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PGA103: Can I use +/- 3.3 V or +/-2.5V for offet trimming?

Qi Liu
Expert 8875 points

Part Number: PGA103

Hi, support team

My customer has the questions as follow:

I am using PGA103U for one of my prototype develpment, but the application notes suggest +/-15V for offset trimming

Can I use +/- 3.3 V or +/-2.5V for offet trimming?

Thanks so much.

Best regards,
Yuki

  • 0
    TI__Guru 55466 points

    Hi Yuki,

    I am assuming you are referring to the supplies on the 50kΩ potentiometer of Figure 3 "Offset Voltage Trim Circuit" below.  Let me know if you are referring to a different circuit.

    The offset adjustment is in generally used to null or cancel  small offset shifts in the overall analog chain circuit/system.  This circuit is typically for offset adjustments less than <100mV.  The max range for VTRIM recommended is ±100mV, because the logic threshold A1,A2 voltage is altered by VTRIM.  If a large offset is used (greater than 0.1V), be sure that the logic input signals provide valid logic levels when referred to the voltage at the ground terminal, pin 3.

    For example, it is possible to easily re-scale the resistors of the voltage divider circuit while using a ±2.5V supplies to power the potentiometer.  Below is one possible example with a 50kΩ pot, using ±2.5V supplies and a voltage divider scaled for a ±5mV VTRIM output. 

    However, If I did not understood your question, kindly clarify which circuit and/or application note you are referring to, and provide a diagram explaining what the requirement is.

    Thank you and Regards,

    Luis

  • 0 in reply to Luis Chioye
    Prodigy 21 points

    The question was regarding replacement. I am trying to change potmeter to digital pot and the available terminal svoltage ratings is of +/-2.5V or 5V. I thought the same while repacing but wanted to be verified by your team. The digital pot i am choosing of +/-2.5V will require power sequncing firstly at Supply terminals of Digital pot ex AD5122A and then at resistor terminals A,B ,W. So, i have the requirements of reduced voltage ratings at Pot meter end.

  • 0 in reply to SHREE PRAKASH
    Prodigy 21 points

    the gains are supposed to be affected by Vtrim as per datasheet. Or putting +/-5mV will not disturb gain stages? How feasible would be to add digital potmeter with increased Power sequencing circuitry and +/-2.5V than the old Potmeter designs?

  • 0 in reply to SHREE PRAKASH
    TI__Genius 15465 points

    Hi Shree,

    Yes as you mentioned, the impedance must be low to maintain gain accuracy.

    This means using a digital potentiometer directly to the trim node, without an amplifier, will not work.  If you add an amplifier between the potentiometer and the trim node, though, it should work similar to the circuit above; you just need to create a variable +/- 5 mV with low impedance.  The potentiometer works to create a +/-5 mV signal, then it is buffered by the amplifier.  It doesn't matter if the +/- 5 mV signal is from a physical potentiometer or a digital pot.

    You would not be able to create a negative signal using only 0-5 V power with the potentiometer, so, you may not be able to use the device across the full trim range.  Note the PGA logic levels are also impacted by the "VTRIM" voltage.  I would be sure your supplies have stabilized before attempting to communicate with the devices.

    Let me know if this answers your question.

    Best Regards,
    Mike