AD7294BCPZRL datasheet(21/48 Pages) AD

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AD7294

Data Sheet

Rev. I | Page 20 of 47

THEORY OF OPERATION

ADC OVERVIEW

The AD7294 provides the user with a 9-channel multiplexer, an

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

based around a capacitive DAC. The analog input range for the

device can be selected as a 0 V to VREF input or a 2 × VREF input,

configured with either single-ended or differential analog inputs.

The AD7294 has an on-chip 2.5 V reference that can be disabled

when an external reference is preferred. If the internal ADC

reference is to be used elsewhere in a system, the output must

first be buffered.

The various monitored and uncommitted input signals are multi-

plexed into the ADC. The AD7294 has four uncommitted

analog input channels, VIN0 to VIN3. These four channels allow

single-ended, differential, and pseudo differential mode

measurements of various system signals.

ADC TRANSFER FUNCTIONS

The designed code transitions occur at successive integer LSB

values (1 LSB, 2 LSB, and so on). In single-ended mode, the

LSB size is VREF/4096 when the 0 V to VREF range is used and

2 × VREF/4096 when the 0 V to 2 × VREF range is used. The ideal

transfer characteristic for the ADC when outputting straight

binary coding is shown in Figure 35.

000...000

111...111

1LSB = VREF/4096

1LSB

VREF – 1LSB

ANALOG INPUT

000...001

000...010

111...110

111...000

011...111

NOTE

1. VREF IS EITHER VREF OR 2 × VREF.

Figure 35. Single-Ended Transfer Characteristic

In differential mode, the LSB size is 2 × VREF/4096 when the 0 V to

VREF range is used, and 4 × VREF/4096 when the 0 V to 2 × VREF

range is used. The ideal transfer characteristic for the ADC when

outputting twos complement coding is shown in Figure 36 (with

the 2 × VREF range).

100...000

011...111

1LSB = 2 × VREF/4096

+VREF – 1LSB

–VREF + 1LSB VREF – 1LSB

ANALOG INPUT

100...001

100...010

011...110

000...001

000...000

111...111

Figure 36. Differential Transfer Characteristic with VREF ± VREF Input Range

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

straight binary, where

VIN = 0 V, DOUT = x000, VIN = VREF − 1 LSB, and DOUT = xFFF

In differential mode, the code is twos complement, where

VIN+ − VIN− = 0 V, and DOUT = x00

VIN+ − VIN− = VREF − 1 LSB, and DOUT = x7FF

VIN+ − VIN− = −VREF, and DOUT = x800

Channel 5 and Channel 6 (current sensor inputs) are twos

complement, where

VIN+ − VIN− = 0 mV, and DOUT = x000

VIN+ − VIN− = VREF/12.5 − 1 LSB, DOUT = x7FF

VIN+ − VIN− = −VREF/12.5, DOUT = x800

Channel 7 to Channel 9 (temperature sensor inputs) are twos

complement with the LSB equal to 0.25°C, where

TIN = 0°C, and DOUT = x000

TIN = +255.75°C, and DOUT = x7FF

TIN = −256°C, and DOUT = x800

ANALOG INPUTS

The AD7294 has a total of four analog inputs. Depending on

the configuration register setup, they can be configured as two

single-ended inputs, two pseudo differential channels, or two

fully differential channels. See the Register Setting section for

further details.

Single-Ended Mode

The AD7294 can have four single-ended analog input channels.

In applications where the signal source has high impedance, it is

recommended to buffer the analog input before applying it to the

ADC. The analog input range can be programmed to be either

0 V to VREF or 0 V to 2 × VREF. In 2 × VREF mode, the input is

effectively divided by 2 before the conversion takes place. Note

that the voltage with respect to GND on the ADC analog input

pins cannot exceed AVDD.

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