AD7666AST datasheet(17/29 Pages) AD

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AD7666

Rev. 0 | Page 16 of 28

CIRCUIT INFORMATION

SWA

COMP

SWB

REF

REFGND

LSB

MSB

32,768C

INGND

16,384C

65,536C

CONTROL

LOGIC

SWITCHES

CONTROL

BUSY

OUTPUT

CODE

CNVST

Figure 24. ADC Simplified Schematic

The AD7666 is a very fast, low power, single supply, precise

16-bit analog-to-digital converter (ADC). The AD7666 is

capable of converting 100,000 samples per second (500 kSPS)

and allows power savings between conversions.

The AD7666 provides the user with an on-chip track/hold,

successive approximation ADC that does not exhibit any

pipeline or latency, making it ideal for multiple multiplexed

channel applications.

The AD7666 can be operated from a single 5 V supply and can

be interfaced to either 5 V or 3 V digital logic. It is housed in

either a 48-lead LQFP or a 48-lead LFCSP that saves space and

allows flexible configurations as either a serial or parallel inter-

face. The AD7666 is pin-to-pin compatible with PulSAR ADCs

and is an upgrade of the AD7661 and AD7664.

CONVERTER OPERATION

The AD7666 is a successive-approximation ADC based on a

charge redistribution DAC. Figure 24 shows a simplified sche-

matic of the ADC. The capacitive DAC consists of an array of

16 binary weighted capacitors and an additional LSB capacitor.

The comparator’s negative input is connected to a dummy

capacitor of the same value as the capacitive DAC array.

During the acquisition phase, the common terminal of the array

tied to the comparator's positive input is connected to AGND

via SWA. All independent switches are connected to the analog

input IN. Thus, the capacitor array is used as a sampling

capacitor and acquires the analog signal on IN. Similarly, the

dummy capacitor acquires the analog signal on INGND.

When CNVST goes LOW, a conversion phase is initiated. When

the conversion phase begins, SWA and SWB are opened. The

capacitor array and dummy capacitor are then disconnected

from the inputs and connected to REFGND. Therefore, the

differential voltage between IN and INGND captured at the end

of the acquisition phase is applied to the comparator inputs,

causing the comparator to become unbalanced. By switching

each element of the capacitor array between REFGND and REF,

the comparator input varies by binary weighted voltage steps

(VREF/2, VREF/4, …VREF/65536). The control logic toggles these

switches, starting with the MSB, to bring the comparator back

into a balanced condition.

After this process is completed, the control logic generates the

ADC output code and brings the BUSY output LOW.

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