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

author: alexscep
author_rank: Contributor
timestamp: April 03, 2015, 02:12:05 pm
content: Dear forum-members, i am quite new to the idea of DDS signal generation. I understand how it functions and how i should program it but I don't really know how to succesfully design an analog output circuitry. I am currently working with an AD9834 DDS, and am using it tot generate sine waves up to a frequency of 2 Mhz. My design has been based upon following open source hardware projecthttp://electronics-lab.com/projects/test/022/index.html. But if you take a closer look at the schematic, you can see that the section named DDS output has a very minimalistic RLC filter. This doesn't quite match up to my specifications, so i designed a 7th order chebyshev filter with cutoff frequency 4Mhz. This cutoff frequency was chosen to obtain an as flat as possible passband from 0 to 2MHz.But i want the generator to have an 50 ohm output, so i will replace the existing filter with my filter and will add an extra drive opamp with gain 2 to drive my 50 ohm output. I am going to use the same opamp LM7171 (since it has an +/- 100mA drive-capability) for this. The filter i designed also has 50 ohm input and output impedance. could someone give me some help to how i should design this last output circuitry.Thanks in advance, Alex

author: Kleinstein
author_rank: Super Contributor
timestamp: April 04, 2015, 07:18:56 pm
content: Typical DDS reconstruction filters for a DDS are Cauer-type RLC filters, right at the output of the DDS Chip. They look simple, but are rather powerful. However the RLC filter get more difficult if the cut off frequency is low, e.g. 1 MHz or less. Often the filters are not 50 Ohms, but more like 200 Ohms, to match the DDS Chip output impedance. 200 Ohm is also ok to drive an amplifier or even a pot if the frequency is not that high.The triangle signal might be slightly better with a different filter (e.g. Bessel or similar) - anyway the quality of the triangular signal is ok only at lower frequencies. So even with 75 MHz system clock the 1 MHz triangle will not be really good. The sine may be good up to about 20 MHz. So even if you don't need high frequency sine waves the triangle may need a fast clock, not just because of the Bandwidth, but also because you can't interpolate the peaks.With just 1 OP-amp for the output, one has to check on the power dissipation and choose the right supply voltage, so that the power limit is not reached, even if the output is short circuit to GND. Depending on the required level, it may help to have 2 OPs with 100 Ohms to output each. Having an amplification of about 2 for the output stage is good, as this increases stability, without to much loss in bandwidth.Often you find a switchable divider (e.g. -20 dB / -40 dB) at the output, to get smaller signals as well. However if you want a DC Bias as well, it may be better to have the divider before adding the DC bias - alternatively add the DC-Bias at the GND Side, if there is only one output.

author: alexscep
author_rank: Contributor
timestamp: April 04, 2015, 07:59:37 pm
content: Dear Kleinstein, thanks for the reply,So i have to design a 200 ohm LC ladder filter with the desired cutoff frequency and then the necessary amplification blocks, isn't it better to filter at a later stadium? Another question. I am looking for a decent way to filter the DC offset output by the DDS, how do i implement this best. I you would answer to this question, would it be possible to give an complete overview of the output circuitry, i mean giving the proper buildingblocks, alias filter, amplification, DDS offset removal.

author: hugo
author_rank: Regular Contributor
timestamp: April 04, 2015, 10:51:23 pm
content: Hi,For your inspiration take a look at this ELV function generator called DDS 20 (german) :

author: Bud
author_rank: Super Contributor
timestamp: April 04, 2015, 11:47:03 pm
content: Search for the following documents from Texas Instruments :slyt360.pdfslyt368.pdfAnd from Analog Devices:MT-019.pdf