2
AD7711A
–17–REV. D
USING THE AD7711A
SYSTEM DESIGN CONSIDERATIONS
The AD7711A operates differently from successive approxima-
tion ADCs or integrating ADCs. Since it samples the signal
continuously, like a tracking ADC, there is no need for a start
convert command. The output register is updated at a rate
determined by the first notch of the filter, and the output can be
read at any time, either synchronously or asynchronously.
Clocking
The AD7711A requires a master clock input, which may be an
external TTL/CMOS compatible clock signal applied to the
MCLK IN pin with the MCLK OUT pin left unconnected.
Alternatively, a crystal of the correct frequency can be con-
nected between MCLK IN and MCLK OUT, in which case the
clock circuit will function as a crystal controlled oscillator. For
lower clock frequencies, a ceramic resonator may be used instead
of the crystal. For these lower frequency oscillators, external
capacitors may be required on either the ceramic resonator or
on the crystal.
The input sampling frequency, the modulator sampling fre-
quency, the –3 dB frequency, the output update rate, and the
calibration time are all directly related to the master clock fre-
quency, f
CLK IN.
Reducing the master clock frequency by a factor
of 2 will halve the above frequencies and update rate and will
double the calibration time.
The current drawn from the DV
DD
power supply is also directly
related to f
CLK IN
. Reducing f
CLK IN
by a factor of 2 will halve the
DV
DD
current but will not affect the current drawn from the
AV
DD
power supply.
System Synchronization
If multiple AD7711As are operated from a common master
clock, they can be synchronized to update their output registers
simultaneously. A falling edge on the SYNC input resets the
filter and places the AD7711A into a consistent, known state. A
common signal to the AD7711As’ SYNC inputs will synchro-
nize their operation. This would normally be done after each
AD7711A has performed its own calibration or has had calibra-
tion coefficients loaded to it.
The SYNC input can also be used to reset the digital filter in
systems where the turn-on time of the digital power supply
(DV
DD
) is very long. In such cases, the AD7711A will start
operating internally before the DV
DD
line has reached its mini-
mum operating level, 4.75 V. With a low DV
DD
voltage, the
AD7711A’s internal digital filter logic does not operate cor-
rectly. Thus, the AD7711A may have clocked itself into an
incorrect operating condition by the time that DV
DD
has
reached its correct level. The digital filter will be reset upon
issue of a calibration command (whether it is self-calibration,
system calibration, or background calibration) to the AD7711A.
This ensures correct operation of the AD7711A. In systems
where the power-on default conditions of the AD7711A are
acceptable, and no calibration is performed after power-on,
issuing a SYNC pulse to the AD7711A will reset the AD7711A’s
digital filter logic. An R, C on the SYNC line, with R, C time
constant longer than the DV
DD
power-on time, will perform the
SYNC function.
Accuracy
Sigma-delta ADCs, like VFCs and other integrating ADCs, do
not contain any source of nonmonotonicity and inherently offer
no missing codes performance. The AD7711A achieves excel-
lent linearity by the use of high quality, on-chip silicon dioxide
capacitors, which have a very low capacitance/voltage coeffi-
cient. The device also achieves low input drift through the use
of chopper stabilized techniques in its input stage. To ensure
excellent performance over time and temperature, the AD7711A
uses digital calibration techniques that minimize offset and
gain error.
Autocalibration
Autocalibration on the AD7711A removes offset and gain errors
from the device. A calibration routine should be initiated on the
device whenever there is a change in the ambient operating
temperature or supply voltage. It should also be initiated if there
is a change in the selected gain, filter notch, or bipolar/unipolar
input range. However, if the AD7711A is in its background
calibration mode, the above changes are all automatically taken
care of (after allowing for the settling time of the filter).
The AD7711A offers self-calibration, system calibration, and
background calibration facilities. For calibration to occur on the
selected channel, the on-chip microcontroller must record the
modulator output for two different input conditions. These are
zero-scale and full-scale points. With these readings, the
microcontroller can calculate the gain slope for the input to
output transfer function of the converter. Internally, the part
works with a resolution of 33 bits to determine its conversion
result of either 16 bits or 24 bits.
The AD7711A also provides the facility to write to the on-chip
calibration registers, and, in this manner, the span and offset for
the part can be adjusted by the user. The offset calibration regis-
ter contains a value that is subtracted from all conversion results,
while the full-scale calibration register contains a value that is
multiplied by all conversion results. The offset calibration coeffi-
cient is subtracted from the result prior to the multiplication by
the full-scale coefficient. In the first three modes outlined here,
the DRDY line indicates that calibration is complete by going
low. If DRDY is low before (or goes low during) the calibration
command, it may take up to one modulator cycle before DRDY
goes high to indicate that calibration is in progress. Therefore,
DRDY should be ignored for up to one modulator cycle after
the last bit of the calibration command is written to the control
register.
