Variable output voltage DDS system

[sahko123]

I'm designing an lcr meter for which I want to use the AD9833 DDS generator and I want to be able to get an output voltage of 50mVpk-pk to 20Vpk-pk.

How would go about doing this?

Should I have a fixed gain followed by variable attenuation or attenuation followed by fixed gain?

What parts/circuit would people recommend for doing this? The frequency range will be 25Hz-1MHz.

[sahko123]

UPDATE:

This applicaiton note seems to describe exactly what i want I dont know how i missed it
https://www.analog.com/media/en/technical-documentation/application-notes/AN-1192.pdf

[Kleinstein]

For a LCR meter on usually does not need a fine adjustable ampliture. So just switching between some attenuator steps could be good enough.
The variable gain amplifier is definitely an option, though possibly overkill.

[MarkF]

I would change to an AD9834 and follow this Circuit Note.

Beware the FS_ADJUST is 0v to 1.15v max.  Where FS_ADJUST of 0v will be max output voltage.
If you use a differential op-amp on the two outputs, the DC offset will remain the same when adjusting the amplitude.

Example circuit:  Re: Looking for <$50 function generator

[sahko123]

The fsadjust is not voltage controlled it has a nominal open circuit voltage of 1.15v and the resistance is used to change the full scale. I want the gain to either be digitally controlled or analog voltage controlled

[MarkF]

The fsadjust is not voltage controlled it has a nominal open circuit voltage of 1.15v and the resistance is used to change the full scale. I want the gain to either be digitally controlled or analog voltage controlled

That's funny.  It works for me (digitally controlled via a MCP4802 DAC).
You can tie the resistor to GND or a voltage less than 1.15v to change the amplitude.
Look at my circuit or the Circuit Note using a DAC and series resistor.
The output voltage can be controlled between approx. 50mV to 500mV.

It does what you want.
 

[Marco]

Also Analog Devices says it works

[sahko123]

Right I misread the schematic, I thought you were feeding voltage directly into the FSadjust pin.

With my max desired range of 20Vpk-pk I would be able to get a range of 2Vpk-pk minimum instead of the 50mV I wanted. I would even be happy with 100mVpk-pk minimum

[MarkF]

I don't know what the optimum range is.  It just wasn't that important to me.
It's been ages since I worked with it and how close I came to 1.15V and if the minimum was 50mV, I just can't remember.
You would have to tweak your own circuit to see what can be accomplished.

Also, I used the MCP4802 8-bit DAC.  You could use the MCP4822 12-bit to get finer detail.
Maybe add a high/low range by having a 2x or 4x op-amp stage switched into the path?

[passedpawn]

I'm designing an lcr meter for which I want to use the AD9833 DDS generator and I want to be able to get an output voltage of 50mVpk-pk to 20Vpk-pk.

How would go about doing this?

Should I have a fixed gain followed by variable attenuation or attenuation followed by fixed gain?

What parts/circuit would people recommend for doing this? The frequency range will be 25Hz-1MHz.

Have you looked at the AD5934?  It's used for measuring reactance and inductance.  I've used it with success in a couple of applications.  I've also used the AD9833 quite a bit, but at lower frequencies than you're looking at.  I generated signals > 150V p-p, but was able to use audio transformers as that was the frequencies I needed.