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AD9706 OTCM

We are using the above part to drive a zero-IF IQ modulator that requires a DC bias of 1.15V. We have set the full scall current to 5mA (Radj = 6.4K). We have set the OTCM to 1V (from REFIO). We have a load resistance of 118 ohms. We are unable to achieve the required DC bias when we set the DAC code to mid-range. A full range DAC code sweep gives a 0.6V swing on each output arm.

  • Hi -

    The voltage on the OTCM pin determines the compliance voltage range of the AD9706. It does not change the half scale voltage at IOUTA and IOUTB. At OTCM = ground, the compliance range is up to +/- 1.0V at IOUTFS = 5ma. At OTCM = 0.5V you can go up to 1.2V +/- 1.0V compliance range at IOUTFS = 5ma. Your circuit needs to provide the bias voltage.

    The data sheet recommends that for bias levels above 0.9V, which you have, OTCM be set to 0.5V. 1.0 volts on OTCM is too large. Also, the OTCM pin demands dynamically changing current and should be driven by a low source impedance to prevent a common-mode signal from appearing on the DAC outputs.

    Thanks,

    Larry