AD7266 datasheet(13/28 Pages) AD

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AD7266

Rev. B | Page 13 of 28

THEORY OF OPERATION

CIRCUIT INFORMATION

The AD7266 is a fast, micropower, dual, 12-bit, single-supply,

ADC that operates from a 2.7 V to a 5.25 V supply. When

operated from a 5 V supply, the AD7266 is capable of

throughput rates of 2 MSPS when provided with a 32 MHz

clock, and a throughput rate of 1.5 MSPS at 3 V.

The AD7266 contains two on-chip, differential track-and-hold

amplifiers, two successive approximation ADCs, and a serial

interface with two separate data output pins. It is housed in a

32-lead LFCSP or a 32-lead TQFP, offering the user considerable

space-saving advantages over alternative solutions. The serial

clock input accesses data from the part but also provides the

clock source for each successive approximation ADC. The

analog input range for the part can be selected to be a 0 V to

VREF input or a 2 × VREF input, configured with either single-

ended or differential analog inputs. The AD7266 has an on-chip

2.5 V reference that can be overdriven when an external reference

is preferred. If the internal reference is to be used elsewhere in a

system, then the output needs to buffered first.

The AD7266 also features power-down options to allow power

saving between conversions. The power-down feature is

implemented via the standard serial interface, as described in

the Modes of Operation section.

CONVERTER OPERATION

The AD7266 has two successive approximation ADCs, each

based around two capacitive DACs. Figure 16 and Figure 17

show simplified schematics of one of these ADCs in acquisition

and conversion phase, respectively. The ADC is comprised of

control logic, a SAR, and two capacitive DACs. In Figure 16 (the

acquisition phase), SW3 is closed, SW1 and SW2 are in Position A,

the comparator is held in a balanced condition, and the sampling

capacitor arrays acquire the differential signal on the input.

CAPACITIVE

DAC

CAPACITIVE

DAC

CONTROL

LOGIC

COMPARATOR

SW3

SW1

SW2

VIN+

VIN–

VREF

Figure 16. ADC Acquisition Phase

When the ADC starts a conversion (see Figure 17), SW3 opens

and SW1 and SW2 move to Position B, causing the comparator

to become unbalanced. Both inputs are disconnected once the

conversion begins. The control logic and the charge redistribu-

tion DACs are used to add and subtract fixed amounts of charge

from the sampling capacitor arrays 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. The output impedances of the

sources driving the VIN+ and VIN− pins must be matched;

otherwise, the two inputs will have different settling times,

resulting in errors.

CAPACITIVE

DAC

CAPACITIVE

DAC

CONTROL

LOGIC

COMPARATOR

SW3

SW1

SW2

VIN+

VIN–

VREF

Figure 17. ADC Conversion Phase

ANALOG INPUT STRUCTURE

Figure 18 shows the equivalent circuit of the analog input

structure of the AD7266 in differential/pseudo differential

mode. In single-ended mode, VIN− is internally tied to AGND.

The four diodes provide ESD protection for the analog inputs.

Care must be taken to ensure that the analog input signals never

exceed the supply rails by more than 300 mV. This causes these

diodes to become forward-biased and starts conducting into the

substrate. These diodes can conduct up to 10 mA without

causing irreversible damage to the part.

The C1 capacitors in Figure 18 are typically 4 pF and can

primarily be attributed to pin capacitance. The resistors are

lumped components made up of the on resistance of the

switches. The value of these resistors is typically about 100 Ω.

The C2 capacitors are the ADC’s sampling capacitors with a

capacitance of 45 pF typically.

For ac applications, removing high frequency components from

the analog input signal is recommended by the use of an RC

low-pass filter on the relevant analog input pins with optimum

values of 47 Ω and 10 pF. In applications where harmonic dis-

tortion and signal-to-noise ratio are critical, the analog input

should be driven from a low impedance source. Large source

impedances significantly affect the ac performance of the ADC

and may necessitate the use of an input buffer amplifier. The

choice of the op amp is a function of the particular application.

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