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MPY634 use as divider

cc57567
Prodigy 20 points
Other Parts Discussed in Thread: MPY634

My question is about the "10 to 1 denominator range" mentioned in the data sheet on page 7.  What exactly is this denominator range?  Is this with reference to the numerator?

Question springs from the fact that in my current circuit, I am getting low accuracy for Z input closer to 0 keeping X inputs constant.

  • 0
    TI__Guru* 93105 points

    Hi,

    The MPY634 transfer function in the divider mode is:

    Vout = ((10(Z2 - Z1) / (X1 - X2)) + Y1

    The Vout accuracy statement is in relation to the difference in the two denominator numbers. It states that the typical accuracy has a range of 1.0 to 2.5%, for a 10 to 1 denominator range. For example, if X1= +5V, and X2= -5V, the denominator has a value of 10. Then, if  X1= +0.5V and X2= -0.5V, the denominator has a value of 1. So for any denominator difference value that is between 1 and 10, the Vout accuracy will typically be between 1.0 and 2.5%.

    Regards, Thomas

    PA - Linear Applications Engineering

  • 0 in reply to Thomas Kuehl
    Prodigy 20 points

    If I'm following your answer correctly, this means that applying a trimming voltage of +/-3.5mV to the low X input allows for a 100V denominator value (further down the data sheet there is a reference to 100 to 1 denominator range) - is this correct?

     

  • 0 in reply to cc57567
    TI__Guru* 93105 points

    Hi,

    Well, my lack of familiarity with the MPY634 operating in divider mode may have led me to some incorrect conclusions. So after giving it more thought here is what I have concluded:

    Data-sheet Figure 4 shows the X-denominator inputs have a voltage range of 0.1V ≤ X ≤ 10V. The X1 input is required to be more positive than X2 for correct feedback polarity. If we consider the input voltage in terms of a range, the 10:1 range could be 10V:1V, 5V:0.5V, 1V:0.1V, etc, and it is across a 10:1 range that the accuracy is typically 1 to 2.5%.

    Indeed the range can be increased to 100:1, such as 10V:0.1V, but the inaccuracy may exceed 2.5% unless the optional trim voltage (+/-3.5mV) is applied to the X2 input.

    Sorry for the intial confusion. Does that answer your question?

    Regards, Thomas

    PA - Linear Applications Engineering

     

  • 0 in reply to Thomas Kuehl
    Prodigy 140 points

    sir,

    How to apply trim voltage to X2. My X1 input is positive always but still I am getting some voltage at output in divider config even when the numerator is 0 volt or Z2-Z1 is 0volt. How to balance this to make output 0 for numerator part 0? Please illustrate with diagram

  • 0 in reply to Ronie Adhiraaj Ghosh
    TI__Guru* 93105 points

    Ronnie,

    Any trim voltage, or offset, is normally applied to the MPY634 Y1 input when it is operated as a divider. I expect that X2 could be trimmed by summing a voltage with X2 before it arrives at the X2 input. However, that added voltage would then be subject to the divider transfer equation because it enters through the X2 input. Using the Y1 input simply sums the applied offset with the divider transfer function.

    Regards, Thomas

    PA - Linear Applications Engineering