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AD7982

Preliminary Technical Data

Rev PrC | Page 10 of 23

SW+

MSB

16,384C

IN+

LSB

COMP

CONTROL

LOGIC

SWITCHES CONTROL

BUSY

OUTPUT CODE

CNV

REF

GND

IN–

32,768C

SW–

MSB

LSB

16,384C

32,768C

Figure 11. ADC Simplified Schematic

CIRCUIT INFORMATION

The AD7982 is a fast, low power, single-supply, precise 18-bit

ADC using a successive approximation architecture.

The AD7982 is capable of converting 1,000,000 samples per

second (1MSPS) and powers down between conversions. When

operating at 10 kSPS, for example, it consumes 75 μW typically,

ideal for battery-powered applications.

The AD7982 provides the user with an on-chip track-and-hold

and does not exhibit any pipeline delay or latency, making it

ideal for multiple multiplexed channel applications.

The AD7982 can be interfaced to any 1.8 V to 5 V digital logic

family. It is housed in a 10-lead MSOP or a tiny 10-lead QFN

(LFCSP) that combines space savings and allows flexible

configurations.

It is pin-for-pin-compatible with the 16-bit AD7980.

CONVERTER OPERATION

The AD7982 is a successive approximation ADC based on a

charge redistribution DAC. Figure 11 shows the simplified

schematic of the ADC. The capacitive DAC consists of two

identical arrays of 18 binary weighted capacitors, which are

connected to the two comparator inputs.

During the acquisition phase, terminals of the array tied to the

comparator’s input are connected to GND via SW+ and SW−.

All independent switches are connected to the analog inputs.

Thus, the capacitor arrays are used as sampling capacitors and

acquire the analog signal on the IN+ and IN− inputs. When the

acquisition phase is complete and the CNV input goes high, a

conversion phase is initiated. When the conversion phase

begins, SW+ and SW− are opened first. The two capacitor

arrays are then disconnected from the inputs and connected to

the GND input. Therefore, the differential voltage between the

inputs IN+ and IN− 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 GND and REF, the comparator input varies by

binary weighted voltage steps (VREF/2, VREF/4 . . . VREF/262144).

The control logic toggles these switches, starting with the MSB,

in order to bring the comparator back into a balanced

condition. After the completion of this process, the part returns

to the acquisition phase and the control logic generates the

ADC output code and a BUSY signal indicator.

Because the AD7982 has an on-board conversion clock, the

serial clock, SCK, is not required for the conversion process.

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