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AD8546 Production
The AD8546 and AD8548 are dual and quad micropower, high input impedance amplifiers optimized for low power and wide operating supply voltage range applications...
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AD8546 on Analog.com

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AD8546 'resistive load' limits

		
	
				

			

									
					    KE5FX
					
							

            
            			
				
									
											
			
			
				
			
			
			    	
			                    
                
				                on Jun 23, 2018 
				
			
			
		

		
		                    	    

        	    
Thread Summary

                The user questions the AD8546's ability to drive resistive loads under 100kΩ, despite its 11 mA short-circuit current and 15Ω output impedance specs. The final answer clarifies that the AD8546 is a micropower opamp, and its output swing degrades at higher currents, which is why it is rated for resistive loads down to 100kΩ. The output swing and load current figures (Figures 18 and 19) show that the opamp can maintain output within the headroom limits at 100kΩ loads, but performance degrades at lower resistances. The user's application, which requires unity-gain stability and a +5V supply, should still work with the AD8546.
                        	    

                
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Recently I needed to specify a dual opamp with uncritical specs, for use in an oven controller:

 

 
(The other half of the amp interfaces the thermistor to the ADC, not shown.) 

 
A quick parametric search suggested that the AD8546 would be a good fit. Now that I'm looking closer at the data sheets prior to ordering, though, I'm seeing an unfamiliar warning:

 

 
What kind of opamp has problems driving resistive loads, and at the 100K level?!  Is this a misprint?  Nothing in the output impedance or short-circuit current specs suggests any limitations along these lines:

 

 
So I wonder if I'm missing something important here.  Any thoughts?  Obviously there are plenty of other opamps that will work in this application but before rejecting this one I'd like to learn more about the reasoning above.

			

			
									
		                             
                        

    

                                            
                 Operational Amplifiers (Op Amps)
                
                                                    
                 amplifiers
                
                                                    
                 Low Power Op Amps (≤ 1mA/amp)
                
                                                    
                 ad8546
                
                                
            

    
    
		


				
				        
        		

			
						
									
												
			
			
                        
                                    
                        
                        
                        
            
			            
						
						
			
			
			
			
		


        
        
        
    
					

			





















	

	

					



			








	
	
	
	



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                on Jun 25, 2018 10:17 AM
            
            
						
			            		
    
		
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    The specs aren't the least bit critical, given unity-gain stability and the ability to run from +5V, as long as the part has the same footprint as the AD8546.  I'm sure the AD8546 will work OK in my (closed-loop) application.  I'm mostly just curious as to the conflict between the 11 mA Isc / 15-ohm Zout specs and the whole notion of being unsuited for driving resistive loads under 100K.
    
 
    How do you reconcile the Zout spec with figure 27, for instance, which shows a 0.3V drop to the supply rail at a load current of 1 mA?  That'd be more like 300 ohms, wouldn't it? 
    
 
    You guys may consider it a specialized "micropower" part, but it's listed at DigiKey as a "General Purpose" device with perfectly reasonable specs:
    
 
    
 
    So the bug may be on the marketing side. 
    
			
    

		
			
	      

		
		
	
    
	
    
		
    
			
		
    
	
    
	
    
		
						
						
				
						
				
				
		
		
    
			
		
    
	
    

    

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                on Jun 25, 2018 10:17 AM
            
            
						
			            		
    
		
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    The specs aren't the least bit critical, given unity-gain stability and the ability to run from +5V, as long as the part has the same footprint as the AD8546.  I'm sure the AD8546 will work OK in my (closed-loop) application.  I'm mostly just curious as to the conflict between the 11 mA Isc / 15-ohm Zout specs and the whole notion of being unsuited for driving resistive loads under 100K.
    
 
    How do you reconcile the Zout spec with figure 27, for instance, which shows a 0.3V drop to the supply rail at a load current of 1 mA?  That'd be more like 300 ohms, wouldn't it? 
    
 
    You guys may consider it a specialized "micropower" part, but it's listed at DigiKey as a "General Purpose" device with perfectly reasonable specs:
    
 
    
 
    So the bug may be on the marketing side. 
    
			
    

		
			
	      

		
		
	
    
	
    
		
    
			
		
    
	
    
	
    
		
						
						
				
						
				
				
		
		
    
			
		
    
	
    

    

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