Datasheet

AD7091R Data Sheet
Rev. 0 | Page 12 of 20
THEORY OF OPERATION
CIRCUIT INFORMATION
The AD7091R is a 12-bit successive approximation analog-to-
digital converter (ADC) that offers ultralow power consumption
(typically 349 µA at 3 V and 1 MSPS) while achieving fast
throughput rates (1 MSPS with a 50 MHz SCLK). The part can be
operated from a single power supply in the range of 2.7 V to 5.25 V.
The AD7091R provides an on-chip track-and-hold ADC with
a serial interface housed in a tiny 10-lead LFCSP and 10-lead
MSOP packages. These packages offer considerable space-saving
advantages compared with alternative solutions. The serial clock
input accesses data from the part. The clock for the successive
approximation ADC is generated internally. The reference voltage
for the AD7091R is generated internally by an accurate on-chip
reference source. The analog input range for the AD7091R is
0 V to V
REF
.
The AD7091R also features a power-down option to save power
between conversions. The power-down feature is implemented
across the standard serial interface as described in the Modes of
Operation section.
CONVERTER OPERATION
The AD7091R is a successive approximation ADC based
around a charge redistribution DAC. Figure 18 and Figure 19
show simplified schematics of the ADC. Figure 18 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
.
Figure 18. ADC Acquisition Phase
When the ADC starts a conversion, SW2 opens and SW1 moves
to Position B (see Figure 19), 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 rebalanced, the conversion
is complete. The control logic generates the ADC output code.
Figure 20 show the ideal ADC transfer function.
Figure 19. ADC Conversion Phase
ADC TRANSFER FUNCTION
The output coding of the AD7091R is straight binary. The designed
code transitions occur midway between successive integer LSB
values, such as 0.5 LSB, 1.5 LSB, and so on. The LSB size for the
AD7091R is V
REF
/4096. The ideal transfer characteristics for the
AD7091R are shown in Figure 20.
Figure 20. AD7091R Ideal Transfer Characteristics
INTERNAL/EXTERNAL VOLTAGE REFERENCE
The AD7091R allows the choice of an internal voltage reference
or an external voltage reference.
The internal reference provides an accurate 2.5 V low temperature
drift voltage reference. The internal reference is available at the
REF
IN
/REF
OUT
pin. When using the internal reference, this pin
should be decoupled using a capacitor with a typical value of 2.2 µF
to achieve the specified performance. With a fully discharged
2.2 µF reference capacitor, the internal reference requires 50 ms
typically to fully charge to the 2.5 V REF
OUT
voltage level.
In power-down mode, the internal voltage reference is shut down.
After exiting power-down mode, adequate time should be allowed
for the reference capacitor to recharge before performing a con-
version. The time required to recharge the reference capacitor is
dependent on the amount of charge remaining on the capacitor
when exiting power-down mode.
If the on-chip reference is used externally to the AD7091R, it is
recommended to buffer this reference before supplying the
external circuitry.
Alternatively, the AD7091R reference voltage can be applied
externally. If an external reference is applied to the device, the
internal reference is automatically overdriven. An externally
applied reference voltage should be in the range of 2.7 V to
5.25 V and should be connected to the REF
IN
/REF
OUT
pin.
CHARGE
REDISTRIBUTION
DAC
CONTROL
LOGIC
COMPARATOR
SW2
SAMPLING
CAPACITOR
ACQUISITION
PHASE
SW1
A
B
GND
LDO/2
V
IN
10494-017
CHARGE
REDISTRIBUTION
DAC
CONTROL
LOGIC
COMPARATOR
SW2
SAMPLING
CAPACITOR
CONVERSION
PHASE
SW1
A
B
GND
LDO/2
V
IN
10494-018
000 ... 000
0V
ADC CODE
ANALOG INPUT
111 ... 111
000 ... 001
000 ... 010
111 ... 110
111 ... 000
011 ... 111
1LSB
V
REF
– 1LSB
1LSB = V
REF
/4096
10494-019