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Original question:

DC offset in output of MPY634 used as multiplier

MPY634: MPY634 output consistent with data sheet

Anuraag Mohan18
Prodigy 40 points
Part Number: MPY634

Hello,

I am using the MPY634 as an analog multiplier in the following configuration. I am having a hard time understanding the SF. In this case, the input voltage for both X1 and Y1 is 1.2V. For this input, I get an output of 0.35V. 

Can you help me understand how this voltage should be the output of square of 1.2V.

  • 0
    TI__Guru* 93105 points

    Hello Anuraag,

    The basic transfer equation for the MPY634 connected as a multiplier is:

    Vout = [ ((X1 - X2) (Y1 -Y2)) / SF ] + Z2

    For your circuit connection:

    X1 = Y1 = 1.2 V,    X2 = Y2 = 0 V,   Z2 = 0 V,   SF = 10 V

    Vout  = [ ((1.2 V - 0 V) (1.2 V)) / 10 V ] + 0 V

    Vout =  [ (1.2 V) (1.2 V) ] /10 V = 0.144 V

    Therefore, if your MPY634 circuit is functioning correctly Vout should be very close to 0.144 V, not 0.35 V.

    The scale factor SF is simply the divisor which is trimmed to 10 V. It can be set externally to another voltage, in which case Vout will scale with the SF value.

    Some things to check:

    • Make sure the MPY634 power supply pins have 100 nF power supply decoupling capacitors to ground.
    • The MPY634 is mounted on a good, clean PC board.
    • Observe the MPY634 Vout with a wideband DSO and 10x probe. Make sure there isn't any oscillation present.
    • Be sure the MPY634 was obtained from an authorized TI distributor.

    Regards, Thomas

    Precision Amplifiers Applications Engineering

  • 0 in reply to Thomas Kuehl
    TI__Guru* 93105 points
    Hello Anuraag,

    Did you get your MPY634 issue resolved? If so, please close this E2E inquiry.

    Regards, Thomas
    Precision Amplifiers Applications Engineering
  • 0 in reply to Thomas Kuehl
    TI__Genius 11135 points
    Anuraag

    We haven't heard back from you so we assume this answered your questions. If you need additional help just post another reply below.

    Thanks
    Dennis