AD7280 datasheet(11/33 Pages) AD

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Preliminary Technical Data

AD7280

Rev. PrD | Page 11 of 33

Vin0

Vin1

Vin2

Vin3

Vin4

Vin5

Vin6

ADC Vin+

ADC Vin-

Figure 5. MUX Configuration During Vin2-Vin1 Sampling

The ADC is a 12-bit successive approximation analog-to-digital

converter. The converter is composed of a comparator, SAR,

some control logic and 2 capacitive DACs. Figure 6 shows a

simplified schematic of the converter. During the acquisition

phase switches SW1, SW2 and SW3 are closed. The sampling

capacitor array acquires the signal on the input during this

phase.

CAPACITIVE

DAC

CONTROL

LOGIC

CAPACITIVE

DAC

COMPARATOR

SW3

SW1

VIN+

SW2

VIN–

Figure 6. ADC Configuration During Acquisition Phase

When the ADC starts a conversion (Figure 7), SW3 opens and

SW1 and SW2 move to position B, causing the comparator to

become unbalanced. The control logic and capacitive DACs are

used to add and subtract fixed amounts of charge 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. This output code

is then stored in the appropriate register for the input that has

been converted.

CAPACITIVE

DAC

CONTROL

LOGIC

CAPACITIVE

DAC

COMPARATOR

SW3

SW1

VIN+

SW2

VIN–

Figure 7. ADC Configuration During Conversion Phase

ANALOG INPUT STRUCTURE

Figure 8 shows the equivalent circuit of the analog input

structure of the AD7280. The two diodes provide ESD

protection. The resistors are lumped components made up of

the on-resistance of the input multiplexer and the track-and-

hold switch. The value of these resistors is typically about 300Ω.

Capacitor C1 can primarily be attributed to pin capacitance

while Capacitor C2 is the sampling capacitor of the ADC. The

total lumped capacitance of C1 and C2 is approximately 13 pF.

VDD

VIN+

VSS

VDD

VIN–

VSS

Figure 8. Equivalent Analog Input Circuit

TRANSFER FUNCTION

The output coding of the AD7280 is straight binary. The

designed code transitions occur at successive integer LSB values

(that is, 1 LSB, 2 LSB, and so on). The LSB size is dependent on

whether the voltage or temperature inputs are being measured.

The analog input range of the voltage inputs is 1V to 5V, the

analog input range of the temperature inputs is 0V to 5V. The

ideal transfer characteristic is shown in Figure 9.

Table 5. LSB Sizes for Each Analog Input Range

Selected

inputs

Input

Range

Full-Scale

Range

LSB Size

Voltage

1 V to 5 V

4 V/4096

976

µV

Temperature

0 V to 5 V

5 V/4096

1.22 mV

111...111

111...110

111...000

011...111

1V + 1LSB

AGND + 1LSB

5V – 1LSB

4V INPUT RANGE

5V – 1LSB

5V INPUT RANGE

ANALOG INPUT

000...010

000...001

000...000

Figure 9. Transfer Characteristic

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