Implements ADC in a DSO ,must have filter before ADC?

[shteo83]

adc dso

I going to use adc device, TCL5540INS with sample frequency up to 40Msps in my oscilloscope project. Is it possible i ignore the filter device before the ADC? Can someone give me some idea or comment? urgently, 10s

 

[sixth]

I going to use adc device, TCL5540INS with sample frequency up to 40Msps in my oscilloscope project. Is it possible i ignore the filter device before the ADC? Can someone give me some idea or comment? urgently, 10s

It is determined by your application. The main consideration here is the spectrum aliasing. If the aliasing spectrum does not affect the desired signal, you can of coarse eliminate the antialiasing filter before the ADC, if not, you must add a filter before it.

sixth

 

[shteo83]

May i know what is the normal frequency will be enter the ADC if we measure the voltage? IS it vey high or low? Because for the nyquist frequency is about 20Mhz if the sample frequecy is 40Msps, so if ADC capture the signal with frequency lower than 20Mhz, the aliasing will still occur?

 

[sixth]

May i know what is the normal frequency will be enter the ADC if we measure the voltage? IS it vey high or low? Because for the nyquist frequency is about 20Mhz if the sample frequecy is 40Msps, so if ADC capture the signal with frequency lower than 20Mhz, the aliasing will still occur?

Signal with all frequency will enter the ADC no matter if there is a filter before the ADC or not, and the signal above fs/2 will be aliased into signal bandwidth and decrease the system performance. The purpose of the antialiasing filter is to attenuate the signal above fs/2. If your desired signal is not pure enough, you must place a antialiasing filter befor the ADC. For example, you signal bandwidth is between 0~5MHz, but together with this signal, there exists a interference or crosstalk at 38MHz, then this 38MHz signal will be aliased into your signal bandwidth. If the interference of crosstalk is strong enough, you can not distingguish the desired signal and the interference signal in digital domain. This will decrease the system performance. If the the interference or crosstalk exist in 30MHz, will it affect the system performance? The answer is notbecause we can attenuate this signal in digital domain. Everything is determined by your applications.

sixth

 

[shteo83]

if my ADC analog inout bandwidth is 75Mhz.. If i use function generator to test my ADC. If i supply the AC signal with frequency just for few kHz, in this case can made aliasing to my ADC output. My ADC Sample frequency is 40Msps. Because my project due date is around the corner, so i try to skip some hardware part if it doens't effect to much to my ADC output. Because when the signal pass through the filter, the amplitude or voltage will be reduce rite? So i need one more device which is preamplifier circuit to my ADC output. I dun hav extra time to do reseach on it ady.