Power supply for handheld DDS synthesiser - Page 1
Forum section: electronics / projects

author: microbug
author_rank: Frequent Contributor
timestamp: July 04, 2014, 08:52:31 pm
content: Hi,I am making a handheld DDS synthesiser which is based around the AD9850 modules commonly found on eBay. Its sine wave output will be run through a precision op-amp (THS3001 –datasheet) whose gain will be adjustable via a pot. I haven't built this up on a breadboard yet, only in parts. The case I chose for my DDS synthesiser has optional (purchased separately) battery compartments – you can get a 1x9V compartment, 2x9V compartment or 4x1.5V AA compartment. I need to pick one of these and I am currently thinking of 4 AAs as they are more common than 9V batteries. Ideally the output of the DDS will be +-10V maximum. I am wondering whether a viable solution would be to use a boost converter likethis oneto 24V and a buck converter likethis onedown to 12V which would serve as the ground, giving my op-amp +-12V to work with (I could make this greater if needed). There will also be a 7805 powering the AD9850 which will not require much current.I am worried that there will be excessive noise injected into the output if I do this – would this be likely? I will add some power supply decoupling on the PCB in addition to what is already present on the DC-DC converters as I know the caps on them will be One-Hung-Low brand.

author: Rerouter
author_rank: Super Contributor
timestamp: July 05, 2014, 02:49:06 am
content: there are pricier chips that take 5V in and produce +-12V (think about $12), with my own project (same AD9850, a device working as a counter / frequency source for automotive, but way over the top input and output protection) i got myself stuck a little as it is harder to find them in the +- 30V i was aiming for (with 50mA drive.... )

author: mij59
author_rank: Frequent Contributor
timestamp: July 05, 2014, 03:25:54 am
content: I am worried that there will be excessive noise injected into the output if I do this – would this be likely? I will add some power supply decoupling on the PCB in addition to what is already present on the DC-DC converters as I know the caps on them will be One-Hung-Low brand.You may need some shielding.Have calculated the total power consumption of the device ?How long do you want the battery to last ?

author: mariush
author_rank: Super Contributor
timestamp: July 05, 2014, 03:43:46 am
content: I'd suggest going with 2 x 18650 batteries , they'd pretty much fit within the space 4aa batteries would take, or two 9v batteries.Depending on how much current you need, you should be fine with a couple of cheap MC34063 regulators, one in inverting mode and one in boost mode. The mc34063 need at least 3v to work, but the higher the input voltage the more efficient they'll be.

author: microbug
author_rank: Frequent Contributor
timestamp: July 05, 2014, 07:51:04 am
content: I would estimate that, taking into account the AD9850's low power consumption, the power consumption of my micro and the LED backlight for the LCD screen, the whole thing will take up ~150 mA while it is running (not including power supply quiescent current) and without the backlight probably much lower; ~30mA max.I like the idea of 18650 batteries as I have a ton of them from old laptop batteries. I think, therefore, I will go with a dual MC34063 configuration (thanks mariush).I will drive the first (boost) MC34063 from two 18650s in series for efficiency. I can add another two 18650 batteries in for the inverter or drive the inverter from the output of the first MC34063; I am currently thinking of doing it from the first MC34603, again for efficiency.