Low Power, 24-Bit Sigma-Delta ADC with In-
Amp and Embedded Reference (6 Channel)
Preliminary Technical Data
AD7794


REV.PrE 6/04.
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FEATURES
Six Differential Analog Inputs
Low Noise Programmable Gain Instrumentation-Amp
RMS noise: 80 nV (Gain = 64)
Bandgap Reference with 5 ppm/ C Drift typ
Power
Supply: 2.7 V to 5.25 V operation
Normal: 400 µA typ
Power-down: 1 µA max
Update Rate: 4 Hz to 500 Hz
Simultaneous 50 Hz/60 Hz Rejection
Internal Clock Oscillator
Reference Detect
Programmable Current Sources (10 µA/200 µA/1 mA)
On-Chip Bias Voltage Generator
100 nA Burnout Currents
Low Side Power Switch
Independent Interface Power Supply
24-Lead TSSOP Package

INTERFACE
3-wire serial
SPI®, QSPI™, MICROWIRE™, and DSP compatible
Schmitt trigger on SCLK
APPLICATIONS
Temperature measurement
Pressure measurement
Weigh scales

GENERAL DESCRIPTION
The AD7794 is a low power, complete analog front end for
low frequency measurement applications. It contains a low
noise 24-bit ∑-∆ ADC with six differential inputs. The on-chip
low noise instrumentation amplifier means that signals of small
amplitude can be interfaced directly to the ADC.
The device contains a precision low noise, low drift internal
reference for absolute measurements. An external reference can
also be used if ratiometric measurements are required. Other
on-chip features include programmable excitation current
sources and a bias voltage generator for temperature applica-
tions along with 100 nA burnout currents. For pressure and
weighscale applications, a low-side power switch is available to
power down the bridge between conversions to minimize the
power consumption of the system. The device can be operated
with the internal clock or, alternatively, an external clock can be
used if synchronizing several devices. The output data rate
from the part is software programmable and can be varied from
4 Hz to 500 Hz.
The part operates with a power supply from 2.7 V to 5.25 V. It
consumes a current of 450 uA maximum and is housed in a 24-
lead TSSOP package.
FUNCTIONAL BLOCK DIAGRAM
AD7794
SERIAL
INTERFACE
AND
CONTROL
LOGIC
INTERNAL
CLOCK
CLK
SIGMA DELTA
ADC
AIN1(+)
AIN1(-)
AIN5(+)/IOUT2
AIN5(-)/IOUT1
MUX
IN-AMP
REFIN1(+) REFIN1(-)
BANDGAP
REFERENCE
GND
V
DD
GND
V
DD
PSW
REFERENCE
DETECT
AIN2(+)
AIN2(-)
AIN6(+)/P1
AIN6(-)/P2
AIN3(+)
AIN3(-)
AIN4(+)/REFIN2(+) AIN4(-)/REFIN2(-)
V
BIAS
GND
GND AV
DD
DOUT/RDY
DIN
SCLK
CS
DVDD
TEMP
SENSOR