REV. A
AD7866
–11–
CIRCUIT INFORMATION
The AD7866 is a fast, micropower, dual 12-bit, single supply,
A/D converter that operates from a 2.7 V to 5.25 V supply.
When operated from either a 5 V supply or a 3 V supply, the
AD7866 is capable of throughput rates of 1 MSPS when provided
with a 20 MHz clock.
The AD7866 contains two on-chip track-and-hold amplifiers,
two successive approximation A/D converters, and a serial inter-
face with two separate data output pins, and is housed in a
20-lead TSSOP package, which offers the user considerable
space-saving advantages over alternative solutions. The serial
clock input accesses data from the part but also provides the
clock source for each successive approximation ADC. The ana-
log input range for the part can be selected to be a 0 V to V
REF
input or a 2 V
REF
input with either straight binary or twos
complement output coding. The AD7866 has an on-chip 2.5 V
reference that can be overdriven if an external reference is pre-
ferred. In addition, each ADC can be supplied with an individual
separate external reference.
The AD7866 also features power-down options to allow power
saving between conversions. The power-down feature is imple-
mented across the standard serial interface, as described in the
Modes of Operation section.
CONVERTER OPERATION
The AD7866 has two successive approximation analog-to-digital
converters, each based around a capacitive DAC. Figures 2 and
3 show simplified schematics of one of these ADCs. The ADC
is comprised of control logic, a SAR, and a capacitive DAC, all
of which are used to add and subtract fixed amounts of charge
from the sampling capacitor to bring the comparator back into a
balanced condition. Figure 2 shows the ADC during its acquisition
phase. SW2 is closed and SW1 is in position A, the comparator
is held in a balanced condition, and the sampling capacitor
acquires the signal on V
A1
, for example.
CAPACITIVE
DAC
CONTROL
LOGIC
COMPARATOR
SW2
SW1
A
B
AGND
V
IN
Figure 2. ADC Acquisition Phase
When the ADC starts a conversion (see Figure 3), SW2 will
open and SW1 will move to position B, causing the comparator
to become unbalanced. The Control Logic and the capacitive
DAC are used to add and subtract fixed amounts of charge
from the sampling capacitor to bring the comparator back into a
balanced condition. When the comparator is rebalanced, the
conversion is complete. The Control Logic generates the ADC
output code. Figures 10 and 11 show the ADC transfer functions.
CAPACITIVE
DAC
CONTROL
LOGIC
COMPARATOR
SW2
SW1
A
B
AGND
V
IN
Figure 3. ADC Conversion Phase
ANALOG INPUT
Figure 4 shows an equivalent circuit of the analog input structure
of the AD7866. The two diodes, D1 and D2, provide ESD
protection for the analog inputs. Care must be taken to ensure
that the analog input signal never exceeds the supply rails by more
than 300 mV. This will cause these diodes to become forward-
biased and start conducting current into the substrate. 10 mA is
the maximum current these diodes can conduct without causing
irreversible damage to the part. The capacitor C1 in Figure 4 is
typically about 10 pF and can primarily be attributed to pin
capacitance. The resistor R1 is a lumped component made up
of the on resistance of a switch. This resistor is typically about
100 Ω. The capacitor C2 is the ADC sampling capacitor and
has a capacitance of 20 pF typically. For ac applications, removing
high frequency components from the analog input signal is
recommended by use of an RC low-pass filter on the relevant
analog input pin. In applications where harmonic distortion and
signal-to-noise ratio are critical, the analog input should be driven
from a low impedance source. Large source impedances will
significantly affect the ac performance of the ADC. This may
necessitate the use of an input buffer amplifier. The choice of the
op amp will be a function of the particular application.
V
DD
V
IN
C1
D1
D2
R1
CONVERT PHASE – SWITCH OPEN
TRACK PHASE – SWITCH CLOSED
C2
Figure 4. Equivalent Analog Input Circuit
When no amplifier is used to drive the analog input, the source
impedance should be limited to low values. The maximum
source impedance will depend on the amount of total harmonic
distortion (THD) that can be tolerated. The THD will increase
as the source impedance increases, and performance will degrade
(see TPC 7).
