Digital control of output amplitude - Page 1
Forum section: electronics / projects

author: space_bob
author_rank: Newbie
timestamp: October 26, 2017, 06:12:13 pm
content: Hello,I'm currently building a small signal generator based on the AD9834.Awaited specs are sin/tri/sqr, 1Hz to 1MHz, +/-10v (10v amplitude and +/-5v offset).The setting of the duty cycle, amplitude and offset should be digitaly controlled (by user interface).I need some help to design the control of the output amplitude.I was wondering if an AD835 (analogic multiplier) coupled with a DAC could be suitable to control the output amplitude.Any advice and suggestions will be greatly appreciated.I'm doing this little project on my free time to improve my skills and learn new things, I'm not looking for commercial solution

author: Kleinstein
author_rank: Super Contributor
timestamp: October 26, 2017, 06:28:39 pm
content: A analog multiplier or voltage controlled amplifier (like AD603) is one option. Some of the cheap (e.g. $50-$100) range DDS generators use this.Another Option is to do fine (e.g. 6 or 10) dB Rangetuning by adjusting the DDS chips reference current/voltage and do coarse adjustment with switches (e.g. relays or due to the rather low frequencies CMOS switched can be an option too). Adjusting the DAC current might require the use of both outputs to avoid DC level shifts.

author: RES
author_rank: Regular Contributor
timestamp: October 26, 2017, 06:40:27 pm
content: Amplitude Control Circuit for AD9834 Waveform Generator (DDS)http://www.analog.com/en/design-center/reference-designs/hardware-reference-design/circuits-from-the-lab/cn0156.html

author: MasterT
author_rank: Frequent Contributor
timestamp: October 26, 2017, 09:43:18 pm
content: You may use digital potentiometer buffered with OPA. Or multiplying DAC, feeding reference input and setting coeff. MCP4922 for example.

author: space_bob
author_rank: Newbie
timestamp: October 28, 2017, 05:00:05 pm
content: Thank you for your help I found some schematics of low cost signal generator and they use muxes with R2R ladder to make an controlled attenuator. Idea given by Kleinstein too if I understood well. I guess resistor must be quite accurate and muxes need to have the lowest resistance to keep the ratios acceptable.

author: jaycee
author_rank: Regular Contributor
timestamp: October 28, 2017, 05:38:54 pm
content: Digital pot such as AD5142 for the attenuation, followed by an opamp to give you gain. You can use another digital pot (or a two channel one) to control the offset by injecting a bias voltage into the -ve of the opamp.

author: space_bob
author_rank: Newbie
timestamp: October 28, 2017, 08:26:47 pm
content: Quote from: jaycee on October 28, 2017, 05:38:54 pmCould you elaborate please i don't see why I should attenuate then use opamp to give gain.

author: MasterBuilder
author_rank: Regular Contributor
timestamp: October 28, 2017, 09:26:36 pm
content: Heres an explanation of some VCA circuits:http://sound.whsites.net/articles/vca-techniques.html

author: jaycee
author_rank: Regular Contributor
timestamp: October 28, 2017, 10:55:28 pm
content: Quote from: space_bob on October 28, 2017, 08:26:47 pmMost digital pots work with max 5V. You will have an I-V conversion stage as the output of the AD9834 is a current output not a voltage output. You could use your digital pot to control the gain of the IV stage fairly easily

author: danadak
author_rank: Super Contributor
timestamp: October 29, 2017, 01:31:20 am
content: Multiplying DAC -http://www.analog.com/en/analog-dialogue/articles/high-resolution-multiplying-dacs.htmlLadder + OpAmp all thats needed. Possibly some compensation to optimize transientbehavior.Regards, Dana.

author: Rerouter
author_rank: Super Contributor
timestamp: October 29, 2017, 09:51:14 am
content: .. always the hard way.. the DDS has a full scale adjustment pin, grab an R2R ladder dac, and an op amp, and set it up as a programmable resistance on the pin, like the attached image,