AD7293 datasheet(21/78 Pages) AD

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Data Sheet

AD7293

Rev. B | Page 21 of 78

THEORY OF OPERATION

ANALOG-TO-DIGITAL CONVERTER (ADC)

OVERVIEW

The AD7293 provides the user with a multichannel multiplexer,

an on-chip track-and-hold, and a successive approximation

ADC based around a capacitive DAC. The analog input range

for the ADC is selectable as a 0 V to REFADC, 0 V to 2 × REFADC,

or 0 V to 4 × REFADC input single-ended input, where REFADC =

1.25 V.

The various monitored and uncommitted input signals are

multiplexed into the ADC. The AD7293 has four uncommitted

analog input channels, VIN0 to VIN3.

ADC TRANSFER FUNCTIONS

The designed code transitions occur at successive integer least

significant bit (LSB) values (1 LSB, 2 × LSB, and so on). The

reference voltage for the ADC is referred from the main 2.5 V

reference through an amplifier that attenuates the voltage by

one half. REFADC = 1.25 V.

In single-ended mode, the LSB size is REFADC/4096 when the

0 V to REFADC range is selected, 2 × REFADC/4096 when the 0 V

to 2 × REFADC range is selected, and 4 × REFADC/4096 when the

0 V to 4 × REFADC range is selected (which is the default value).

Figure 35 shows the ideal transfer characteristic for the ADC

when outputting straight binary coding.

Figure 35. Single-Ended Transfer Characteristics

In differential mode, the LSB size is 2 × REFADC/4096 when the

0 V to REFADC range is selected, 4 × REFADC/4096 when the 0 V to

2 × REFADC range is selected, and 8 × REFADC/4096 when the 0 V

to 4 × REFADC range is selected. Figure 36 shows the ideal transfer

characteristic for the ADC when outputting differential coding

(with the 2 × REFADC range).

Figure 36. Differential Transfer Characteristics

Table 12. Code Transition and Voltage

Code

Transition

Single-Ended

Voltage (VIN)

Differential Voltage

(VIN+ − VIN−)

0x000 to 0x001

REFADC × Range/4096

−REFADC × Range ×

2047/2048

0x7FF to 0x800

REFADC × Range/2

0 V

0xFFE to 0xFFF

REFADC × Range ×

4095/4096

+REFADC × Range ×

2047/2048

For VIN0 to VIN3 in single-ended mode, the output code is

straight binary, and the ideal input voltage is given by

VIN = ((Code + 0.5)/ 4096) × REFADC × Range

The differential code is shown in Table 12, and the associated

voltage is calculated by

VIN+ − VIN− = ((Code − 2047.5)/2048) × REFADC × Range

where:

Code is the decimal equivalent of the binary code read from the

ADC register.

REFADC = 1.25 V.

Range = 1 when in the 0 V to REFADC range.

Range = 2 when in the 0 V to 2 × REFADC range.

Range = 4 when in the 0 V to 4 × REFADC range.

Table 13. ADC Range Selected vs. LSB Size

Range

Value

Single-Ended

ADC LSB

Differential

ADC LSB

4 × REFADC1

4 × REFADC/4096

8 × REFADC/4096

2 × REFADC1

2 × REFADC/4096

4 × REFADC/4096

2 × REFADC1

2 × REFADC/4096

4 × REFADC/4096

REFADC1

REFADC/4096

2 × REFADC/4096

REFADC = 1.25 V.

000...000

111...111

1LSB

REFADC – 1LSB

ANALOG INPUT

000...001

000...010

111...110

111...000

011...111

1LSB = REFADC/4096

NOTES

1.REFADC IS REFADC, 2 × REFADC, OR 4 × REFADC.

ANALOG INPUT

100...000

011...111

REFADC – 1LSB

–REFADC + 1LSB

100...001

100...010

011...110

000...001

000...000

111...111

1LSB = 2 × REFADC/4096

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