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AD7927 View Datasheet(PDF) - Unspecified

Part NameDescriptionManufacturer
AD7927 8-Channel, 200 kSPS, 12-Bit ADC with Sequencer in 20-Lead TSSOP ETC
Unspecified 
AD7927 Datasheet PDF : 20 Pages
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AD7927
POWER-ON
DUMMY CONVERSION
DIN = ALL 1s
DIN: WRITE TO CONTROL REGISTER,
CS
WRITE BIT = 1,
SELECT CODING, RANGE, AND POWER MODE.
SELECT CHANNEL A2–A0 FOR CONVERSION.
SEQ = 1 SHADOW = 1
VIN0
VIN7
AGND
A
SW1
B
4k
SW2
CAPACITIVE
DAC
COMPARATOR
CONTROL
LOGIC
Figure 5. ADC Acquisition Phase
When the ADC starts a conversion (see Figure 6), SW2 will
open and SW1 will move to position B, causing the comparator
DOUT: CONVERSION RESULT FROM CHANNEL 0
to become unbalanced. The Control Logic and the Capacitive
DAC are used to add and subtract fixed amounts of charge
CS
CONTINUOUSLY CONVERTS ON A CONSECUTIVE
SEQUENCE OF CHANNELS FROM CHANNEL 0 UP WRITE BIT = 0
from the sampling capacitor to bring the comparator back into a
TO AND INCLUDING THE PREVIOUSLY SELECTED
A2–A0 IN THE CONTROL REGISTER
balanced condition. When the comparator is rebalanced, the
conversion is complete. The Control Logic generates the ADC
output code. Figures 8 and 9 show the ADC transfer functions.
CONTINUOUSLY CONVERTS ON THE SELECTED
CS
SEQUENCE OF CHANNELS BUT WILL ALLOW
RANGE, CODING AND SO ON, TO CHANGE IN THE
CONTROL REGISTER WITHOUT INTERRUPTING
WRITE BIT = 1,
THE SEQUENCE, PROVIDED SEQ = 1 SHADOW = 0 SEQ = 1,
SHADOW = 0
Figure 4. SEQ Bit = 1, SHADOW Bit = 1 Flowchart
VIN0
.
.
VIN7
AGND
A
SW1
B
4k
SW2
CAPACITIVE
DAC
COMPARATOR
CONTROL
LOGIC
CIRCUIT INFORMATION
The AD7927 is a high speed, 8-channel, 12-bit, single supply,
A/D converter. The part 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 AD7927
is capable of throughput rates of 200 kSPS. The conversion time
may be as short as 800 ns when provided with a 20 MHz clock.
The AD7927 provides the user with an on-chip track-and-hold,
A/D converter, and a serial interface housed in a 20-lead TSSOP
package. The AD7927 has eight single-ended input channels
with a channel sequencer, allowing the user to select a channel
sequence through which the ADC can cycle with each consecu-
tive 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 A/D
converter. The analog input range for the AD7927 is 0 V to
REFIN or 0 V to 2 ¥ REFIN, depending on the status of Bit 1 in
the Control Register. For the 0 to 2 ¥ REFIN range, the part
must be operated from a 4.75 V to 5.25 V supply.
The AD7927 provides 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
Power Management bits, PM1 and PM0, in the Control Register.
CONVERTER OPERATION
The AD7927 is a 12-bit successive approximation analog-to-
digital converter based around a capacitive DAC. The AD7927
can convert analog input signals in the range 0 V to REFIN or 0 V
to 2 ¥ REFIN. Figures 5 and 6 show simplified schematics of the
ADC. The ADC is comprised of Control Logic, SAR, and a
Capacitive DAC that are used to add and subtract fixed amounts
of charge from the sampling capacitor to bring the comparator
back into a balanced condition. Figure 5 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 VIN channel.
Figure 6. ADC Conversion Phase
Analog Input
Figure 7 shows an equivalent circuit of the analog input structure
of the AD7927. The two diodes D1 and D2 provide ESD pro-
tection 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 7 is
typically about 4 pF and can primarily be attributed to pin capaci-
tance. The resistor R1 is a lumped component made up of the on
resistance of a switch (track-and-hold switch) and also includes
the on resistance of the input multiplexer. The total resistance is
typically about 400 W. The capacitor C2 is the ADC sampling
capacitor and has a capacitance of 30 pF typically. For ac appli-
cations, 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.
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 5.)
VIN
C1
4pF
AVDD
D1
C2
R1
30pF
D2
CONVERSION PHASE: SWITCH OPEN
TRACK PHASE: SWITCH CLOSED
Figure 7. Equivalent Analog Input Circuit
–12–
REV. 0
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