REV. A

AD7675

–10–

IN+

REF

REFGND

IN–

32,768C 16,384C

MSB

4C

2C

C

C

LSB

SWITCHES

CONTROL

32,768C 16,384C

MSB

4C

2C

C

C

LSB

BUSY

OUTPUT

CODE

CNVST

CONTROL

LOGIC

COMP

Figure 3. ADC Simplified Schematic

CIRCUIT INFORMATION

The AD7675 is a fast, low power, single-supply, precise 16-bit

analog-to-digital converter (ADC). The AD7675 is capable of

converting 100,000 samples per second (100 kSPS) and allows

power saving between conversions. When operating at 100 SPS,

for example, it consumes typically only 15

µW. This feature

makes the AD7675 ideal for battery-powered applications.

The AD7675 provides the user with an on-chip track/hold,

successive approximation ADC that does not exhibit any pipe-

line or latency, making it ideal for multiple multiplexed channel

applications.

The AD7675 can be operated from a single 5 V supply and be

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

48-lead LQFP package that combines space savings and allows

flexible configurations as either serial or parallel interface. The

AD7675 is pin-to-pin compatible with the AD7660.

CONVERTER OPERATION

The AD7675 is a successive approximation analog-to-digital

converter based on a charge redistribution DAC. Figure 3 shows

the simplified schematic of the ADC. The capacitive DAC con-

sists of two identical arrays of 16 binary-weighted capacitors that

are connected to the two comparator inputs.

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

All independent switches are connected to the analog inputs.

Thus, the capacitor arrays are used as sampling capacitors and

acquire both analog signals.

When the acquisition phase is complete and the

CNVST input

goes or is low, a conversion phase is initiated. When the con-

capacitor arrays are then disconnected from the inputs and

connected to the REFGND input. Therefore, the differential

voltage between the output of 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

REFGND or REF, the comparator input varies by binary

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

in order to bring the comparator back into a balanced condition.

After the completion of this process, the control logic generates

the ADC output code and brings BUSY output low.

Transfer Functions

Using the OB/

2C digital input, the AD7675 offers two output

codings: straight binary and two’s complement. The ideal trans-

fer characteristic for the AD7675 is shown in Figure 4.

000...000

000...001

000...010

111...101

111...110

111...111

ANALOG INPUT

+FS – 1.5 LSB

+FS – 1 LSB

–FS + 1 LSB

–FS

–FS + 0.5 LSB

Figure 4. ADC Ideal Transfer Function

TEMPERATURE – C

5

3

–5

–55

135

–35

–15

–5

15

35

55

75

95

1

–1

–3

115

4

2

0

–2

–4

–FS

OFFSET

+FS

TPC 13. Drift vs. Temperature