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MPY634: Divider application - how to disable multiplying by 10V

Wojciech Endzelm
Prodigy 20 points
Part Number: MPY634

Hello,

At the beginning I would like to highlight that I am at my beggining of the journey with electronic circuits, so my apologies for very basic and maybe not clear explanations.

I would like to use the MPY634 as a analog divider in my circuit. I have two signals (let's call them X and Y) for which I would like to do the following calculation:

Vout=(X-Y)/(X+Y)

Amplitude of X and Y may vary from 0V to 2V.

I have used MPY634 application as shown on below screenshot:

In this configuratiotion on the output of this device I should receive: Vout = 10V(Z2- Z1 )/(X1-X2 ) + Y1.
Y1 is grounded. To get the sum in the denominator I am using inverting amplifier.

The problem that I face is, when I apply various signals on the inputs of MPY634, device is reaching saturation point every time.
For example if (Z2-Z1) = 1 and (X1-X2)=2V on output I measure Vout=13,35
For counter=0 Vout=0
However, when denominator is at 10V and counter at 2V Vout=1,81V, but from application point of view I cannot use still 10V in denominator.

My question is, is there a way to disable multiplying the counter by 10V to avoid reaching saturation point?

Thank you in advance for any help.

Kind Regards,

Wojtek

  • 0
    TI__Mastermind 31685 points
    Hello Wojciech,

    I see you are using the MPY634 in the divider configuration and that there are some immediate considerations to make.

    1. You are using a power supply of +-5V, but according to the datasheet the minimum operating power is +-8V. Your +-5V power supply is hurting the design in two areas. You are approaching the 10V output swing limit much more quickly and you are not operating the part according to the datasheet. Consider raising the power supply to +-8V. Note that all of the specs for the MPY634 were specified with the MPY634 at +-15V, so you will get optimum performance with this power supply.

    2. The 10V multiplier is the scale-factor (SF). You cannot disable it, but it can be reduced to 3V by connecting an external resistor (Rsf) between -Vs and the SF pin of the part. This can be seen in the datasheet and the equation to determine Rsf is: Rsf = 5.4kOhms*(SF/(10-SF)).

    Peter Iliya
    Precision Amplifiers Product Line
  • 0 in reply to Peter Iliya
    Prodigy 20 points
    Hello Peter,

    Thank you very much for such a quick and helpful answer.

    In my final project I have used power supply of +-15V. By mistake I have made a screen shot of old schematics, my apologies.
    Measurements that I have mentioned in previous post were made with +-15V power supply.

    Can you please write something more about this SF pin, how does is operate and what is for?
    After reading datasheet several times it is still not clear for me.

    Thank you in advance.

    Kind Regards,
    Wojciech
  • 0 in reply to Wojciech Endzelm
    TI__Mastermind 31685 points
    Hello Wojciech,

    I could not replicate your problem with a MPY634 DIP and breadboard. Using your schematic, I connected the X and Z sources to DC power supplies and could see the part successfully divide the signals even for low signals such as (Z2-Z1) = 1V, and (X1-X2) = 1V.

    Here's a couple things to consider.
    1. Make sure that the signal sources for Z and X and your power supplies for +Vs and -Vs are all connected to the same common node. This could be pull down resistors from your Z and X inputs to the common ground or connecting the commons of the X,Z sources to your power supply common. I did the latter, but overall there needs to be a DC return path for bias current.

    2. I had the circuit working with both 1uF X7R and 47uF electrolytic capacitors on the +Vs and -Vs supplies to GND. When I didn't have these capacitors, I did see some noise and ringing from the power supply, so if your supply is noiser than mine for whatever reason, you could switch in 47uF electrolytic capacitors here and see if this helps.

    It took some digging into decades-old documentation to know, but the SF pin is connected to the transistor bases of the X, Y, and Z current sources inside the IC. When a resistor connects the SF pin and the Vs- pin, this pulls all of these bases to a different voltage. This will change the current level in all three X, Y, and Z current sources, which changes the transconductance and effectively the gain. Thus this is how the overall transfer function gain is scaled when pulling the SF pin to a different voltage.

    Peter Iliya
    Precision Amplifiers Tucson