I am doing a DDS function generator with AD 9850 enter image description here

,because of the charactericts of DAC converter http://www.analog.com/static/imported-files/data_sheets/AD9850.pdf, page 9 figure 5, the output of amplitude of waveform is decresing when increasing frequency, due to my requirement of DDS function generator a) operate within range 0 to 40 MHZ with 5V amplitude

So any opamp circuit can be suggested? as I am newbie to opamp device.

  • \$\begingroup\$ Pick any 5v opamp, look at it's datasheet for typical circuits. \$\endgroup\$
    – Passerby
    Mar 1, 2014 at 3:17
  • \$\begingroup\$ Wikipedia has a few different amplifier circuits: en.wikipedia.org/wiki/… However, if you need 40 MHz, you need to make sure your op-amp is fast enough. Also, what kind of signal are you generating? This chip can certainly generate a 40 MHz signal, but you won't have a lot of resolution (i.e. a 40 MHz sine wave should really have at minimium a 400 MHz DAC behind it.) \$\endgroup\$ Mar 1, 2014 at 5:37
  • \$\begingroup\$ I think what a few people are missing is that you really need an AGC (automatic gain control) circuit to keep it constant over the voltage range. I was expecting to be able to find a suitable circuit pretty quickly but couldn't. Most were based on obsolete parts or had other problems. \$\endgroup\$
    – PeterJ
    Mar 1, 2014 at 5:54
  • \$\begingroup\$ I ended up using LM7171 opamp from TI, it has a big GBWP and huge current output. Achieving 40MHz is a bold goal. ACG is probably a way to go there, but I couldn't find quick and easy solution. Please share if you come up with one. \$\endgroup\$
    – miceuz
    Mar 1, 2014 at 11:25
  • \$\begingroup\$ @PeterJ, ya, a AGC is needed for this circuit,but AGC I have googled it and found information on its,and it just too complicated,basically while reading the information I am lost,and some IC like CLC IC has obsolete. \$\endgroup\$
    – user37970
    Mar 2, 2014 at 13:13

1 Answer 1


The problem you are seeing is "natural" for DACs - you are probably best advised to use a sinc-compensation filter like this: -

enter image description here

I took this circuit from a very good (and readable) article by Maxim entitled "Equalizing Techniques Flatten DAC Frequency Response"

  • \$\begingroup\$ hi Andy aka, this article clear all the cloud of my question,thanks so much...and then post equalisation can solve my problem, then could you recommend the suitable ic for the post equalisation technique? \$\endgroup\$
    – user37970
    Mar 2, 2014 at 14:16
  • \$\begingroup\$ you need an opamp with bandwidth greater than the highest frequency you are trying to equalize, preferably twice as high. You probably need to run it from a 5v rail and ideally you should choose one with rail to rail input and output but how big is your signal and what power rails do you use. \$\endgroup\$
    – Andy aka
    Mar 2, 2014 at 15:50
  • \$\begingroup\$ my output signal from the sensor is sine waveform around 1.1V fro frequency below 10 MHz, <br/>Output Frequency Amplitude of the Output waveform 10.006Hz 1.13V 100.01Hz 1.16V 1kHz 1.16V 10.002kHz 1.13V 100.01kHz 1.13V 1MHz 1.09V 10MHz 1.17V 20MHz 680mV 30.1MHz 350mV 40.0MHz 143mV 50.6MHz 88mV \$\endgroup\$
    – user37970
    Mar 2, 2014 at 16:31
  • \$\begingroup\$ above were some result I collected,any chip or ic can be suggested for me?thanks in advance =) \$\endgroup\$
    – user37970
    Mar 2, 2014 at 16:44
  • \$\begingroup\$ power rails? LT6202 is good for 5V rails. \$\endgroup\$
    – Andy aka
    Mar 2, 2014 at 17:02

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