AD7705/AD7706
Placing the part in standby mode reduces the total current to
9 µA typical with VDD = 5 V and 4 µA with VDD = 3 V when the
part is operated from an external master clock provided this
master clock is stopped. If the external clock continues to run in
standby mode, the standby current increases to 150 µA typical
with 5 V supplies and 75 µA typical with 3.3 V supplies. If a
crystal or ceramic resonator is used as the clock source, the total
current in standby mode is 400 µA typical with 5 V supplies and
90 µA with 3.3 V supplies. This is because the on-chip oscillator
circuit continues to run when the part is in its standby mode.
This is important in applications where the system clock is pro-
vided by the AD7705/AD7706’s clock, so that the AD7705/
AD7706 produces an uninterrupted master clock even when it is
in its standby mode.
Supply Current
The current consumption on the AD7705/AD7706 is specified
for supplies in the range +2.7 V to +3.3 V and in the range +4.75 V
to +5.25 V. The part operates over a +2.7 V to +5.25 V supply
range and the IDD for the part varies as the supply voltage varies
over this range. There is an internal current boost bit on the
AD7705/AD7706 that is set internally in accordance with the
operating conditions. This affects the current drawn by the
analog circuitry within these devices. Minimum power consump-
tion is achieved when the AD7705/AD7706 is operated with an
fCLKIN of 1 MHz or at gains of 1 to 4 with fCLKIN = 2.4575 MHz
as the internal boost bit is off reducing the analog current con-
sumption. Figure 15 shows the variation of the typical IDD with
VDD voltage for both a 1 MHz crystal oscillator and a 2.4576 MHz
crystal oscillator at +25°C. The AD7705/AD7706 is operated in
unbuffered mode. The relationship shows that the IDD is mini-
mized by operating the part with lower VDD voltages. IDD on the
AD7705/AD7706 is also minimized by using an external master
clock or by optimizing external components when using the
on-chip oscillator circuit. Figures 3, 4, 6 and 7 show variations
in IDD with gain, VDD and clock frequency using an external
clock.
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 AD7705/AD7706 achieves
excellent linearity by the use of high quality, on-chip capacitors,
which have a very low capacitance/voltage coefficient. The de-
vice 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 AD7705/AD7706
uses digital calibration techniques that minimize offset and gain
error.
1600
MCLK IN = CRYSTAL OSCILLATOR
1400
T
= +25؇C
A
UNBUFFERED MODE
GAIN = 128
Drift Considerations
1200
1000
800
600
400
200
0
The AD7705/AD7706 uses chopper stabilization techniques to
minimize input offset drift. Charge injection in the analog
switches and dc leakage currents at the sampling node are the
primary sources of offset voltage drift in the converter. The dc
input leakage current is essentially independent of the selected
gain. Gain drift within the converter depends primarily upon
the temperature tracking of the internal capacitors. It is not
affected by leakage currents.
fCLK = 2.4576MHz
fCLK = 1MHz
Measurement errors due to offset drift or gain drift can be
eliminated at any time by recalibrating the converter. Using
the system calibration mode can also minimize offset and gain
errors in the signal conditioning circuitry. Integral and differen-
tial linearity errors are not significantly affected by temperature
changes.
2.5
3.0
3.5
4.0
4.5
5.0
5.5
V
DD
Figure 15. IDD vs. Supply Voltage
Grounding and Layout
Since the analog inputs and reference input are differential, most
of the voltages in the analog modulator are common-mode volt-
ages. The excellent common-mode rejection of the part will
remove common-mode noise on these inputs. The digital filter
will provide rejection of broadband noise on the power supplies,
except at integer multiples of the modulator sampling frequency.
The digital filter also removes noise from the analog and refer-
ence inputs provided those noise sources do not saturate the
analog modulator. As a result, the AD7705/AD7706 is more
immune to noise interference than a conventional high resolu-
tion converter. However, because the resolution of the AD7705/
AD7706 is so high, and the noise levels from the AD7705/
AD7706 so low, care must be taken with regard to grounding
and layout.
POWER SUPPLIES
The AD7705/AD7706 operates with a VDD power supply be-
tween 2.7 V and 5.25 V. While the latch-up performance of the
AD7705/AD7706 is good, it is important that power is applied
to the AD7705/AD7706 before signals at REF IN, AIN or the
logic input pins in order to avoid excessive currents. If this is not
possible, the current that flows in any of these pins should be
limited. If separate supplies are used for the AD7705/AD7706
and the system digital circuitry, the AD7705/AD7706 should be
powered up first. If it is not possible to guarantee this, current
limiting resistors should be placed in series with the logic
inputs to again limit the current. Latch-up current is greater
than 100 mA.
–22–
REV. A
ADI [ ADI ]
