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AD7690

Data Sheet

THEORY OF OPERATION

Figure 24. ADC Simplified Schematic

CIRCUIT INFORMATION

The AD7690 is a fast, low power, single-supply, precise, 18-bit

ADC using a successive approximation architecture.

The AD7690 is capable of converting 400,000 samples per

second (400 kSPS) and powers down between conversions.

When operating at 1 kSPS, for example, it consumes 50 µW

typically, ideal for battery-powered applications.

The AD7690 provides the user with an on-chip track-and-hold

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

for multiple multiplexed channel applications.

The AD7690 is specified from 4.75 V to 5.25 V and 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 LFCSP that allows space

savings and flexible configurations.

It is pin-for-pin compatible with the 18-bit AD7691 and AD7982

and the 16-bit AD7687, AD7688, and AD7693.

CONVERTER OPERATION

The AD7690 is a successive approximation ADC based on a

charge redistribution DAC. Figure 24 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

IN+ and IN− inputs 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/262,144).

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

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 AD7690 has an on-board conversion clock, the

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

SW+

MSB

65,536C

IN+

LSB

COMP

CONTROL

LOGIC

SWITCHES CONTROL

BUSY

OUTPUT CODE

CNV

REF

GND

IN–

131,072C

SW–

MSB

65,536C

LSB

131,072C

Rev. C | Page 12 of 24

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