REV. D–16–
AD7713
look like the AIN1 analog input (see Figure 7). In this case, R
INT
is 5 kΩ typ and C
INT
varies with gain. The input sample rate is
f
CLK IN
/256 and does not vary with gain. For gains of 1 to 8,
C
INT
is 20 pF; for a gain of 16, it is 10 pF; for a gain of 32, it is
5 pF; for a gain of 64, it is 2.5 pF; and for a gain of 128, it is
1.25 pF.
The digital filter of the AD7713 removes noise from the reference
input just as it does with the analog input, and the same limita-
tions apply regarding lack of noise rejection at integer multiples of
the sampling frequency. The output noise performance outlined
in Tables I and II assumes a clean reference. If the reference
noise in the bandwidth of interest is excessive, it can degrade the
performance of the AD7713. A recommended reference source
for the AD7713 is the AD680, a 2.5 V reference.
USING THE AD7713 SYSTEM
DESIGN CONSIDERATIONS
The AD7713 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 AD7713 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 connected
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 frequency,
the –3 dB frequency, output update rate, and calibration time
are all directly related to the master clock frequency, f
CLK IN
.
Reducing the master clock frequency by a factor of two will halve
the above frequencies and update rate and will double the cali-
bration 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 two will halve
the DV
DD
current but will not affect the current drawn from the
AV
DD
power supply.
System Synchronization
If multiple AD7713s are operated from a common master clock,
they can be synchronized to update their output registers simul-
taneously. A falling edge on the SYNC input resets the filter
and places the AD7713 into a consistent, known state. A com-
mon signal to the AD7713’s SYNC inputs will synchronize their
operation. This would normally be done after each AD7713 has
performed its own calibration or has had calibration 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 AD7713 will start operat-
ing internally before the DV
DD
line has reached its minimum
operating level, 4.75 V. With a low DV
DD
voltage, the AD7713’s
internal digital filter logic does not operate correctly. Thus, the
AD7713 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 AD7713. This ensures correct
operation of the AD7713. In systems where the power-on default
conditions of the AD7713 are acceptable, and no calibration is
performed after power-on, issuing a SYNC pulse to the AD7713
will reset the AD7713’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
⌺-⌬ ADCs, like VFCs and other integrating ADCs, do not contain
any source of nonmonotonicity, and inherently offer no missing
codes performance. The AD7713 achieves excellent linearity by
the use of high quality, on-chip silicon dioxide capacitors, which
have a very low capacitance/voltage coefficient. The device also
achieves low input drift through the use of chopper stabilized tech-
niques in its input stage. To ensure excellent performance over
time and temperature, the AD7713 uses digital calibration tech-
niques that minimize offset and gain error.
Autocalibration
Autocalibration on the AD7713 removes offset and gain
errors from the device. A calibration routine should be initi-
ated 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 AD7713 is in
its background calibration mode, the above changes are all
automatically taken care of (after the settling time of the filter
has been allowed for).
The AD7713 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 AD7713 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 register
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 coefficient is sub-
tracted 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, the DRDY line should be
ignored for up to one modulator cycle after the last bit of the cali-
bration command is written to the control register.
Self-Calibration
In the self-calibration mode with a unipolar input range, the
zero-scale point used in determining the calibration coefficients
is with both inputs shorted and the full-scale point is V
REF
. The
zero-scale coefficient is determined by converting an internal
shorted inputs node. The full-scale coefficient is determined
from the span between this shorted inputs conversion and a
conversion on an internal V
REF
node. The self-calibration mode
