude and offset control: Part 1

The Basics

These days Direct Digital Synthesizers (DDS) are commonly used in many applications. Analog Devices (ADI) offer wide range of DDS ICs with 1 to 4 output channels, 10 to 14 bits output DAC, sampling rate up to 1Gsps and even higher.
Many information about the principle of DDS can be found in an A Technical Tutorial on Digital Signal Synthesis by ADI.
I decided to design a circuit for a waveform generator, which I will use to test other circuits I design. I will stick on the following parameters of the output signal:

• Frequency from 0 to 5MHz;
• Amplitude from 0 to 10Vpp;
• Offset from -5V to +5V.

The DDS IC provides the frequency control. A DAC will control the amplitude of the output signal by changing the reference voltage of the DDS's output DAC. Another DAC will provide an offset, which will be summed with the waveform signal at the output stage of the circuit. The block diagram of the generator can be found on figure 1.

Figure 1. Block diagram of the waveform generator

Many articles about DDS can be found on Direct Digital Synthesis (DDS) & Modulators page. Useful for me were Circuit Note CN-0156 and Application Note AN-1070. CN-0156 describes how to control the amplitude of the output signal and AN-1070 describes how to set the desired frequency. For more information you can follow the links.

Components pick-up

The previous links made me AD9834 as the heart of this project. It's main characteristics are:

• Output frequency up to 37.5MHz (25MHz for the grade I will use. We will see how close to the truth is this when I begin testing it);
• Sine or triangular and square wave output (using the on-board comparator);
• 2.3V to 5.5V power supply;
• 3 wire SPI interface;
• 20 pins TSSOP package.

And one more, very important for this project specification of this IC - the full scale current of the output DAC can be controlled externally.
I've already chosen the DDS IC, so the next steps are to choose the DACs for amplitude and offset control. As I need two of them, it's better to choose one IC with two DACs inside. I want to be able to control the amplitude and offset with at least 1mV, so I need
 
10V/1mV = 10000

combinations of each DAC. The resolution in bits is
 
2^bits = 10000

which is 13,29. There is no such DAC ever made, so if I take 14bits I will have

10/2^14 =  0,61mV

With 16 bits DAC I will have

10/2^16 = 0,15mV resolution.

I selected AD5663R

• On-chip 1.25V/2.5V, 5ppm/°C reference;
• 2.7V to 5.5V power supply;
• 3 wire SPI interface;
• 10 pins MSOP package.

While I was digging ADI website for a solution of the output stage I found AD8130. It's 270MHz differential receiver amplifier. One of it's typical application is as high speed instrumentation amplifier, which is exactly what I need - differential input, as AD9834 output is the same, offset input, 2 resistors defined gain (as a normal inverting/non-inverting amplifier, based on operational amplifier). I'm so exited to see what will be the results on my lab desk.