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INA1650: Input voltage range then unpowed

Part Number: INA1650

Tool/software:

We're looking at using the INA1650's XLR example circuit (8.2.1 / page 22 from the datasheet), but are worried about audio being fed to the device when it's in an unpowered state. 

Here's an excerpt from the datasheet:

7.3.3 (electrical overstresses) mentions keeping the current through the device's protection diodes less than 10mA and shows the following image:

We are however unable to find a mention of the value of the resistance from IN+/IN- to the diodes. 

Our question: is the 8.2.1 application circuit safe to be used as shown due to the inherent current limiting of those resistors, or is an additional strategy required? We are worried adding series resistors would deteriorate the CMRR performance. 

  • Hi David,

    is the 8.2.1 application circuit safe to be used as shown due to the inherent current limiting of those resistors, or is an additional strategy required?

    This is INA1650's absolute max. ratings for the part. If the audio input exceeds V+ and/or V- and audio input signal is higher than Vcc or Vee by +/-0.5V, it can potential damage the input front. 

    There are two different abs. max. ratings in the table. One is input current, and other one is input voltage. if any one of the parameters are exceeded, the input front could be damaged.  

    If you are able to measure the resistance between input to Vcc and/or Vee, it will tell you the possible issues vs. good INA1650. If it is shorted, it is likely that the part is damaged. 

    If you have other questions, please let us know. 

    Best,

    Raymond

  • Hey Raymond,

    Thank you for your answer! I however don't think it answers my question. We also don't have any damaged devices - we're still in the design phase. 

    The example application in 8.2.1 shows a circuit using XLR connectors - any audio device built like this will at some point in its life experience an input signal while being unpowered. This being given as an example application implies that not to be a problem.

    The current through the input pins will depend on the internal series resistors, is this is high enough I can imagine not easily reaching the mentioned 10mA with common audio voltages. This will might indeed exceed the absolute maximum specified in the datasheet depending on the signal generators output impedance, but with the input stage primarily consisting of resistors I'm not sure what would be damaged having a volt or 2 over them. 

    These make me think it's fine to use in an application like this. Blindly sticking to the super generalized "Vcc / Vee +- 0.5V would not. So what should I believe here? And if it's not safe to use it like this, are there any examples of high CMRR input circuits that would allow you to build a real world product with them? 

  • Hi David, 

    The current through the input pins will depend on the internal series resistors, is this is high enough I can imagine not easily reaching the mentioned 10mA with common audio voltages.

    When you mentioned INA1650 is in unpowered state, I interpreted that Vcc and Vee are not powered or in its rest state (low impedance, near GND potential). 

    +/-10mA is specified to protect ESD diodes at the input. So the current can be limited at the input terminals readily. As long as the input resistor is large enough, the input current can be limited for the worst case scenario (max. input signal swing) for your application. This current limits are applicable for both operating and nonoperating states. 

    Blindly sticking to the super generalized "Vcc / Vee +- 0.5V would not. So what should I believe here?

    For input overvoltage scenario, the application has to stay below +/-0.5V in order to be kept below Vcc and Vee rail potential, which are near GND potential. 

    The abs. max. ratings are not suggestion. It is the absolute max. ratings for potential damage, typically we called it as EIPD (External Induced Physical Damage). There are certain protective margins that are specified for such ratings. I agree that the input is not readily damaged, and our product is rugged in these protective circuitry, but these are the max. electrical overstress (voltage) on the op amp's input terminals. It will damage over time if the designer is ignoring the abs. max. ratings. EIPD related damages are not warranted. 

    As user, you can do whatever you want to do. If the abs. max. ratings are violated, we are unable to help you, if the op amp is damaged. We can only recommend the safe operating conditions. 

    if it's not safe to use it like this, are there any examples of high CMRR input circuits that would allow you to build a real world product with them? 

    I am not sure what you are trying to do. If it is CMRR voltage, you can couple the input with a capacitor or switch or something similar. 

    If you have other questions, please let us know. 

    Best,

    Raymond

  • Hey Raymond,

    Since there is no way to prevent an end user from accidentally applying an audio signal to an unpowered device, making an actual product with the schematic in 8.2.1 will by definition violate the absolute maximum ratings specified. 

    What I mean with:

    if it's not safe to use it like this, are there any examples of high CMRR input circuits that would allow you to build a real world product with them? 

    Is that we would like to build a real world product, while keeping the high CMRR of the INA1650 / example circuit. I'm not sure what you mean by "CMRR voltage", the suggestions you do are incredibly vague, and the coupling capacitor you mention is already in the circuit I showed you (and does nothing to protect the device against an input signal overvoltage).

    The only thing the datasheet mentions in the electrical overstress section is: 

    A series input resistor can typically limit the current. Some signal sources are inherently current-limited and do not require limiting resistors.

    I worry adding non-high-precision series resistors will degrade the CMRR due to the mismatch it could cause. Is there an application engineer that would be able to take a quick look at this and help us out with the math behind both the current limit and the resulting CMRR? 

    Much appreciated,
    David

  • Hi David,

    The current limiting resistors before the ESD cells are 500 Ohms-  this means you can apply 5V above the supply voltage on the inputs before the 10 mA limit is reached.  I agree with your point - you don't want to add just basic resistors for current limiting before the inputs, they should be matched because the CMRR depends on it.  

    So, as long as your input signal is < 5V, and the power supply is low-impedance (which typically is not the case - the power supply itself will increase when current flows back through the ESD cells into the supply) there should not be any problems to connect directly.

    Regards,
    Mike

  • Hey Michael,

    Awesome - that's exactly the information we've been looking for. We'll run some simulations with these numbers and decide how to move forward. 

    Much appreciated!