DDS generator output amplifier - Page 1
Forum section: electronics / projects

author: Dago
author_rank: Frequent Contributor
timestamp: November 06, 2009, 01:37:29 pm
content: I'm making a DDS based function generator (sine and squarewave for something like 50Hz-8MHz). I have the DDS part working and I'm working on the output amplifier section (since the DDS chip outputs around 200mV p-p). The first output amp version I built was actually (unknowingly at the time) based on Dave's schematic (http://alternatezone.com/electronics/images/DDSFreqGenSchematic.gif)I used a AD847 for the op-amp but it was way too slow (and I suspect the EL2044/LM6361 will be as well) and got like -20dB attenuation @ 8MHz (due to opamp bandwidth product limit). Well I made a new version with way faster LT1225 op-amps and used two amplifying stages to get around the bandwidth limit problem (then only thing really limiting is the slew rate and these opamps have a pretty good slew rate). Well it's working WAY better but uhh... the problem this time is that the gain actually goes UP with frequency which is uhh sort of bafflingAtm. I have two non-inverting amplifying stages with gain = 5 back-to-back, with a 33k feedback resistor and a 8,2k resistor going to ground from the minus input. Pretty much the same schematic as Dave's. What I'd essentially want is AROUND (its not THAT critical, +-20% is fine with me) 1V p-p from the first amplifying stage and 5V from the second one (I'm going to use an analog multiplexer to select which voltage I want on the output).Here's a capture of the first stage at 100kHz and everything is working as calculated (Yellow or Vpp(1) is the input and blue or Vpp(2) is the output):Then at 8MHz gain goes over 8:Anybody got any ideas whats happening and what can I do to get rid of it? Only thing I can think of right now is that I'm hitting some resonance and getting extra gain due to that.Edit: Here's my PCB layout, the two SO-8 up from the lower left corner are the two op-amps:http://www.dgkelectronics.com/storage/electronics/function_generator/dds_output_layout.pngRest of the stuff isn't even soldered in yet.

author: Dago
author_rank: Frequent Contributor
timestamp: November 06, 2009, 05:18:22 pm
content: Found itAlways remember to read the datasheet: "Design Note 50. Feedback resistor values greater than 5k are not recommended because a pole is formed with the input capacitance which can cause peaking. If feedback resistors greater than 5k are used, a parallel capacitor of 5pF to 10pF should be used to cancel the input."I reduced the resistor values to 4,7k and 1k and now I get ~1.2V first stage @ 100kHz, 920mV @ 8MHz and second stage 6,9V @ 100kHz, ~5.3V @ 8MHz.

author: Dago
author_rank: Frequent Contributor
timestamp: November 08, 2009, 12:36:29 am
content: Hmm, weird. I attached a NPN + PNP (BC639 + BC640) totem-pole to the opamp output (bases fed through a 470ohm resistor) and the NPN collector is attached to a 15V supply and the PNP collector is attached to the ground, but its not working correctly. I just get around 3,5V out from the emitters (output varies with frequency a bit but I tried everything from 100kHz to 8MHz). I'm 100% sure the transistors are fine and are wired correctly. What could be wrong?

author: Dago
author_rank: Frequent Contributor
timestamp: November 24, 2009, 06:34:09 pm
content: Finished it, heres a page about it:http://www.dgkelectronics.com/page.cgi?page=dds_gen