DDS Output Circuitry (AD9834) - Page 1
Forum section: electronics / projects

author: xandmann
author_rank: Contributor
timestamp: April 11, 2015, 01:07:26 am
content: Hello All,I have seen quite a few solutions for cleaning and reconstructing the output fromthe AD9834 from Analog Devices.Some ELV designs posted here on this blog, the tinyDDS project or thereference design CN-0304.Most of it boils down to these three alternatives:1. dds -> rf transformer -> elliptic filter2. dds -> opamp -> elliptic filter3. dds -> elliptic filter -> opamp(opamp used for offset and gain, both current and voltage feedback)Would be grateful if an expert could elaborate on the above mentionedsolutions and their pros and cons...?

author: Kleinstein
author_rank: Super Contributor
timestamp: April 11, 2015, 10:35:49 am
content: The first solution is good, if you need power from a DDS Chip with differential outputs. This way you can get a 50 Ohms output and use 50 Ohm filters without amplification. However the transformer sets a lower frequency limit. The transformer also helps to avoid ground loops.The second solution with first amplification and then the filter is a bad idea - no real advantage, but HF noise can upset the OP and produce a variable offset and other trouble. One may need an amplifier at the filter output anyway.The third method is often the best solution if you need low frequencies as well: the filter is usually slightly higher impedance (e.g. 200 Ohms), this has the slight disadvantage of needing higher inductance values. The output of the filter is well suited for amplification and Offset correction, so that operation down to DC is possible.

author: Bud
author_rank: Super Contributor
timestamp: April 11, 2015, 02:04:57 pm
content: Having an amplifier in the dds signal chain will worsen harmonic contents. Usually raw filtered dds signal is cleaner in terms of harmonics. If to put an amplifier after the filter, the complete range of DDS generated frequencies will have increased harmonics due to unfiltered amplifier output. Putting amplifier first followed by a filter will mitigate this issue as DDS will sweep towards filter cut off frequency. Either of the two topologies will in total have worse harmonics than just a transformer + filter. So it depends on what your requirements are on signal purity and power.

author: xandmann
author_rank: Contributor
timestamp: April 12, 2015, 12:49:04 am
content: Thank you guys for your input.My goal is to make a general purpose signal generator, (as a learning exercise/project).Wouldn't one get better SNR by amplifying before filtering?If one would follow the Analog Reference Design CN-0304 where they accually used alternative 2 (dds -> op-amp -> elliptic filter).Would it be overkill to put an rf transformer before the op-amp? (dds -> rf transformer -> opamp (dc offset and gain) -> elliptic filter).

author: Liv
author_rank: Regular Contributor
timestamp: April 12, 2015, 08:30:19 am
content: Quote from: Bud on April 11, 2015, 02:04:57 pmAfter the amplifier is also desirable to add a filter, because the harmonics of the amplifier increases at high frequencies.P.S. I do not understand why today use such bad DDS as AD9834. Now there DDS with 14-bit DAC and clock frequency of 400 MHz and above, which have a much better output spectrum. The lifetime should be spent only on them.