- #1
Jdo300
- 554
- 5
Hello All,
I've been playing worth some DDS output filter and amplifier circuits in the simulator and have run into an interesting little problem. I want to make an output that can range from 0.2Hz to 20Mhz and so far I have built a very nice reconstruction filter that can give me a very clean sine wave through that whole range. The problem is that I want to also have offset and amplitude adjustments for the output. I am going with a couple of high speed op-amps with digital pots for the adjustments but things still get complicated because at the high frequency end (10-20Mhz) where the filter begins to attenuate the signal a bit, the offset changes. Also, I have a simple digital pot on the end of the filter with a center tap that I use as the amplitude adjustment. When ever that changes, that effectively changes the DC offset of the signal as well. I was thinking that in the absolute worse case, I could include all these factors in the programming of the microcontroller so that it can compensate for it. But I'm wondering if there is a better way to deal with this problem?
I know it is common to just use a capacitor in series with the output going to the op-amp but i have tried this and it doesn't seem to work so well at very high frequencies or very low frequencies (I tried a 1uF cap with a 47Ohm resistor in shunt to ground). My design does use a bipiolar supply so I'm wondering if I could somehow use this to my advantage. The DDS raw output sine wave is 0 to 600mV though that can vary a bit after it goes through the filter circuit. What do you all think?
Thanks,
Jason O
I've been playing worth some DDS output filter and amplifier circuits in the simulator and have run into an interesting little problem. I want to make an output that can range from 0.2Hz to 20Mhz and so far I have built a very nice reconstruction filter that can give me a very clean sine wave through that whole range. The problem is that I want to also have offset and amplitude adjustments for the output. I am going with a couple of high speed op-amps with digital pots for the adjustments but things still get complicated because at the high frequency end (10-20Mhz) where the filter begins to attenuate the signal a bit, the offset changes. Also, I have a simple digital pot on the end of the filter with a center tap that I use as the amplitude adjustment. When ever that changes, that effectively changes the DC offset of the signal as well. I was thinking that in the absolute worse case, I could include all these factors in the programming of the microcontroller so that it can compensate for it. But I'm wondering if there is a better way to deal with this problem?
I know it is common to just use a capacitor in series with the output going to the op-amp but i have tried this and it doesn't seem to work so well at very high frequencies or very low frequencies (I tried a 1uF cap with a 47Ohm resistor in shunt to ground). My design does use a bipiolar supply so I'm wondering if I could somehow use this to my advantage. The DDS raw output sine wave is 0 to 600mV though that can vary a bit after it goes through the filter circuit. What do you all think?
Thanks,
Jason O