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Original question:

TLV9064: input current limit resistor

TLV9064: Signal Input Pins Beyond ABS Voltage Rating, but use Current Limiting Resistor

Darren (FAE)
Expert 5483 points
Part Number: TLV9064

I have a customer using the TLV9064 device.

They have an input voltage of 11.6V max, and this passes through at 43kΩ resistor to the device input.

The voltage at the input is 5.9V, which is beyond the V+ + 0.5 specification in the the "Absolute Maximum Ratings"

However, there is a note (2) which states the voltage at V+ can be higher than this V+ + 0.5V limit, if the current is kept below 10mA.

In their setup, the current would be (11.6 - 5.9) / 43000 = 132uA

(V+ is 5V)

Is this 5.9V allowable, given their current limiting?

Would you have a comment on the high input resistance? (Section 11.1 in the DS mentions: The added resistor contributes thermal noise at the amplifier input and the value must be kept to a minimum in noise-sensitive applications.)

Thanks,
Darren

  • Former Member
    0 Former Member

    Hi Darren,

    Good question.  We get these all the time.

    So, if you go beyond the V+ + 500mV limit, you are technically out of spec.  The note on the current is really to help prevent damage.  So, the question is always "how much is too much?"

    Typically, the listed maximum is 10mA.  However, we'll often recommend staying below 1mA.  100uA is even better.  Here's a good thread from just earlier this week on the topic.  I think your customer's current is fairly safe here.

    Noise is a more complex topic.  You can run a full noise simulation to account for current noise, voltage noise, and thermal noise from the resistors.  You can also run a hand estimate.  We have a sample calculation for a simpler circuit here.

    To get a quick idea of whether or not a set of resistors are acceptable from a noise perspective, I like to compare the thermal noise spectral density of the resistor value to the broadband noise spectral density of the part.  This gives me a rough idea of whether or not the thermal noise from the resistors is significant enough affect the overall noise of the system.  That's assuming the current noise is not the dominant noise source, which is typically the case with CMOS amplifiers.  I'd rather have the noise intrinsic to the part be the dominant noise source.  Keep in mind that parallel resistors will drastically the equivalent resistance and corresponding noise.

    If you go to page 63 of this pdf, you can get a plot of noise spectral density vs resistance.  Let me know if you need any further help here.

    Regards,

    Daniel

  • 0
    Guru 140200 points

    Hi Darren,

    is noise an issue in your application at all? The use of an additional external diode clamp can help to reduce the current limiting resistor and by this the noise. Is the input bias current critical in your application? What input bias current can you allow?

    Kai

  • Former Member
    0 Former Member

    Hello Darren,

    I hope your question has been sufficiently answered.  If you need further assistance, please let me know.  If I don't hear back, then I'll assume the issue has been resolved.

    Thanks,

    Daniel