Q
We have a DVB-T upconverter Design in progress, in which we use the AD9773.This DAC needs a digital and an analog supply and therefor a digital and a
analog ground. In the layout of the evalboard which is mentioned in the
datasheet, there is no connection between digital and analog ground. My
question: In the most applications with DACs in know,there is a connection
directly unter the device. Is this not necessary by applications with the
AD9773?
A
We do recommend that the digital ground on ADC/DACs be connected to analogground. The reason for treating ADCs and DACs as analogue components is that
the digital circuitry on many ADCs and DACs is very quiet. Indeed it has to be
because there are various ways for noise to couple from the digital to the
analogue circuitry inside the package (capacitance between bond wires and
coupling through the silicon itself). The separate AGND and DGND pins refer to
the analogue and digital circuitry inside the IC, not to the analogue and
digital grounds on the printed circuit board. The IC designer often wishes to
minimise the effect of bond wire inductance and resistance by bonding the IC
analogue ground to a separate pin than the digital ground. If the two grounds
are connected together to a good quality, low impedance, quiet analogue ground
plane on the PCB, then this will cause much less coupling from digital to
analogue. A "typical" digital ground plane on a circuit board will be far more
noisy than the "typical" digital ground on an ADC and connecting the ADCs DGND
pin to the noisy digital ground plane will usually degrade the ADCs performance.
Since the digital circuitry on some of our evaluation boards is minimal, the
DGND of the ADC/DAC will be connected to the digital ground plane rather than
the analogue ground plane. The digital ground plane on the evaluation board is
not as noisy as the digital ground plane of a typical application board
containing more noisy digital circuits.
