Datasheet

AD7908/AD7918/AD7928
Rev. D | Page 18 of 32
POWER-ON
DUMMY CONVERSION
DIN = ALL 1s
CS
DOUT: CONVERSION RESULT FROM
CHANNEL 0.
CONTINUOUSLY CONVERTS ON A
CONSECUTIVE SEQUENCE OF
CHANNELS FROM CHANNEL 0 UP TO,
AND INCLUDING, THE PREVIOUSLY
SELECTED A2 TO A0 IN THE CONTROL
REGISTER.
CONTINUOUSLY CONVERTS ON THE
SELECTED SEQUENCE OF CHANNELS
BUT WILL ALLOW RANGE, CODING, AND
SO ON, TO CHANGE IN THE CONTROL
REGISTER WITHOUT INTERRUPTING
THE SEQUENCE, PROVIDED SEQ = 1
SHADOW = 0
DIN: WRITE TO CONTROL REGISTER,
WRITE BIT = 1, SELECT CODING, RANGE,
AND POWER MODE.
SELECT CHANNEL A2 TO A0
FOR CONVERSION.
SEQ = 1 SHADOW = 1
CS
WRITE BIT = 1
SEQ = 1, SHADOW = 0
WRITE BIT = 0
CS
03089-013
Figure 13. SEQ Bit = 1, SHADOW Bit = 1 Flowchart
CIRCUIT INFORMATION
The AD7908/AD7918/AD7928 are high speed, 8-channel, 8-bit,
10-bit, and 12-bit, single-supply ADCs, respectively. The parts
can be operated from a 2.7 V to 5.25 V supply. When operated
from either a 5 V or 3 V supply, the AD7908/AD7918/AD7928
are capable of throughput rates of 1 MSPS when provided with
a 20 MHz clock.
The AD7908/AD7918/AD7928 provide the user with an on-
chip, track-and-hold ADC, and a serial interface housed in a
20-lead TSSOP package. The AD7908/AD7918/ AD7928 each
have eight single-ended input channels with a channel
sequencer, allowing the user to select a channel sequence that
the ADC can cycle through with each consecutive
CS
falling
edge. The serial clock input accesses data from the part, controls
the transfer of data written to the ADC, and provides the clock
source for the successive approximation ADC. The analog input
range for the AD7908/AD7918/ AD7928 is 0 V to REF
IN
or 0 V
to 2 × REF
IN
, depending on the status of Bit 1 in the control
register. For the 0 to 2 × REF
IN
range, the part must be operated
from a 4.75 V to 5.25 V supply.
The AD7908/AD7918/AD7928 provide flexible power
management options to allow the user to achieve the best power
performance for a given throughput rate. These options are
selected by programming the PM1 and PM0 power
management bits in the control register.
CONVERTER OPERATION
The AD7908/AD7918/AD7928 are 8-, 10-, and 12-bit
successive approximation analog-to-digital converters based
around a capacitive DAC, respectively. The AD7908/AD7918/
AD7928 can convert analog input signals in the range 0 V to
REF
IN
or 0 V to 2 × REF
IN
. Figure 14 and Figure 15 show
simplified schematics of the ADC. The ADC is comprised of
control logic, SAR, and a capacitive DAC, 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 14 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 the selected V
IN
channel.
V
IN
7
V
IN
0
AGND
A
B
SW1
SW2
COMPARATOR
CONTROL
LOGIC
4kΩ
CAPACITIVE DAC
03089-014
Figure 14. ADC Acquisition Phase
When the ADC starts a conversion (see Figure 15), SW2 opens
and SW1 moves 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. Figure 17 and Figure 18 show the ADC transfer
functions.
V
IN
7
V
IN
0
AGND
A
B
SW1
SW2
COMPARATOR
CONTROL
LOGIC
4kΩ
CAPACITIVE DACCAPACITIVE DAC
03089-015
Figure 15. ADC Conversion Phase
Analog Input
Figure 16 shows an equivalent circuit of the analog input
structure of the AD7908/AD7918/AD7928. 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 causes 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 16 is typically about 4 pF and can
primarily be attributed to pin capacitance. The Resistor R1 is a
lumped component made up of the on resistance of the track-
and-hold switch and also includes the on resistance of the input
multiplexer. The total resistance is typically about 400 Ω. The
Capacitor C2 is the ADC sampling capacitor and has a
capacitance of 30 pF typically. For ac applications, removing
high frequency components from the analog input signal is
recommended by use of an RC lowpass filter on the relevant
analog input pin. In applications where harmonic distortion