Electronics > Beginners
DIY Function Generator
MarkF:
Here's my DDS design with only a tenths worth of stuff around it.
eeFearless:
Is VOUTB from the MCP DAC for adjusting DC offset?
Wouldn't you want a big honking cap at the 470/100/2.7K junction, so the AC differential gain is symmetric?
Kleinstein:
It looks like VoutB is for adjusting the offset. For symmetry one could just use slightly different resistor values (e.g. around 2.78 K in the feedback of the OP) to include the extra resistance. One might even save one ore resistor and possibly the lower OP.
However this circuit is still missing two parts: the reconstruction filter is very low grade. Also there is no coarse adjustment for the amplitude (like the relay part) in the other circuit. Also the upper frequency limit and amplitude is rather low. In addition the output impedance is not at 50 Ohms in the simple circuit.
So that simpler circuit has some good ideas (like having the offset) and using an DAC to set the amplitude. However there is also a oversimplification in using a slow OP as an filter and the low output amplitude.
MarkF:
Yes. VoutB is the offset adjustment and VoutA is the amplitude control. The amplitude gain is set for 2Vpp with a +/- 1V offset control. The op-amp gain could be adjusted for a larger output if desired. I see no reason for a coarse amplitude adjustment with noise prone relay contacts. The coarse/fine adjustment is part of the GUI with finer adjustments achieved with a 12 bit DAC (MCP4822) instead of the 8 bit DAC (MCP4802) I used.
As for the reconstruction filter, I found it almost unnecessary with the 1Hz to 1MHz frequency range I limited the DDS to. A FFT shows the DDS output to be very clean without one.
The 50 ohm output impedance is a weak point for me. Any suggestion for a fix is welcome.
Edit: See the attached application note on the amplitude control for the AD9834. It is a coarse adjustment in itself.
ziplock9000:
--- Quote from: Kleinstein on January 17, 2017, 01:12:06 pm ---External RAM to sample data can be rather fast, but this would need something like an FPGA / CPLD to generate the address sequences. To have a good arb.- generator one might also needs a more or less fine adjustable clock one when to update the clock / data - this is different from an DDS where the clock is fixed.
--- End quote ---
Ah balls. It's looking more and more like even a modest 10Mhz range arb gen is out of range and better to just buy one :/
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