amplifierddsoperational-amplifier
I am doing a DDS function generator with AD 9850 
,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.
Try these linear amplifiers made by Apex. They are designed specifically for ultrasound aplications.
There are several points in the AD843 datasheet that appear to be relevant.
I first spotted the "Overdrive Recovery" time (under the general category of "Frequency Response"). Note that the recovery time for positive overdrive (found in your negative peak detector) is significantly longer than the recovery time for negative overdrive (found in your positive peak detector). Half a microsecond when your pulse width is on the order of 2.5 µs (half-cycle @ 200 kHz) could be significant.
Second, the datasheet specifically mentions that the AD843 has trouble driving capacitive loads. Your 10 nF capacitor is more than an order of magnitude larger than any example load they mention in the datasheet.
Third, there is a peak detector circuit given in the datasheet. The topology is slightly different from yours, but more significantly, they use the AD843 in the output stage, but use an AD847 in the input stage "since the AD847 can drive an arbitrarily large value of capacitance".
Best Answer
The problem you are seeing is "natural" for DACs - you are probably best advised to use a sinc-compensation filter like this: -
I took this circuit from a very good (and readable) article by Maxim entitled "Equalizing Techniques Flatten DAC Frequency Response"