| ??? 01/09/06 23:49 Read: times |
#106940 - Proper Op-Amp Responding to: ???'s previous message |
A big misinterpretation of sub-sampling technique is, that by sub-sampling a higher bandwidth can be achieved. This is wrong! Bitscope designer says clearly here that sub-sampling is usefull when you have invariant periodic waveforms and if you are about to see frequency contents higher than half the sample rate you'll have to go for a faster ADC. From Shannon-Nyquist theorem this means that a maximum signal frequency of 20MHz can be handled. So, the analog part of the circuit must at least have a bandwidth of 20MHz But could we reproduce a sine-wave 10 MHz with only 2 samples (taken by the 20MSPs ADC) ,Of course not and Bitscope designer says we have to at least take 8 samples to reproduce the captured signal on the screen. Again, this does not at all mean, that the bandwidth is increased in any way!
Yes , This is some sort of interpolation which e.g. is used in Digital cameras to produce a higher resolution than their exact CCD resolution. Today i just found some more Op-Amps and specially this goody goody : AD9632 : BW : 320 MHz(small signal),175 MHz(Large Signal) SR : 1100 V/us Input res. : 500K and let me correct a mistake on AD829 : BW : 600 Mhz SR : 220 v/us Input res. : 13K I'll need a 2V swing (0.6 to 2.6V) at the ADC input.So provided by an x10 gain I would be able to measure down to 200mV peak-peak which i think would be enough. A bandwidth of 20MHz means a dampening of 3dB at 20MHz. If you would use OPamps with 20MHz bandwidth, then using four such OPams in series would result in a dampening of 4 x 3dB = 12dB. That's too bad , I don't like it :( . I thought that if i feed a signal throught a series of high BW Op-Amps i wouldn't lose much. Dear Kai where could i read on this '3 db dampening' (Hopefully on the net) ? I can't find anything releated in the datasheets. After all How about one AD844 as a -1 gain buffer followed by an AD844 with at -10 Gain (where it delivers 33 Mhz according to the datasheet)? This is what currently i can think of, Not considering the '3 db dampening' problem. Thanks alot for your great and detailed replies Farshid |
