Variable output voltage DDS system - Page 1
Forum section: electronics / projects

author: sahko123
author_rank: Frequent Contributor
timestamp: June 21, 2022, 05:53:48 pm
content: 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.

author: sahko123
author_rank: Frequent Contributor
timestamp: June 21, 2022, 06:19:43 pm
content: UPDATE:This applicaiton note seems to describe exactly what i want I dont know how i missed ithttps://www.analog.com/media/en/technical-documentation/application-notes/AN-1192.pdf

author: Kleinstein
author_rank: Super Contributor
timestamp: June 21, 2022, 06:26:07 pm
content: 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.

author: MarkF
author_rank: Super Contributor
timestamp: June 21, 2022, 06:31:25 pm
content: I would change to an AD9834 and follow this Circuit Note.Beware the FSADJUSTis 0v to 1.15v max. Where FSADJUSTof 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

author: sahko123
author_rank: Frequent Contributor
timestamp: June 21, 2022, 06:47:58 pm
content: 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

author: MarkF
author_rank: Super Contributor
timestamp: June 21, 2022, 07:07:46 pm
content: Quote from: sahko123 on June 21, 2022, 06:47:58 pmThat'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.

author: Marco
author_rank: Super Contributor
timestamp: June 21, 2022, 07:29:04 pm
content: Also Analog Devices says it works

author: sahko123
author_rank: Frequent Contributor
timestamp: June 21, 2022, 08:05:18 pm
content: 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

author: MarkF
author_rank: Super Contributor
timestamp: June 21, 2022, 08:24:03 pm
content: 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?

author: passedpawn
author_rank: Contributor
timestamp: June 22, 2022, 12:09:27 am
content: Quote from: sahko123 on June 21, 2022, 05:53:48 pmHave 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.