AD7819_15 Datasheet, PDF (6/11 Pages) Analog Devices – 2.7 V to 5.5 V, 200 kSPS 8-Bit Sampling ADC

AD7819
CIRCUIT DESCRIPTION
Converter Operation
The AD7819 is a successive approximation analog-to-digital
converter based around a charge redistribution DAC. The ADC
can convert analog input signals in the range 0 V to V_DD. Fig-
ures 2 and 3 below show simplified schematics of the ADC.
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 sig-
nal on V_IN+.
V_IN
AGND
SAMPLING
A ^CAPACITOR
SW1
B
ACQUISITION
PHASE
V_DD/3
CHARGE
RESTRIBUTION
DAC
SW2
COMPARATOR
CONTROL
LOGIC
CLOCK
OSC
Figure 2. ADC Track 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 Charge Redis-
tribution 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 rebal-
anced the conversion is complete. The Control Logic generates
the ADC output code. Figure 7 shows the ADC transfer function.
SAMPLING
A ^CAPACITOR
V_IN
SW1
B
CONVERSION
PHASE
AGND
V_DD/3
CHARGE
RESTRIBUTION
DAC
SW2
COMPARATOR
CONTROL
LOGIC
CLOCK
OSC
Figure 3. ADC Conversion Phase
TYPICAL CONNECTION DIAGRAM
Figure 4 shows a typical connection diagram for the AD7819. The
parallel interface is implemented using an 8-bit data bus, the
falling edge of CONVST brings the BUSY signal high and at
the end of conversion, the falling edge of BUSY is used to
initiate an ISR on a microprocessor. (See Parallel Interface
section for more details.) V_REF is connected to a well decoupled
V_DD pin to provide an analog input range of 0 V to V_DD. When
V_DD is first connected the AD7819 powers up in a low current
mode, i.e., power-down. A rising edge on the CONVST input
will cause the part to power up. (See Power-Up Times section.)
If power consumption is of concern, the automatic power-down
at the end of a conversion should be used to improve power
performance. See Power vs. Throughput Rate section of the
data sheet.
SUPPLY
2.7V TO 5.5V _10␮F
0.1␮F
0V TO V_REF
INPUT
V_DD V_REF
DB0-DB7
AD7819
V_IN
BUSY
RD
GND
CS
CONVST
PARALLEL
INTERFACE
␮C/␮P
Figure 4. Typical Connection Diagram
Analog Input
Figure 5 shows an equivalent circuit of the analog input struc-
ture of the AD7819. 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 200 mV. This will cause these diodes to become
forward biased and start conducting current into the substrate.
20 mA is the maximum current these diodes can conduct with-
out causing irreversible damage to the part. The capacitor C2
is typically about 4 pF and can be primarily attributed to pin
capacitance. The resistor R1 is a lumped component made up of
the on resistance of a multiplexer and a switch. This resistor is
typically about 125 Ω. The capacitor C1 is the ADC sampling
capacitor and has a capacitance of 3.5 pF.
V_DD
V_IN
C2
4pF
D1
R1
125⍀
C1
3.5pF
V_DD/3
D2 _{CONVERT PHASE – SWITCH OPEN}
TRACK PHASE – SWITCH CLOSED
Figure 5. Equivalent Analog Input Circuit
DC Acquisition Time
The ADC starts a new acquisition phase at the end of a conver-
sion and ends on the falling edge of the CONVST signal. At the
end of a conversion there is a settling time associated with the
sampling circuit. This settling time lasts approximately 100 ns.
The analog signal on V_IN is also being acquired during this
settling time. The minimum acquisition time needed is approxi-
mately 100 ns. Figure 6 shows the equivalent charging circuit
for the sampling capacitor when the ADC is in its acquisition
phase. R2 represents the source impedance of a buffer amplifier
or resistive network, R1 is an internal multiplexer resistance and
C1 is the sampling capacitor.
R2
V_IN
R1
125⍀
C1
3.5pF
Figure 6. Equivalent Sampling Circuit
–6–
REV. B