Comparison of THD in Analog Multiplexers

Question

Hi 
 I require an 8:1 analog multiplexer in a new design. Ideally a dual 8:1 multiplexer would be best & I am considering the ADG1407BRUZ, which I have used before. The THD spec of this part is given as 0.07% typical for RL=110R. Previously I had to lower my input signal in order to achieve the level of THD I wanted. 
 I am now considering using two ADG5208F 8:1 multiplexers rather than the dual ADG1407 part. It has a THD spec of 0.005% typical. However, this is into a 10k load rather than he heavier 110R for the ADG1407 device. 
 I would like to be able to compare the 2 specs in order to decide whether one part is better than the other. I understand that THD is related to RON Flatness, hence the reason I reduced my signal level previously. 
 Is there a way for me to approximate THD for different loads based on the Ron Flatness & Ron values, maybe an approximate formula? if not how can I compare the ADG1407 & ADG5208F in order to decide which part has better THD performance? 
 regards 
 Steve

ADIApproved · Accepted Answer

Hi Steve, 
 
 Currently, we are working on a THD calculator for our switches and multiplexers that should help people with similar questions. 
 You are correct. THD is directly related to RON Flatness and the device's on-resistance, circuit input resistance, and load resistance, as shown here: https://www.analog.com/media/en/technical-documentation/application-notes/AN-251.pdf 
 To summarize what it says there, THD is proportional to Rflat / (Rin + Ron + Rload). Then, let's see the switches you are interested in: 
 
 Rload = 110 &Omega; (Rin = 0 &Omega; )
 
 ADG1407 -> proportion = 0.013389 
 ADG5208F -> proportion = 0.018056

Rload = 10 k &Omega; (Rin = 0 &Omega;) 
 
 ADG1407 -> proportion = 0.00016 
 ADG5208F -> proportion = 0.000634

Looking at those proportions, even though the ADG5208F has a better (RON Flatness / RON) than the ADG1407, its RON is still very high and will worsen more the distortion than the ADG1407. Therefore, the ADG1407 will still have a better distortion performance than the ADG5208F. 
 That simple equation above and the AN-251 are a good way of comparing THD performance between devices. However, bear in mind that it is lacking any capacitance and frequency information that your circuit has. So, it should only be used as a guide. 
 Now, for the recommendation, you can have a look at the ADG1408 or ADG5408 (both 8:1 x 1, not 8:1 x 2), where the ADG1408 will have the best distortion performance overall. 
 
 Rload = 110 &Omega; (Rin = 0 &Omega; )
 
 ADG1408 -> proportion = 0.004386 
 ADG5408 -> proportion = 0.014575

Rload = 10 k &Omega; (Rin = 0 &Omega;) 
 
 ADG1408 -> proportion = 4.998e-5 
 ADG5408 -> proportion = 0.00018

Please, let us know if any of these parts still do not meet your requirements. 
 
 Yours sincerely, 
 Igor Ono.)

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Comparison of THD in Analog Multiplexers