Preliminary Technical Data

AD7794

REV.PrE 6/04 | Page 21

Bit Location

Bit Name

Description

IEXCEN0

excitation currents.

IEXCEN1

IEXCEN0

Current Source Value

0

0

Excitation Currents Disabled

0

1

10 uA

1

0

200 uA

1

1

1 mA

OFFSET REGISTER (RS2, RS1, RS0 = 1, 1, 0; POWER-ON/RESET = 0x800000)

The offset register holds the offset calibration coefficient for the ADC. The power-on-reset value of the internal zero-scale calibration

coefficient register is 800000 hex. The AD7794 has 4 offset registers. Channels AIN1 to AIN3 have dedicated offset registers while chan-

nels AIN4, AIN5 and AIN6 share an offset register. Each of these registers is a 24-bit read/write register. This register is used in

conjunction with its associated full-scale register to form a register pair. The power-on-reset value is automatically overwritten if an in-

ternal or system zero-scale calibration is initiated by the user. The AD7794 must be placed in power down mode or idle mode when

writing to the offset register.

FULL-SCALE REGISTER (RS2, RS1, RS0 = 1, 1, 1; POWER-ON/RESET = 0x5XXXX5)

The full-scale registers is a 24-bit register that holds the full-scale calibration coefficient for the ADC. The AD77794 has 4 full-scale

registers. Channels AIN1, AIN2 and AIN3 have dedicated full-scale registers while channels AIN4, AIN5 and AIN6 share a register. The

full-scale registers are read/write registers. However, when writing to the full-scale registers, the ADC must be placed in power down

mode or idle mode. These registers are configured on power-on with factory-calibrated internal full-scale calibration coefficients, the

factory calibration being performed with the gain set to 1 and using the internal reference. Therefore, every device will have different

default coefficients. These default values are used when the device is operated with a gain of 1 and when the internal reference is selected.

For other gains or when the external reference is used at a gain of 1, these default coefficients will be automatically overwritten if an

internal or system full-scale calibration is initiated by the user. A full-scale calibration should be performed when the gain is changed.

TYPICAL APPLICATION (FLOWMETER)

GND

AVDD

AD7794

SERIAL

INTERFACE

AND

CONTROL

LOGIC

INTERNAL

CLOCK

CLK

SIGMA DELTA

ADC

IN-AMP

DOUT/RDY

DIN

SCLK

CS

DVDD

AIN1(+)

AIN1(-)

VDD

GND

VDD

PSW

GND

AIN3(+)

AIN2(+)

AIN2(-)

IN+

IN-

OUT+

OUT-

IN+

IN-

OUT+

OUT-

VDD

REFIN1(+)

REFIN1(-)

AIN3(-)

MUX

RCM

IOUT1