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AM26LS32A: Maximum voltage on 1A/1B port

Nam, Dino
Expert 6445 points
Part Number: AM26LS32A
Other Parts Discussed in Thread: LM339, TLV1814, ISO7740, AM26C32, AM26LV32E

Dear team, 

The customer would like to use AM26LS32A for their Encoder application. Please review below customer question.  

1. The absolute maximum rating of differential input voltage is +-25V at datasheet. How about recommended operating condition of differential input voltage? 

2. The max operating voltage on 1A/1B port? Operating max voltage 1A or 1B is 7V? 

3. It the customer use 0 to 12V on 1A, 12V to 0V on 1B port, is it operate well without damage? 

Please let me know your opinion. 

Thank you. 

  • 0
    Guru 310880 points

    1./2. The common-mode input voltage range is ±7 V.

    3. 12 V is inside the absolute maximum ratings, so it will not damage the device. But above 7 V, correct operation is not guaranteed.

    Why are there 12 V signals? Are the lines not supposed to be terminated? It sounds as if you could just use comparators like the LM339 or TLV1814.

  • 0
    TI__Mastermind 49742 points

    Hi Nam,

    1. +/-7V - same as the common mode input range (as Clemens indicated)

    2. +/-7V for recommended input range - higher than that and the device may not work correctly - it shouldn't be damaged until abs max is violated however. 

    3. 12V shouldn't damage it - but that is outside our suggested operational ranges so we wouldn't guarantee that type of operation and all risk is assumed by customer. 

    Best,

    Parker Dodson

  • 0 in reply to Parker Dodson
    TI__Expert 6445 points

    Hello Parker, 

    Thank you for your comment. more questions. 

    1.  If the customer use  12V on 1A,  0V on 1B port , The common voltage is (12V+0V) /2 = 6V, is it right?

    - If yes, Is it spec-in our +/-7V  as the common mode input range? 

    - And our application note(SLLA070D), https://www.ti.com/lit/pdf/slla070?keyMatch=SLLA070D 

    we can see below table. As below application note, +-10V input range is available under +7V to -7V common mode voltage range. Is the +-7V input range  only for our AM26LS32A? Please review it again and let me know your opinion. 

    2. If you don't recommend above 7V on 1A port, Why our datasheet show +-15V at test condition of electric characteristics like below? This condition makes it seem like the customer can use more than 7V input. please let me know your opinion. 

    3. The customer would like to use AM26LS32A + digital isolator(ISO7740) configuration. 

    Isolator Vih is 0.7xVCC  = 3.5V under 5V VCC but as you can see below, The Voh value of AM26LS32A is Min 2.5V 

    - Is it available to use AM26LS32A + digital isolator(ISO7740) configuration? How can we remain Voh value above 3.5V? 

    - Is the VI(G) is input voltage of  G port? If this voltage is 5V, what is output voltage? is it available above 3.5V output? 

    Thank you. 

  • 0 in reply to Nam, Dino
    Guru 310880 points

    1./2. It indeed appears that the datasheet contradicts itself.

    3. The AM26LS32A is a very old device with TTL-compatible outputs. I would strongly recommend to use the AM26C32, or the AM26LV32E with a 3.3 V supply.

  • +1 in reply to Nam, Dino
    TI__Mastermind 49742 points

    Hi Nam,

    First - there is no  contradiction in the datasheet - I will get into the details a little later in my reply as its not super straightforward.

    1. common mode input range and common mode voltages are actually slightly different things. The equation that you are using is how you find common mode voltage - but on, at least, RS-485 and RS-422 devices common mode input range is the voltage at any differential input with respect to GND. So 7V on A is within common mode input range (regardless of state at B) while 12V is out of range - it shouldn't damage the device we just don't guarantee operation. 

    As for the app note you are misunderstanding - there is one paragraph that explains why the 10V shows up

    RS-422 specifies bus leakage over the range of -10V to 10V that is why they included that. In practice it makes very little sense and if you can't keep the input voltage range within +/-7V than we don't guarantee operation beyond that range. 

    2. This has everything to do with how the product was defined - it is from 1980 and we largely have kept the part definition largely the same.  This datasheet supports the 32A and and the 33A - the 33A has a +/-15V common mode input range and for some reason they decided to specify the devices input bias current as 1 spec that is shared between the 33A and the 32A. The 32A will not be damaged at +/-15V but we do have the current defined at that point - but they really shouldn't go over +/-7V because nothing else would be defined at that point - and if they tried to and something went we' point to that they violated common mode input range. I understand how this can be frustrating - but it is just how it is. This is a very old device (> 40 years) so information / definitions are not the as clear or useful as some of our newer devices. 

    3. Don't use this device - use the AM26C32 - it was designed to be a better replacement for the AM26LS32A - honestly when it comes to quad device systems the AM26LS32A is not one I'd suggest - the AM26C32 has much better performance. 

    Best,

    Parker Dodson