AD7720
–13–REV. 0
Offset and Gain Calibration
The analog inputs of the AD7720 can be configured to measure
offset and gain errors. Pins MZERO and GC are used to config-
ure the part. Before calibrating the device, the part should be
reset so that the modulator is in a known state at calibration.
When MZERO is taken high, the analog inputs are tied to
AGND in unipolar mode and V
REF
in bipolar mode. After
taking MZERO high, 1000 MCLK cycles should be allowed for
the circuitry to settle before the bit stream is read from the
device. The ideal ones density is 50% when bipolar operation is
selected and 37.5% when unipolar mode is selected.
When GC is taken high, VIN(–) is tied to ground while VIN(+)
is tied to V
REF
. Again, 1000 MCLK cycles should be allowed for
the circuitry to settle before the bit stream is read. The ideal
ones density is 62.5%.
The calibration results apply only for the particular analog input
mode (unipolar/bipolar) selected when performing the calibra-
tion cycle. On changing to a different analog input mode, a new
calibration must be performed.
Before calibrating, ensure that the supplies have settled and that
the voltage on the analog input pins is between the supply
voltages.
Standby
The part can be put into a low power standby mode by taking
STBY high. During standby, the clock to the modulator is
turned off and bias is removed from all analog circuits.
Reset
The RESET pin is used to reset the modulator to a known state.
When RESET is taken high, the integrator capacitors of the
modulator are shorted and DVAL goes low and remains low
until 20 MCLK cycles after RESET is deasserted. However, an
additional 1000 MCLK cycles should be allowed before reading
the modulator bit stream as the modulator circuitry needs to
settle after the reset.
DVAL
The DVAL pin is used to indicate that an overrange input signal
has resulted in invalid data at the modulator output. As with all
single bit DAC high order sigma-delta modulators, large overloads
on the inputs can cause the modulator to go unstable. The
modulator is designed to be stable with signals within the input
bandwidth that exceed full scale by 20%. When instability is
detected by internal circuits, the modulator is reset to a stable
state and DVAL is held low for 20 clock cycles.
Grounding and Layout
Since the analog inputs are differential, most of the voltages in
the analog modulator are common-mode voltages. The excellent
common-mode rejection of the part will remove common-mode
noise on these inputs. The analog and digital supplies to the
AD7720 are independent and separately pinned out to minimize
coupling between analog and digital sections of the device.
The printed circuit board that houses the AD7720 should be
designed so that the analog and digital sections are separated
and confined to certain areas of the board. This facilitates the
use of ground planes which can easily be separated. A minimum
etch technique is generally best for ground planes as it gives the
best shielding. Digital and analog ground planes should only
be joined in one place. If the AD7720 is the only device requir-
ing an AGND-to-DGND connection, the ground planes should
be connected at the AGND and DGND pins of the AD7720.
If the AD7720 is in a system where multiple devices require
AGND-to-DGND connections, the connection should still be
made at one point only, a star ground point that should be
established as close as possible to the AD7720.
Avoid running digital lines under the device as these will couple
noise onto the die. The analog ground plane should be allowed
to run under the AD7720 to avoid noise coupling. The power
supply lines to the AD7720 should use as large a trace as pos-
sible to provide low impedance paths and reduce the effects of
glitches on the power supply line. Fast switching signals like
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
feedthrough through the board. A microstrip technique is by far
the best, but is not always possible with a double-sided board.
In this technique, the component side of the board is dedi-
cated to ground planes while signals are placed on the other
side.
Good decoupling is important when using high resolution ADCs.
All analog and digital supplies should be decoupled to AGND
and DGND respectively, with 100 nF ceramic capacitors in
parallel with 10 µF tantalum capacitors. To achieve the best
from these decoupling capacitors, they should be placed as close
as possible to the device, ideally right up against the device. In
systems where a common supply voltage is used to drive both
the AVDD and DVDD of the AD7720, it is recommended that
the system’s AVDD supply is used. This supply should have the
recommended analog supply decoupling between the AVDD
pin of the AD7720 and AGND and the recommended digital
supply decoupling capacitor between the DVDD pins and DGND.
