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OPA602: Offset Voltage Trim

Y.Ottey
Mastermind 6145 points
Part Number: OPA602
Other Parts Discussed in Thread: OPA145,

Dear all,

Our customer would like to ask questions about datasheet.

According to Figure.2 "Offset Voltage Trim" on datasheet p.8, The voltage range that can be trimmed is indicated as ± 100mV.

Is this value the value when a 100kΩ trim potentiometer is attached?

In that case, what is the adjustment range when other resistance values are connected?

By the way, our customers plan to use a 200kΩ potentiometer.

Best Regards,

Y.Ottey

  • 0
    Guru 47535 points

    Hello Y.Ottey

    Why would you be looking at a 1987 device introduction? 

    The offset trim included in the 1980's parts did nothing to improve offset drift and was largely abandoned moving beyond the 1980's. 

    The online pricing of this part suggests a limited remaining lifetime

    Many better parts available today, the OPA602 is not even in the TI cross reference tool -but the OPA602 web folder suggests the OPA145. 

  • 0
    TI__Guru* 93105 points

    Hello Y.Ottey,

    Michael's assessment of the OPA602 product status and availability are correct. The OPA602 is a legacy Op amp that has been available for over 30 years and at some point in the future its availability will become less assured because of higher performance, lower cost replacements.

    I have attempted to find internal details about the OPA602 design, but being a decades old device I don't have direct access to the archived documentation. However, regarding your questions about the offset control resistance Burr-Brown commonly specified a 100 kilohm potentiometer. That potentiometer as connected in the data sheet shunts a portion of the low resistance emitter degeneration resistors in the PNP dynamic load pair. The potentiometer adjusts the current through each side of the input differential pair which results in minimum offset voltage at the null point.

    The 100 kilohm potentiometer will provide the approximate +/-10 mV offset adjustment range. If you review the way the potentiometer ends connect between those two emitter degeneration resistors with the wiper to -Vs, you can see that when the potentiometer is adjusted to one extreme or the other that the emitter resistor on one side becomes completely shorted out and the other emitter resistor sees the full 100 kilohms in parallel with it.

    So if you use a potentiometer with a different resistance value such as 200 kilohms, the extreme end setting will be with one emitter resistor shorted and other having 200 kilohms shunted across it. The adjustment range will be very nearly the same as with the 100 kilohm potentiometer. The OPA602, 10 kilohm to 1 Megohm potentiometer range mostly affects the offset adjustment sensitivity. 

    Regards, Thomas

    Precision Amplifiers Applications Engineering