For the AD9222 is the input span of the ADCs (V_p-V_n) -1->1V or (0 ->2V)?

I've got a unipolar signal 0-1V for which I will using a AD8132 as a differential driver (differential gain x2) .  Do I have to offset the input voltage by 0.5V to utilize the full scale of the ADC (AD9222)?

  • Thank you for a complete answer.  I'll add a DC offset to my unipolar signal to match the swing from 0.4-1.4V.

  • 0
    •  Analog Employees 
    on Aug 12, 2016 10:44 PM

    Hi Metelko,

    Thank you for using the AD8132 and AD9222 in your design.

    Regarding the need to offset your input signal, because you are using an amplifier (AD8132), your unipolar signal needs to satisfy the input requirements of the amplifier. The AD8132 can handle a 0V - 1V signal if it is configured properly.

    If you are DC coupling the amplifier to the ADC, the amplifier must set its output common mode voltage to match the input requirement of the AD9222.

    The AD8132 has a VOCM pin by which you set the output common mode voltage of the amplifier. The input common mode requirement of the AD9222 is VCM = AVDD/2 = 0.9V (nominally). So, 0.9V needs to be applied to the VOCM pin of the AD8132 to set the output common mode voltage of the amplifier. Please keep in mind that the AD8132 could require a negative power supply for its output common mode to be set to 0.9V.

    For the AD9222, the full scale input span is -1V to +1V differential, which is 2Vpp differential. As previously stated, the input common mode voltage requirement for the AD9222 is VCM = AVDD/2, which is nominally 0.9V. This means that the inverting and non-inverting components of the signal will each swing 1Vpp above and below VCM. So, full scale signals at the ADC inputs each will swing from (0.9V - 0.5V) to (0.9V + 0.5V) with respect to ground, which is 0.4V to 1.4V with respect to ground. Each single-ended component swings 1Vpp, which results in 2Vpp differential.

    Please let me know if I misunderstood your question, or if you need more information regarding this.

    Thanks again.

    Doug

  • 0
    •  Analog Employees 
    on Aug 12, 2016 11:08 PM

    Hi Metelko,

    Will your unipolar signal drive the amplifier or the ADC? Please note that if it is driving the amplifier, changing the DC offset at the input to the amplifier will not affect the output common mode of the amplifier in the same way.

    Thanks,

    Doug