Amplitude control for AD9833 function generator

[golf32]

Hi, I've been looking into building a function generator based on an AD9833 DDS generator. The specs I want to hit are:

• 0-12.5MHz
• Sin, Square, Triangle
• 3-12v amplitude
• -9-9v DC offset

I don't think these specs are too hard to meet, and I have most of the design fleshed out, as there is a lot of prior work with this chip.

The one part I am having problems with is amplitude control. Most designs use a potentiometer to vary the gain of the output amp, but I want to try and add digital control. I've explored many ways of varying the gain, but this does not seem to be the right approach.

Instead, I think the best way to go about it is to have a fixed, high, gain and to attenuate the signal to vary the output. I've seen many variable attenuator building blocks, but I can't get any of them to simulate correctly. Any help would be great, Thanks.

[HB9EVI]

direct control over the amplitude is not possible on the AD9833, potentiometer, also digital ones I'd skip on MHz-frequencies; also 12.5MHz is far away from being feasible

the AD9834 is a much better choice for this project; it can be slightly overclocked with 80MHz, 12.5MHz is possible to reach; the amplitude control can be done over the Iset pin with a DAC controlled current source. for the different wave forms it makes sense to switch a LPF for the sine signal

for the DC offset, you find here a simple sketch how to get a bipolar voltage from an unipolar voltage like a DAC: https://www.ti.com/lit/pdf/slau525

I did basically the same with a AD9850, just without tri/squ