AD7811/AD7812
CIRCUIT DESCRIPTION
Converter Operation
SUPPLY
2.7V TO 5.5V
10 F
0.1 F
10nF
THREE-WIRE
SERIAL
INTERFACE
The AD7811 and AD7812 are successive approximation analog-
to-digital converters based around a charge redistribution DAC.
The ADCs can convert analog input signals in the range 0 V to
V
DD
. Figures 2 and 3 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
signal on V
IN
.
CHARGE
REDISTRIBUTION
DAC
V
DD
V
REF
C
REF
V
IN1
V
IN2
0V TO
V
REF
INPUT
SCLK
DOUT
AD7811/
AD7812
DIN
µC/µP
V
IN4(8)
CONVST
RFS
A
V
IN
SW1
B
AGND
SAMPLING
CAPACITOR
AGND
TFS
A0
CONTROL
LOGIC
ACQUISITION
PHASE
SW2
COMPARATOR
CLOCK
OSC
DGND
Figure 4. Typical Connection Diagram
Analog Input
V
DD
/3
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 Charge
Redistribution 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 10 shows the ADC transfer
function.
CHARGE
REDISTRIBUTION
DAC
A
V
IN
SW1
B
AGND
SAMPLING
CAPACITOR
CONTROL
LOGIC
CONVERSION
PHASE
SW2
COMPARATOR
CLOCK
OSC
Figure 5 shows an equivalent circuit of the analog input struc-
ture of the AD7811 and AD7812. 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 without causing irreversible damage to the part. How-
ever, it is worth noting that a small amount of current (1 mA)
being conducted into the substrate due to an overvoltage on an
unselected channel can cause inaccurate conversions on a
selected channel. The capacitor C2 in Figure 5 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 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
DD
/3
D1
R1
125
V
IN
Figure 3. ADC Conversion Phase
TYPICAL CONNECTION DIAGRAM
C2
4pF
D2
C1
3.5pF
V
DD
/3
Figure 4 shows a typical connection diagram for the AD7811/
AD7812. The AGND and DGND are connected together at
the device for good noise suppression. The serial interface is
implemented using three wires with RFS/TFS connected to
CONVST
see Serial 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
. If the AD7811 or AD7812 is not
sharing a serial bus with another AD7811 or AD7812 then A0
(package address pin) should be hardwired low. The default
power up value of the package address bit in the control register
is 0. For applications where power consumption is of concern,
the automatic power down at the end of a conversion should be
used to improve power performance. See Power-Down Options
section of the data sheet.
CONVERSION PHASE – SWITCH OPEN
TRACK PHASE – SWITCH CLOSED
Figure 5. Equivalent Analog Input Circuit
The analog inputs on the AD7811 and AD7812 can be config-
ured as single ended with respect to analog ground (AGND),
as pseudo differential with respect to a common, and also as
pseudo differential pairs—see Control Register section.
–10–
REV. B
AD [ ANALOG DEVICES ]
