AD7993 datasheet(13/28 Pages) AD

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AD7992

Rev. 0 | Page 13 of 28

CIRCUIT INFORMATION

The AD7992 is a low power, 12-bit, single-supply, 2-channel

analog-to-digital converter (ADC). The part can be operated

from a 2.7 V to 5.5 V supply.

The AD7992 provides the user with a 2-channel multiplexer,

an on-chip track-and-hold, an ADC, an on-chip oscillator,

internal data registers, and an I2C-compatible serial interface,

all housed in a 10-lead MSOP package that offers the user

considerable space-saving advantages over alternative solutions.

The AD7992 requires an external reference in the range of 1.2 V

to VDD.

The AD7992 normally remains in a power-down state while not

converting. When supplies are first applied, the part comes up

in a power-down state. Power-up is initiated prior to a con-

version, and the device returns to power-down upon

completion of the conversion. Conversions can be initiated on

the AD7992 by pulsing the CONVST signal, using an automatic

cycle interval mode or a command mode where wake-up and a

conversion occur during a write address function (see the

Modes of Operation section). On completion of a conversion,

the AD7992 again enters power-down mode. This automatic

power-down feature allows power saving between conversions.

This means any read or write operations across the I2C interface

can occur while the device is in power-down.

CONVERTER OPERATION

The AD7992 is a successive approximation, analog-to-digital

converter based around a capacitive DAC. Figure 17 and

Figure 18 show simplified schematics of the ADC during its

acquisition and conversion phases, respectively. Figure 17 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 VIN.

CAPACITIVE

DAC

VIN

COMPARATOR

CONTROL

LOGIC

SW1

SW2

AGND

Figure 17. ADC Acquisition Phase

When the ADC starts a conversion, as shown in Figure 18,

SW2 opens and SW1 moves to position B, causing the

comparator to become unbalanced. The input is disconnected

once the conversion begins. The control logic and the capacitive

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 19 shows the ADC transfer function.

VIN

COMPARATOR

CONTROL

LOGIC

SW1

SW2

AGND

CAPACITIVE

DAC

Figure 18. ADC Conversion Phase

ADC Transfer Function

The output coding of the AD7992 is straight binary. The

designed code transitions occur at successive integer LSB values

(i.e., 1 LSB, 2 LSB, and so on). The LSB size for the AD7992 is

REFIN/4096. Figure 19 shows the ideal transfer characteristic for

the AD7992.

000...000

ANALOG INPUT

0V TO REFIN

111...111

000...001

000...010

111...110

111...000

011...111

AGND + 1LSB

+REFIN – 1LSB

AD7992 1LSB = REFIN/4096

Figure 19. AD7992 Transfer Characteristic

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