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TLV4171: what will happen if input signal common-mode voltage is out of range of spec ?

Part Number: TLV4171
Other Parts Discussed in Thread: OPA172, LM7705

Hi Team,

My customer has a question when use TLV4171. Their input signal common-mode voltage has chance to beyond the spec range (V-)-0.1 ~ (V+)+2. Though customer know the performance of TLV4171 cannot be guarantee when out of specification, they hope to know what is (statistic) probability the ICs become abnormal when input signal common-mode voltage is out of range.  Pls help check this. Thanks.

B.R,

Garrick Dai

  • Hi Garrick,

    Typical value is plus or minus one standard deviation (68% of the devices). But, min or max means most of the devices fall under this range. Hence, it is NOT advisable to operate outside the min or max range because most of the devices will show abnormal behavior. I would recommend using a different op-amp that meets your specification.

    Best Regards,
    Bala Ravi
  • Hi Garrick,

    what is what you don't want to happen?

    Kai
  • Hi Garrick,

    Your customer is right in that the performance will not be guaranteed when operating outside of the common mode voltage range. However, the TLV4171 will operate and function correctly when used with a common mode voltage between (V+) - 2V and (V+)+0.1V but with significantly degraded performance. To give and idea on how much the performance will degrade I recommend taking a look at section 8.4.1 in the OPA172 datasheet (compare the typical performance in the electrical characteristic table with the performance that is shown in Table 3). I also recommend reading "Op amps with complementary-pair input stages: What are the design trade-offs?" for an explanation on why the performance degrades and why it will still operate outside the common mode range.

    BUT the customer cannot apply an input signal that is 0.5V greater than the positive supply (see Absolute Maximum Ratings Table) because that will cause the internal ESD diodes to conduct. If this condition is expected to occur the current into the device must be limited to less than 10mA (preferably less than 1mA if it is expected to occur frequently) or damage may occur.

    Thank you,

    Tim Claycomb

  • Thanks Tim. I will explain to customer.

  • Please let us know if you need any additional information.
  • Hi Tim,

    Customer want to double confirm -- whether or not the TLV4171 can operate and function correctly when used with a common mode voltage between (V-) – 0.5V and (V-) (but with significantly degraded performance). Customer application scenario will have common-mode voltage drop 0.5V below V-, so customer need to make sure the TLV4171 still work correctly (even though performance degrade).

    Thanks,

    Garrick Dai

  • Hi Garrick,

    it is not correctly working, if the performance degrades. And what is the difference between degraded and significantly degraded?

    It's rather simple, the device is specified to work properly with a common mode input voltage of more than (V–) – 0.1V. If you go more negative, you will do this on your own risk.

    By the way, you could solve the issue by the help of LM7705.

    Kai

  • Hi Garrick,

    The device will not be damaged with an input 0.5V below the negative supply but the device will not be able to output the correct output voltage due to the output voltage swing limitation of the device. 

    Thank you,

    Tim Claycomb