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AD7921ARM
(Rev.:Rev0)

2-Channel, 2.35 V to 5.25 V 250 kSPS, 10-/12-Bit ADCs

Analog Devices 

CIRCUIT INFORMATION

The AD7911/AD7921 are fast, 2-channel, 10-/12-bit, single

supply, analog-to-digital converters (ADCs), respectively. The

parts can be operated from a 2.35 V to 5.25 V supply. When

operated from either a 5 V supply or a 3 V supply, the

AD7911/AD7921 are capable of throughput rates of 250 kSPS

when provided with a 5 MHz clock.

The AD7911/AD7921 provide the user with an on-chip track-

and-hold, an ADC, and a serial interface, all housed in a tiny 8-

lead TSOT package or an 8-lead MSOP package, which offer the

user considerable space-saving advantages over alternative

solutions. The serial clock input accesses data from the parts,

controls the transfer of data written to the ADC, and provides

the clock source for the successive approximation ADC. The

analog input range is 0 to VDD. An external reference is not

required for the ADC, and neither is there a reference on-chip.

The reference for the AD7911/AD7921 is derived from the

power supply and, therefore, gives the widest dynamic input

range.

The AD7911/AD7921 feature a power-down option that allows

power saving between conversions. The power-down feature is

implemented across the standard serial interface as described in

the Modes of Operation section.

CONVERTER OPERATION

The AD7911/AD7921 are 10-/12-bit successive approximation

ADCs based around a charge redistribution DAC. Figure 19 and

Figure 20 show simplified schematics of the ADC. Figure 19

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.

CHARGE

REDISTRIBUTION

DAC

VIN0

VIN1

SAMPLING

A CAPACITOR

SW1 ACQUISITION

B

PHASE

AGND

VDD/2

SW2

COMPARATOR

CONTROL

LOGIC

Figure 19. ADC Acquisition Phase

AD7911/AD7921

When the ADC starts a conversion (see Figure 20), SW2 opens

and SW1 moves 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

rebalanced, the conversion is complete. The control logic

generates the ADC output code. Figure 21 shows the ADC

transfer function.

CHARGE

REDISTRIBUTION

DAC

VIN0

VIN1

SAMPLING

A CAPACITOR

SW1 CONVERSION

B

PHASE

AGND

VDD/2

SW2

COMPARATOR

CONTROL

LOGIC

Figure 20. ADC Conversion Phase

ADC TRANSFER FUNCTION

The output coding of the AD7911/AD7921 is straight binary.

The designed code transitions occur at the successive integer

LSB values, that is, 1 LSB, 2 LSB, and so on. The LSB size is

VDD/4096 for the AD7921 and VDD/1024 for the AD7911. The

ideal transfer characteristic for the AD7911/AD7921 is shown

in Figure 21.

111...111

111...110

111...000

011...111

000...010

000...001

000...000

0V 1LSB

1LSB = VDD/4096 (AD7921)

1LSB = VDD/1024 (AD7911)

+VDD – 1LSB

ANALOG INPUT

Figure 21. AD7911/AD7921 Transfer Characteristic

Rev. 0 | Page 15 of 28

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