Can AD8475 configured to a gain other than 0.4 & 0.8 using external resistors

Hi,

I am working on multi-channel DAS. I am using AD7982 in the signal line. The dynamic range of this ADC is 5V p-p differential with a reference of +5V. The receiving signal dynamic range is 20V p-p. In a application circuit given in the datasheet of AD8475, it is used to drive AD7982. But AD8475 with a gain of 0.4 the signal dynamic range is attenuated to 4V p-p. So, I am loosing a dynamic range of 1V. So, I am thinking of using external resistors to configure AD8475 to give a gain of 0.25. I can choose the values of resistors easily. But how to select the proper resistor which has similar drift coefficients of these internal resistors used.

Please suggest on this.

Thanks,

Jebas. 

  • 0
    •  Analog Employees 
    on Jan 13, 2014 10:25 PM

    Hi Jebas,

    Thank you for question. Theoretically, the idea of using external resistors to adjust the gain of AD8475 should work. However, the internal resistors are laser trimmed to have the specified ratios, but the absolute values of these resistors may vary +/-20%. Therefore, if you add an external resistor calculated using the nominal values of the internal resistors may cause very huge errors. Also, the drift caused by the external resistor will really be a source of another error on your design, since the AD8475 is designed to operate without adding external resistors.

    In addition, operating the AD8475 to have an output close to its rails (Vsupply=+/-5V, Vin=+/-10V, G=0.5x) has a very bad effect on its distortion performance. Please see Figure 41 of the AD8475 datasheet.

    A suggestion for you is to use an amplifier that is designed for use with external resistors. Probably ADA4940 may be a good fit.

    Best regards,

    Neil