Preliminary Technical Data

AD7794

REV.PrE 6/04 | Page 19

Table 11. Configuration Register Bit Designations

Bit

Location

Bit Name

Description

CON15–

CON14

0

Bias Voltage Generator Enable. The negative terminal of the analog inputs can b e biased up to VDD/2.

VBIAS1

VBIAS0

Bias Voltage

0

0

Bias Voltage Generator Disabled

0

1

Bias Voltage Generator connected to AIN1(-)

1

0

Bias Voltage Generator connected to AIN2(-)

1

1

Bias Voltage Generator connected to AIN3(-)

CON13

BO

This bit must be programmed with a Logic 0 for correct operation.

Burnout Current Enable Bit. When this bit is set to 1 by the user, the 100 nA current sources in the signal

path are enabled. When BO = 0, the burnout currents are disabled. The burnout currents can be enabled

only when the buffer or in-amp is active.

CON12

U/B

Unipolar/Bipolar Bit. Set by user to enable unipolar coding, i.e., zero differential input will result in

0x000000 output and a full-scale differential input will result in 0xFFFFFF output. Cleared by the user to

enable bipolar coding. Negative full-scale differential input will result in an output code of 0x000000,

zero differential input will result in an output code of 0x800000, and a positive full-scale differential

input will result in an output code of 0xFFFFFF.

CON11

0

This bit must be programmed with a Logic 0 for correct operation.

CON10-

CON8

G2-G0

Gain Select Bits.

Written by the user to select the ADC input range as follows

G2

G1

G0

Gain

ADC Input Range (2.5V Reference)

0

0

0

1 (In-Amp not used)

2.5 V

0

0

1

2 (In-Amp not used)

1.25 V

0

1

0

4

625 mV

0

1

1

8

312.5 mV

1

0

0

16

156.2 mV

1

0

1

32

78.125 mV

1

1

0

64

39.06 mV

1

1

1

128

19.53 mV

CON7-

CON6

REFSEL1/REFSEL0

Reference Select Bits. The reference source for the ADC is selected using these bits.

REFSEL1

REFSEL0

Reference Source

0

0

External Reference applied between REFIN1(+) and REFIN1(-)

0

1

External Reference applied between REFIN2(+) and REFIN2(-)

1

0

Internal 1.17 V Reference

1

1

Reserved

CON5

REF_DET

Enables the Reference Detect Function.

When set, the NOXREF bit in the status register indicates when the external reference being used by the

ADC is open circuit or less than 0.5 V.

When cleared, the reference detect function is disabled.

CON4

BUF

Configures the ADC for buffered or unbuffered mode of operation. If cleared, the ADC operates in

unbuffered mode, lowering the power consumption of the device. If set, the ADC operates in buffered

mode, allowing the user to place source impedances on the front end without contributing gain errors

to the system. For gains of 1 and 2, the buffer can be enabled or disabled. For higher gains, the buffer is

automatically enabled.

CON3-

CON0

CH3-CH0

Channel Select bits.

Written by the user to select the active analog input channel to the ADC.

CH3

CH2

CH1

CH0

Channel

Calibration Pair

0

0

0

0

AIN1(+) – AIN1(-)

0

0

0

0

1

AIN2(+) – AIN2(-)

1

0

0

1

0

AIN3(+) – AIN3(-)

2

0

0

1

1

AIN4(+) – AIN4(-)

3