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OPA211 start-up issue

Collin Wells
Guru 62055 points
Other Parts Discussed in Thread: OPA211

From field engineer David Chan:

Hi all,

The customer has run into a settling problem with the OPA221 for a microwave detector buffer application. ~30% of the parts that they have tested take 2-3sec to settle to the final voltage in a 10x non-inverting configuration under a 1MHz BW. They are looking to replace this device with a different part to make the design work. Attached is a partial schematic.

 Is there any particular reason why they would see only some of these devices encountering this problem?

5554.5500766-03r2 OPA211 settling problem.pdf

  • 0
    TI__Guru 60570 points

    David,

    It is hardly ever a good idea to place large resistors (2 x 600ohms) in series with the supply voltage; not only do they modulate the supply voltage directly on the supply pins due to changing load current (feeding into signal path due to PSRR limitation) but even more importently they may cause start-up issue, which I believe is behind long settling time problem you see.

    Under power-up conditions, OPA211 draws much higher quiescent current, IQ, from its supplies than typical 3.6mA specified in PDS - before the total supply voltage (directly on supply pins) reaches the minimum 4.5V specified in the datasheet, the internal transistors pseudo-randomely turning on and off (due to internal offsets), while operating under non-linear conditions, resulting in the shoot-thru current that is much higher than quiescent current.  If the start-up current is just 3x higher, 10.8mA, (might be even higher) than a typical IQ of 3.6mA, the supply voltage directly on the supply pins, Vss, may have difficulties reaching 4.5V minimum required, which would result in a very long start-up time you see: Vss = [12V-(-5)-(10.8mA*1.2k)] = 4.04V

    Please remove, or decrease by 10x, two 600 ohms resistors currently in series with OPA211 supplies - this should solve your issue; simultaneously, you may want to increase the bypass caps by 10x (to 1uF) in order to maintain the supply pins filter's corner frequency.

  • 0 in reply to Marek Lis
    TI__Mastermind 49906 points

    Hi Marek,

    FB1 and FB2 are Murata SMT ferrite beads...600 ohm impedance at 100MHz....smells like about 800nH.

    DC-wise, they are 0.25 Ohms...should not hurt anything..

    David,

    What is the "detector"? A diode? What is the expected input signal levels? Does the problem follow the op-amp? Are they sure it is not the detector? Have they 'Scoped the output to make sure it is not oscillating? Is the amp getting warm?

    Is the OPA211 in a place where it can get hit with microwaves/RF or other harsh EMI? Is the output railed before the final output? Have they monitored the supplies to make sure they are not drooping for some reason? Is the detector "biased", and is that supply drooping?

    Where are they measuring the settling issue? At the output of U2 (first stage) or U1 (second stage)? U1 stage looks to have some transistors in the feedback loop...they can also add some settling time if they have some current flowing through them or if they are not properly matched.

    Please describe the "settling"? Is it exponential-ish or linear-ish? Can they get a scope photo or plot?

    Do other op-amps show the same behavior? Has this been in production for a while? Or is this a new design?

    Regards,