• State Resolved
  • Locked Locked
  • Replies 2 replies
  • Subscribers 46 subscribers
  • Views 216 views
  • Users 0 members are here
Support feedback
Options
Options
Similar topics

This thread has been locked.

If you have a related question, please click the "Ask a related question" button in the top right corner. The newly created question will be automatically linked to this question.

Original question:

INA381: Input Impedance CMPREF pin

INA381: Comparator Input Bias Currents, Startup Behavior

Nathan Langford
Prodigy 50 points
Part Number: INA381
Other Parts Discussed in Thread: INA302, INA303

Hello,

We are considering implementing the INA381 in a design and have two questions.

1) We are planning to use a large RC filter (1M + 2.2uF) between Vout and CMPIN to filter out load transients in our application. However we're concerned about the undocumented comparator input bias currents creating error over this resistor. Could you provide the comparator input bias currents of this device?

In another thread it is reported that the comparator input impedance is "hundreds of MΩ", which suggests the 1M resistor should impose minimal (1% max) error. However, we're not sure if this impedance figure takes includes the bias currents.

We're aware that the internal 4uA current source to provide hysteresis and the 1M resistor will cause the ALERT signal to latch low once asserted until the power is cycled. This is acceptable in our application.

2) In simulation when ramping the supply voltage rail that powers Vs+, CMPREF, and the ALERT signal while there is already an input signal to the amplifier, we are seeing OUT glitch high and thus ALERT glitch low during startup. Is this accurate? Figure 31 in the datasheet does not show this behavior.

Thank you.

  • +1
    TI__Mastermind 30880 points

    Hi,

    Thank you for choosing INA381:

    • The 1M resistor could be too big, and comparator most likely will have trouble functioning properly (depending on settings) due to the extra-large hysteresis introduced. The comparator input is high impedance. While this is true for the CMPREF pin, the hysteresis circuit on CMPIN pin has implications when an external resistor is added. If you need configurable delay, INA302 or INA303 make it convenient for such applications.
    • The simulation is ramping very slowly, this is probably why the behavior is different. If you speed up the ramping from seconds to uS, the behavior should be correct.

    Regards, Guang  

  • 0 in reply to Guang Zhou
    Prodigy 50 points

    Thank you. While it seemed like the hysteresis circuit might not be a problem in our application since the comparator asserting will cause the entire system to shutdown anyways, we'll switch to a discrete comparator just to be on the safe side.