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AD 7822 Confusing ~PD and ~CONVST



		

			

									
					    mike3886
					
							

			
				
									
											
			
			
				
    
			
			
			    	
			                    
                
				                on Sep 5, 2017 
				
			
			
		
    

				
		
    
		    		    
		    
		    	
			
    Good day everyone!
    
 
       I'm about to use AD 7822 but as I read the datasheet, I got confused regarding the ~PD and ~CONVST. The AD 7822 powers up (getting ready) if ~PD is logic high OR ~CONVST is logic high (after #ns of ~EOC duration). The word OR here confuses me.
    
 
    1) If I tied ~PD permanently high to Vcc, does that mean the AD 7822 will not power down at all even if after ~EOC, ~CONVST is low? Is the state of ~CONVST ignored regarding power down if ~PD is always high?
    
 
    2) Does that mean that a 1µs power up time will always be needed before every conversion start even if ~PD is tied to Vcc?
    
 
    3) Can I use a continuous crystal oscillator (50/50) for ~CONVST, or does ~CONVST just accepts pulses or "per request" conversion? If yes, does that mean frequency of ~CONVST and sampling frequency is equal?
    
 
       I've wanted to use 7822 with a continuous crystal oscillator for clock (50% duty cycle) and I am trying to compute for the frequency to achieve a little more than 1.5MSPS (my signal is 750kHz). I need a continuous ADC and the 1µs power up time (if it powered down during the signal) will mean that AD 7822 is not for the job.
    
 
    Thank you very much.
    
 

    ps. I will use ad 7822 with arduino.
    
									
			        
		
    

				
							ad7822.pdf
					
		
    
			
						
									
									
						
						
			
			
			
			
		
    

        
        

					
    
			
    


    


	















    
	
    
	
    
					



			








	
	
	
	



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      •  Analog Employees  
      
            
			                            
                        
                on Sep 5, 2017 8:36 PM
            
            
					
      
	
      

		
		
      
					
      Hi Miek3886,
      
 
         Please refer to response to each question below.
      
 
         1) If I tied ~PD permanently high to Vcc, does that mean the AD 7822 will not power down at all even if after ~EOC, ~CONVST is low? Is the state of ~CONVST ignored regarding power down if ~PD is always high?
      
 
         The AD7822 can still enter power down with PD always high. The AD7822 can enter mode 2, automatic power down at the end of conversion. You can refer to figure 25 of the datasheet.
      
 
        
      
 
        2) Does that mean that a 1µs power up time will always be needed before every conversion start even if ~PD is tied to  
      
 
        Vcc? power up time is not needed every conversion. Power up time is needed during when VDD is first connected and    when it has been from a power down (like mode 2).
      
 
      3) Can I use a continuous crystal oscillator (50/50) for ~CONVST, or does ~CONVST just accepts pulses or "per request" conversion? If yes, does that mean frequency of ~CONVST and sampling frequency is equal?
      
 
         I am not sure on how would you do it. But if you are going to use crystal oscillator at 50% duty cycle that means high time and low time are equal. Checking on the figure 24 and figure 25, there would be different high time and low time of the CONVST. The low time is much longer as it will consider the conversion time plus the reading of the data time. CONVST can accept pulses, but proper timing must be followed for conversion and reading of data.
      
 
      Regards,
      
 
      Jonathan
      
			
      

		
			
	      

		
		
	
      
	
      
		
      
			
		
      
	
      
	
      
		
						
						
						
						
				
				
		
		
      
			
		
      
	
      

      

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    • 
	
      



	
			
		
	



      
	
      
		
      
							
					
      
		
      
			
													0
							
			
				
									
							        
			
			
			
      •  Analog Employees  
      
            
			                            
                        
                on Sep 5, 2017 8:36 PM
            
            
					
      
	
      

		
		
      
					
      Hi Miek3886,
      
 
         Please refer to response to each question below.
      
 
         1) If I tied ~PD permanently high to Vcc, does that mean the AD 7822 will not power down at all even if after ~EOC, ~CONVST is low? Is the state of ~CONVST ignored regarding power down if ~PD is always high?
      
 
         The AD7822 can still enter power down with PD always high. The AD7822 can enter mode 2, automatic power down at the end of conversion. You can refer to figure 25 of the datasheet.
      
 
        
      
 
        2) Does that mean that a 1µs power up time will always be needed before every conversion start even if ~PD is tied to  
      
 
        Vcc? power up time is not needed every conversion. Power up time is needed during when VDD is first connected and    when it has been from a power down (like mode 2).
      
 
      3) Can I use a continuous crystal oscillator (50/50) for ~CONVST, or does ~CONVST just accepts pulses or "per request" conversion? If yes, does that mean frequency of ~CONVST and sampling frequency is equal?
      
 
         I am not sure on how would you do it. But if you are going to use crystal oscillator at 50% duty cycle that means high time and low time are equal. Checking on the figure 24 and figure 25, there would be different high time and low time of the CONVST. The low time is much longer as it will consider the conversion time plus the reading of the data time. CONVST can accept pulses, but proper timing must be followed for conversion and reading of data.
      
 
      Regards,
      
 
      Jonathan
      
			
      

		
			
	      

		
		
	
      
	
      
		
      
			
		
      
	
      
	
      
		
						
						
						
						
				
				
		
		
      
			
		
      
	
      

      

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