ltc1668 output impedance tolerance

Question

Hello! 
 
 I am designing a differential AFE with this differential DAC driving the feedback into a high-gain instrumentation amplifier. The differential output impedance of this DAC, ( RioutA || 5pF, and RioutB || 5pF ), will affect the common-mode gain of my circuit. If each Riout and 5pF cap are well matched to one another, then the DM to CM gain can be minimized, but if there is a large tolerance on any or both of these on-chip components, then I'd likely need to change my architecture somehow. 
 The datasheet specifies the min and max values that RioutA,B to be 700 ohm to 1.5kohm with 1.1k typical, and the datasheet says the output cap is exactly 5pF. I want to know a bit more about this if possible - I assume that the range of RioutA,B is across different chips, but is not telling us the matching spec, right? I.e. we shouldn't expect the worst-case scenario to be RioutA = 700 and RioutB = 1.5k on a single chip, right? 
 
 Any additional details would be much appreciated! 
 
 Best, 
 Andy

dendalisay · Accepted Answer

Hi, 
 The specified range of output impedances is given to account for variations across different chips. This range represents the possible values you might encounter due to manufacturing tolerances. However, the datasheet does not provide explicit information about the matching tolerance between RioutA and RioutB on a single chip. Given the wide range specified, it is unlikely that you would encounter the extreme case of RioutA = 700 ohms and RioutB = 1.5k ohms on the same chip. More realistically, these values should be closer to each other but within the overall specified range across different units. To minimize common-mode gain and potential mismatches in your circuit, you might consider adding external matching components or design adjustments that can tolerate these variations. 
 Best Regards, 
 Den)

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ltc1668 output impedance tolerance

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