AD737 scale factor and linerity

Dear all,

It seems that our measurement and the contents of the data sheet are not in agreement.

If you may notice, please let me know.


  VR=200ohm is used for Fig.30 in the data sheet (page16)

  What kind of formula is the scale corresponding to this resistance?

  * in the case supply voltage +/-3.3V


  If 22-ohms resistance is inserted between AD737-PIN4 and -Vs of schematics in Fig.30 in the data sheet,

  Linerity (Fig.14 in the data sheet page.9) will improve.

  Actual measurement when following Fig.14 :

    Input Voltage 100mV ==> Output difference error +0.3%

    Input Voltage 400mV ==> Output difference error -0.4%

    * Difference with error 0.7%

  When 22-ohms is inserted on Fig.14 :

    Input Voltage 100mV ==> Output difference error +0.4%

    Input Voltage 400mV ==> Output difference error 0.0%

    * Difference with error 0.4%

  When inserting 22-ohms in schematics, is there any problem?

  And, why does linerity improve?

Best Regards,


  • Dear all,

    I correct it about Q2.

    The actual measurement in my customer is as a data sheet.

    My customer found the circuit where 22 ohms was inserted in -Vs pin with the product of the other company.

    When they also tried it, the characteristic has improved.

    They will want to insert it and to improve the characteristic, if 22 ohms does not do some mischief to others.

    Please give some comment.

    Best Regards,


  • 0
    •  Analog Employees 
    on Jan 5, 2014 7:25 PM

    Hi George,

    Apologies for the delay of response. The AD737 is a very old part and the original designer is unavailable for comment. You can also suggest your customer to use our newer and better alternative part for True  RMS to DC Converter which is the AD8436. In the meantime, I will try to replicate the setup of your customer and will give you an update as soon as I can.

    Best regards,


  • 0
    •  Analog Employees 
    on Jan 5, 2014 7:37 PM

    Hi George,

    I also want to verify the schematic of your customer. Are you referring to the Figure 29: External Scale factor Trim or Figure 30: dB Output Connection?  Also where the 22 ohms was inserted, between the -Vs(Pin 4) to Cav or Pin 4 to ground? If you could provide the actual schematic then it would be very helpful.

    Thanks and best regards,


  • Thanks Emman,

    We were waiting for the reply from you.

    Just now, I requested to show a schematics to my customer.

    Please wait a while.

    Best Regards,


  • Dear Emman,

    The schematic is attached here.

    It cuts only a required part from the schematics of their product.

    I will add an explanation later.

    Best Regards,