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OPA192: output glitch as Vs change

Charles Zhang1
Expert 3080 points
Part Number: OPA192

Hi Team,

In my customer's case, OPA192 is configured as high side current sensing circuit, and the Vs+ is coming from the upstream Flyback converter aux winding. The aux winding is unregulated, so Vs+ will shift between 18V and 21V based on the different loading on aux winding, and my customer observed that OPA192's output voltage will also shift when Vs+ is shifting.

Is this behavior related to P/N transition region? Or it it more related to PSRR?

According to OPA192 datasheet, P/N transition happens betwwen (V+) – 3 V and  (V+) – 1.5 V, will this rule apply to all supply voltage range?   

Regards,

Charles

  • 0
    TI__Mastermind 21337 points

    Charles,

    1. The P/N transition range of (V+) – 3 V and  (V+) – 1.5 V, applies to the entire specified supply voltage range.
    2. One key point to consider is that the P/N transition happens when the common mode voltage moves to a voltage that is from 1.5V to 3V away from the positive supply.  I do not see the details in your post regarding the common mode signal.  However, high-side sense is normally done with a differential-amplifier as shown in this document:  https://www.ti.com/lit/ab/sboa347a/sboa347a.pdf .  Note that the common mode voltage of an op amp is the voltage at the non-inverting input of the op amp.  The differential-amplifier is normally in high gain and the common-mode voltage on the amplifier is near the voltage that the shunt resistor is at.  So, if the op amp supply is the same supply voltage that is being monitored, than you are likely in the N-region (near V+).
    3. For the OPA192 the PSRR causes an offset shift of 1uV/V.   When the common mode transitions through the crossover region, the offset can move by 70uV in the transition region (see figure 13 and 14).  This shift would be multiplied by the amplifiers gain.  If the customer is seeing a shift that is much greater than this, I think there is another issue. Common mode and output swing limitations are covered in this document:  https://www.ti.com/lit/wp/sboa583/sboa583.pdf 
    4. To provide a better answer I need to see the schematic of the current sensing circuit.  Please include voltage and current levels through the shunt and supply voltage of the amplifier.  Also, how much does the amplifier output shift by?  Finally, I would suggest that the customer look at the amplifier output with an oscilloscope and look for oscillations or other abnormal AC output signals.  I understand that the circuit is likely measuring DC or very low frequency signals, but amplifiers sometimes have AC oscillations due to capacitive loading and other issues.  This kind of issue is often overlooked because the circuit if for DC operation and AC problems are not expected.

    Best regards,  Art