Using multiple RS232 isolation channels

Question

Hi, 
 We were considering the use of ADM3251E for multiple channel RS232 isolation, in which we may use upto 9 chips (A future products may use 17 channels). But I find that use of 9 x ADM3251E would need more than 1A supply current which is not acceptable. I am also worried about the EMI issue if there are so many isolation chips on a single PCB. Can you suggest a way to achieve this by using lesser current? The overall cost of the solution should not exceed the cost of the ADM3251E based solution. I was considering ADM3252E which appears to be more efficient, but I found the cost to be on the higher side at $ 8.49 compared to $ 2.99 for ADM3251 even if I were to use the ADM3252 for 2 channels. Please suggest a way out. Also please let me know the EMI related precautions that we may have to take in handling so many isolated channels. 
 Regards, 
 Hemant

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ADIApproved · Answer

Hi Hemant, 
 As you suggest the ADM3252E will be more efficient solution than the ADM3251E. It also has other advantages in that it is a more compact dual channel solution, can run off 3.3V without powering VISO side, etc. 
 In your case with such a large number of channels the other solution would be to use separate RS232 and icoupler devices. There are a number of RS232 parts to choose from depending on your combination of Tx and Rx channels, Vcc voltage, ESD protection, etc. The ADM3315E x 3 may be a suitable option as it has 3 Tx and 3 Rx channels, runs off 3.3V supply and has 15kV ESD protection on RS232 I/O pins. There are other parts with more channels as well. 
 In terms of iCouplers, the choice depends on the isolation rating required and whether you need isolated power or not. Assuming just isolated RS232 at 2.5kV then something like the ADuM1402 which has 2 isolation channels in each direction. 
 Rgds, 
 Paul

ADIApproved · Answer

Hi Hemant, 
 The ADM101E works off a 5V supply and has a lower Txout output voltage swing so doesn't require the additional voltage doubler capacitors that you need on other RS232 parts. If you have a suitable 5V supply from the ADuM5201 and the output levels are sufficient (which they probably are as you mention you only require low speed transmit/receive) then the ADM101E is probably a good fit. The ADM101E also has 15kV ESD protection on the I/O pins so I don't think any extra protection will be required. Just ensure that you use low ESR capacitors for Vcc and V- pins (e.g. X5R, X7R series). 
 Rgds, 
 Paul

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Using multiple RS232 isolation channels