DDS output circuitry with filtering, DC rejection and 50 ohm line-driving - Page 1
Forum section: electronics / projects

author: alexscep
author_rank: Contributor
timestamp: May 12, 2015, 02:57:09 pm
content: Hey everybody,i was wondering if anyone could give me some insight in the following schematic i found of an arbitrary waveform generator. The DAC generates two complementary current outputs, which are converted to a voltage through each resistor to ground. These voltages have a DC offset equal tot the amplitude of the signal. Then this is followed by a differential amplifier, which amplifies the AC-component and rejects the DC-component between its two inputs. After obtaining a pure AC signal, this is passed through a LPF which acts as an anti-aliasing filter. This removes the frequency-images embedded in the signal due to the digital nature of the signal. Then this low pass filtered signal is passed to the buffer stage, where 3 high current, high speed amplifiers are connected in parallel to achieve very high drive capability. Each of these amplifiers are given a gain of +2, to drive 50 ohm loads.Some of the resistors in this design are meant for 50 ohm termination. Could someone point out to me which ones or which design considerations i should consider. I am pretty sure the filter should have a 50 ohm input and output termination. Is the resistor after the THS4271 a 50 ohm resistor? I guess so because the datasheet states the output resistance of the opamp is about 0.06 ohms at 1Mhz operation. And the resistor after the filter, is also a 50 ohm resistor, i suspect. And which resistance value should the output-resistances of the THS3091 have to be, to achieve 50 ohms of termination?

author: Liv
author_rank: Regular Contributor
timestamp: May 12, 2015, 03:10:15 pm
content: The filter does not necessarily have to be input and output impedance of 50 ohms. It can be calculated for any impedance, for example 200 ohms. Resistors after the THS4271 and after the filter are used to matching and should have the same value. The output resistors must be 50/3 ohms, to obtain an output impedance of 50 ohms.

author: janaf
author_rank: Frequent Contributor
timestamp: May 12, 2015, 03:37:58 pm
content: you may find this usefulhttps://www.eevblog.com/forum/projects/simple-amplifier-for-ad9850-dds/

author: alexscep
author_rank: Contributor
timestamp: May 12, 2015, 04:47:09 pm
content: Thx Liv for the fast reply, i also found this in the THS4271 datasheet, but i can't really figure out which purpose the 124 ohm resistor serves. Could you have any guess?

author: Liv
author_rank: Regular Contributor
timestamp: May 12, 2015, 05:29:27 pm
content: Interesting question. In general, this resistor is not needed. For example, in a similar scheme for the AD9744 DAC does not have this resistor. Probably, this resistor - is an attempt to match the impedances. But the differential amplifier based on a conventional op amp has different impedances for their inputs, good matching is still not succeed. In such cases it is better to use a fully-differential amplifiers, such as ADA4927 and the like.You can look at the output path ofAgilent 33220Agenerator (pp. 174, 176). Note that the filter is commed immediately after the DAC (that do not pass on the opamp RF components). And the filter is very strange, is not symmetrical. Most likely, this is due to the different input impedance of the differential amplifier. But how they calculated this filter?

author: alexscep
author_rank: Contributor
timestamp: May 12, 2015, 06:01:26 pm
content: The mentioned schematic is very interesting but maybe somewhat complex for what i want to achieve, do you suggest any fully differential amplifiers that could replace the THS4271. I don't need to generate that high frequencies, 10 MHz is more than adequate range. The mentioned ADA4927 is a decent component but it has an anoying footprint. I am looking for something with SOIC-like package.

author: Liv
author_rank: Regular Contributor
timestamp: May 12, 2015, 06:15:58 pm
content: For example, AD8138.If the differential output is not required, it is possible to use differential receiver like AD8130.

author: alexscep
author_rank: Contributor
timestamp: May 12, 2015, 07:01:29 pm
content: But i want a differential to single ended conversion, can i achieve this with this AD8138

author: Liv
author_rank: Regular Contributor
timestamp: May 12, 2015, 08:04:39 pm
content: You can achieve this with both opamps.

author: alexscep
author_rank: Contributor
timestamp: May 14, 2015, 06:13:32 pm
content: Ah ok Liv, i didn't quite understand youre approach until i read this application note (http://www.analog.com/media/en/technical-documentation/application-notes/AN-1214.pdf) where they use a differential reciever amplifier (AD8130) to convert a differential signal to a single-ended signal. Thank god application notes exist, these things are life-savers sometimes. I will try and use this solution, it will give a better conversion than using a traditional difference amplifier.

author: alexscep
author_rank: Contributor
timestamp: May 15, 2015, 02:35:20 pm
content: Btw, i am not really that familiar with the use of current feedback amplifiers, both the AD8130 and THS3095 are CFB amplifiers. Are there things i should be aware of when designing this output circuitry. What are their main advantages and disadvantages in comparison to voltage feedback amplifiers?