AD7790 Data Sheet
Rev. A | Page 18 of 20
REFIN(+) without introducing gain errors in the system. Deriv-
ing the reference input voltage across an external resistor will
mean that the reference input sees a significant external source
impedance. External decoupling on the REFIN pins would not
be recommended in this type of circuit configuration.
V
DD
MONITOR
Along with converting external voltages, the analog input chan-
nel can be used to monitor the voltage on the V
DD
pin. When
the CH1 and CH0 bits in the communications register are set to
1, the voltage on the V
DD
pin is internally attenuated by 5 and
the resultant voltage is applied to the ∑-∆ modulator using an
internal 1.17 V reference for analog to digital conversion. This
is useful because variations in the power supply voltage can be
monitored.
GROUNDING AND LAYOUT
Since the analog inputs and reference inputs of the ADC are
differential, most of the voltages in the analog modulator are
common-mode voltages. The excellent common-mode rejec-
tion of the part will remove common-mode noise on these
inputs. The digital filter will provide rejection of broadband
noise on the power supply, except at integer multiples of the
modulator sampling frequency. The digital filter also removes
noise from the analog and reference inputs, provided that these
noise sources do not saturate the analog modulator. As a result,
the AD7790 is more immune to noise interference than a con-
ventional high resolution converter. However, because the
resolution of the AD7790 is so high, and the noise levels from
the AD7790 are so low, care must be taken with regard to
grounding and layout.
The printed circuit board that houses the AD7790 should be
designed such that the analog and digital sections are separated
and confined to certain areas of the board. A minimum etch
technique is generally best for ground planes because it gives
the best shielding.
It is recommended that the AD7790’s GND pin be tied to the
AGND plane of the system. In any layout, it is important that
the user keep in mind the flow of currents in the system, ensur-
ing that the return paths for all currents are as close as possible
to the paths the currents took to reach their destinations. Avoid
forcing digital currents to flow through the AGND sections of
the layout.
The AD7790’s ground plane should be allowed to run under the
AD7790 to prevent noise coupling. The power supply lines to
the AD7790 should use as wide a trace as possible to provide
low impedance paths and reduce the effects of glitches on the
power supply line. Fast switching signals such as clocks should
be shielded with digital ground to avoid radiating noise to other
sections of the board, and clock signals should never be run
near the analog inputs. Avoid crossover of digital and analog
signals. Traces on opposite sides of the board should run at
right angles to each other. This will reduce the effects of feed-
through through the board. A microstrip technique is by far the
best, but it is not always possible with a double-sided board. In
this technique, the component side of the board is dedicated to
ground planes, while signals are placed on the solder side.
Good decoupling is important when using high resolution
ADCs. V
DD
should be decoupled with 10 µF tantalum in parallel
with 0.1 µF capacitors to GND. To achieve the best from these
decoupling components, they should be placed as close as
possible to the device, ideally right up against the device. All
logic chips should be decoupled with 0.1 µF ceramic capacitors
to DGND.
