Preliminary Technical Data
FEATURES
AD7798: 16-Bit
AD7799: 24-Bit
Three Differential Analog Inputs
Low Noise Programmable Gain Amp
RMS noise: 80 nV (Gain = 64) at 16.6 Hz update rate (AD7798)
65 nV (Gain = 64) at 16.6 Hz update rate (AD7799)
30 nV (Gain = 64) at 4 Hz update rate (AD7799)
Update Rate: 4 Hz to 500 Hz
Power
Supply: 2.7 V to 5.25 V operation
Normal: 330 µA typ (AD7798)
400 µA typ (AD7799)
Power-down: 1 µA max
Simultaneous 50 Hz/60 Hz Rejection
Two Programmable Digital Outputs
Internal Clock Oscillator
Reference Detect
100 nA Burnout Currents
Low Side Power Switch
Independent Interface Power Supply
16-Lead TSSOP
Resolution:
Low Power, 24-Bit/16-Bit
Sigma-Delta ADC with In-Amp
AD7798/AD7799
FUNCTIONAL BLOCK DIAGRAM
AD7799/AD7798
AVDD
AIN1(+)
AIN1(-)
AIN2(+)
AIN2(-)
AIN3(+)/P1
AIN3(-)/P2
PWRSW
GND
REFERENCE
DETECT
MUX
IN-AMP
SIGMA DELTA
ADC
SERIAL
INTERFACE
AND
CONTROL
LOGIC
DOUT/RDY
DIN
SCLK
CS
GND
INTERNAL
CLOCK
DVDD
Figure 1.
GENERAL DESCRIPTION
The AD7798/AD7799 is a low power, complete analog front end
for low frequency measurement applications. The device
contains a low noise 24-bit (AD7799)/ 16-bit (AD7798) ∑-∆
ADC with three differential inputs. The on-chip low noise
instrumentation amplifier means that signals of small
amplitude can be interfaced directly to the ADC. With a gain
setting of 64, the rms noise is 80 nV for AD7798 and 65 nV for
the AD7799 at 16.6 Hz.
The
device contains a low side power switch which is useful in
bridge applications. The switch allows the bridge to be
disconnected from the power supply when conversions are not
being performed and this will minimise power consumption.
The device also has 100 nA burnout currents. These currents
are used to detect if sensors connected to the analog inputs are
burnt out. Other on-chip features include an internal clock so
the user does not have to supply a clock to the device. This
reduces the component count in a system and provides board
space savings. The update rate is programmable on the
AD7798/99. It can be varied from 4 Hz to 500 Hz.
The part operates with a single power supply from 2.7 V to 5.25
V. It consumes a current of 380 uA maximum for the AD7798
and 450 uA maximum for the AD7799. The AD7799/AD7798 is
housed in a 16-lead TSSOP package.
INTERFACE
3-wire serial
SPI®, QSPI™, MICROWIRE™, and DSP compatible
Schmitt trigger on SCLK
APPLICATIONS
Pressure measurement
Weigh scales
Rev. PrD.
Information furnished by Analog Devices is believed to be accurate and reliable.
However, no responsibility is assumed by Analog Devices for its use, nor for any
infringements of patents or other rights of third parties that may result from its use.
Specifications subject to change without notice. No license is granted by implication
or otherwise under any patent or patent rights of Analog Devices. Trademarks and
registered trademarks are the property of their respective companies.
One Technology Way, P.O. Box 9106, Norwood, MA 02062-9106, U.S.A.
www.analog.com
Tel: 781.329.4700
Fax: 781.326.8703
© 2004 Analog Devices, Inc. All rights reserved.
AD7798/AD7799
TABLE OF CONTENTS
AD7799/AD7798—Specifications.................................................. 3
Timing Characteristics
,
.................................................................... 6
Absolute Maximum Ratings............................................................ 8
Pin Configuration and Function Descriptions............................. 9
Typical Performance Characteristics ........................................... 11
On-chip Registers ........................................................................... 12
Communications Register (RS2, RS1, RS0 = 0, 0, 0) .............. 12
Status Register (RS2, RS1, RS0 = 0, 0, 0; Power-on/Reset =
0x88)............................................................................................. 13
Mode Register (RS2, RS1, RS0 = 0, 0, 1; Power-on/Reset =
0x000A)........................................................................................ 13
Preliminary Technical Data
Configuration Register (rs2, RS1, RS0 = 0, 1, 0; Power-
on/Reset = 0x0710) .................................................................... 15
Data Register (RS2, RS1, RS0 = 0, 1, 1; Power-on/Reset =
0x0000 (AD7798)/ 0x000000 (AD7799))................................ 16
ID Register (RS2, RS1, RS0 = 1, 0, 0; Power-on/Reset = 0xX8
(AD7798)/ 0xX9 (AD7799))..................................................... 16
OFFSET Register (RS2, RS1, RS0 = 1, 1, 0; Power-on/Reset =
0x8000 (AD7798)/0x800000 (AD7799))................................. 17
FULLSCALE Register (RS2, RS1, RS0 = 1, 1, 1; Power-
on/Reset = 0x5XXX (AD7798)/0x5XXX000 (AD7799)) ..... 17
TYPICAL APPLICATION........................................................ 17
REVISION HISTORY
Prelim D, June 2004: Initial Version
REV. PrD. Page 2 of 17
Preliminary Technical Data
AD7799/AD7798—SPECIFICATIONS
1
AD7798/AD7799
Table 1. (AV
DD
= 2.7 V to 5.25 V; DV
DD
= 2.7 V to 5.25 V; GND = 0 V; REFIN(+) = 2.5 V; REFIN(-) = 0 V; all specifications T
MIN
to
T
MAX
, unless otherwise noted.)
Parameter
ADC CHANNEL SPECIFICATION
Output Update Rate
ADC CHANNEL
No Missing Codes
2
Resolution
AD7798/AD7799B
4
500
24
16
16
19
16
18.5
See Tables in ADC
Description
±15
±25
±3
±10
±10
±0.5
±3
90
±REFIN/Gain
GND + 30 mV
AV
DD
– 30 mV
GND + 100 mV
AV
DD
– 100 mV
GND + 300 mV
V
DD
– 1.2
0.5
Unit
Hz min nom
Hz max nom
Bits min
Bits min
Bits p-p
Bits p-p
Bits p-p
Bits p-p
AD7799, f
ADC
≤
125 Hz
AD7798
Gain = 128, 16.6 Hz Update Rate
Gain = 1, 16.6 Hz Update Rate, AD7799
Gain = 1, 16.6 Hz Update Rate, AD7798
Gain = 64, 4 Hz Update Rate, AD7799
Test Conditions/Comments
Output Noise and Update Rates
Integral Nonlinearity
Offset Error
3
Offset Error Drift vs. Temperature
4
Full-Scale Error
5
Gain Drift vs. Temperature
4
Power Supply Rejection
ANALOG INPUTS
Differential Input Voltage Ranges
Absolute AIN Voltage Limits
2
Unbuffered Mode
Buffered Mode
In-Amp Enabled
Common Mode Voltage
In-Amp Enabled
Analog Input Current
Buffered Mode or In-Amp Enabled
Average Input Current
2
ppm of FSR max
ppm of FSR max
µV typ
nV/°C typ
µV typ
ppm/°C typ
ppm/°C typ
dB min
V nom
V max
V min
V min
V max
V min
V max
V min
3.5 ppm typ, Gain 1 to 32
Gain = 64 or 128
Gain = 1, 2
Gain = 4 to 128
100 dB typ, AIN = 50 % of full scale
REFIN = REFIN(+) – REFIN(–), Gain = 1 to 128
Gain = 1 or 2
Gain = 1 or 2
Gain = 4 to 128
Gain = 4 to 128
Average Input Current Drift
Unbuffered Mode
Average Input Current
Average Input Current Drift
Normal Mode Rejection
2
@ 50 Hz, 60 Hz
@ 50 Hz
@ 60 Hz
Common Mode Rejection
@DC
@ 50 Hz, 60 Hz
2
±200
±1
±2
±400
±50
70
84
90
90
100
pA max
nA max
pA/°C typ
nA/V typ
pA/V/°C typ
dB min
dB min
dB min
dB min
dB min
AIN1(+) – AIN1(-), AIN2(+) – AIN2(-) only.
AIN3(+) – AIN3(-).
Gain = 1 or 2
Input current varies with input voltage.
73 dB typ, 50 ± 1 Hz, 60 ± 1 Hz, FS[3:0] = 1010
6
90 dB typ, 50 ± 1 Hz, FS[3:0] = 1001
6
90 dB typ, 60 ± 1 Hz, FS[3:0] = 1000
6
AIN = 50% of FS
80 dB typ, FS[3:0] = 1010
6
50 ± 1 Hz (FS[3:0] = 1001
6
), 60 ± 1 Hz (FS[3:0] =
1000
6
)
REV. PrD. Page 3 of 17
AD7798/AD7799
Parameter
REFERENCE INPUT
REFIN Voltage
Reference Voltage Range
AD7798/AD7799B
2.5
0.1
AV
DD
GND – 30 mV
AV
DD
+ 30 mV
400
±0.03
See ANALOG INPUTS
See ANALOG INPUTS
0.3
0.65
5
7
20
64 ±2%
0.01
Unit
V nom
V min
V max
V min
V max
nA/V typ
nA/V/°C typ
Preliminary Technical Data
Test Conditions/Comments
REFIN = REFIN(+) – REFIN(–)
2
Absolute REFIN Voltage Limits
2
Average reference Input Current
Average Reference Input Current Drift
Normal Mode Rejection
Common Mode Rejection
Reference Detect
2
V min
V max
Ω max
Ω max
mA max
KHz nom
%/°C typ
NOREF bit Inactive if VREF < 0.3 V
NOREF bit Active if VREF > 0.65 V
AV
DD
= 5V
AV
DD
= 3V
Continuous Current
LOW SIDE POWER SWITCH
R
ON
Allowable Current
INTERNAL CLOCK
Drift
LOGIC INPUTS
All Inputs Except SCLK and DIN
V
INL
, Input Low Voltage
V
INH
, Input High Voltage
SCLK and DIN Only (Schmitt-
Triggered Input)
V
T
(+)
V
T
(–)
V
T
(+) – V
T
(–)
V
T
(+)
V
T
(–)
V
T
(+) - V
T
(–)
Input Currents
Input Capacitance
LOGIC OUTPUTS
V
OH
, Output High Voltage
V
OL
, Output Low Voltage
V
OH
, Output High Voltage
V
OL
, Output Low Voltage
2
2
2
2
2
0.8
0.4
2.0
V max
V max
V min
DV
DD
= 5 V
DV
DD
= 3 V
DV
DD
= 3 V or 5 V
1.4/2
0.8/1.4
0.3/0.85
0.9/2
0.4/1.1
0.3/0.85
±1
10
DV
DD
– 0.6
0.4
4
0.4
±1
10
Offset Binary
V min/V max
V min/V max
V min/V max
V min/V max
V min/V max
V min/V max
µA max
pF typ
V min
V max
V min
V max
µA max
pF typ
DV
DD
= 5 V
DV
DD
= 5 V
DV
DD
= 5 V
DV
DD
= 3 V
DV
DD
= 3 V
DV
DD
= 3 V
V
IN
= DV
DD
or GND
All Digital Inputs
DV
DD
= 3 V, I
SOURCE
= 100 µA
DV
DD
= 3 V, I
SINK
= 100 µA
DV
DD
= 5 V, I
SOURCE
= 200 µA
DV
DD
= 5 V, I
SINK
= 1.6 mA
Floating-State Leakage Current
Floating-State Output Capacitance
Data Output Coding
DIGITAL OUTPUTS
P1 and P2
V
OH
, Output High Voltage
2
V
OL
, Output Low Voltage
V
OH
, Output High Voltage
2
V
OL
, Output Low Voltage
2
2
AV
DD
– 0.6
0.4
4
0.4
V min
V max
V min
V max
AV
DD
= 3 V, I
SOURCE
= 100 µA
AV
DD
= 3 V, I
SINK
= 100 µA
AV
DD
= 5 V, I
SOURCE
= 200 µA
AV
DD
= 5 V, I
SINK
= 800 µA
REV. PrD. Page 4 of 17
Preliminary Technical Data
Parameter
SYSTEM CALIBRATION
2
Full-Scale Calibration Limit
Zero-Scale Calibration Limit
Input Span
AD7798/AD7799B
1.05 x FS
-1.05 x FS
0.8 x FS
2.1 x FS
Unit
V max
V min
V min
V max
AD7798/AD7799
Test Conditions/Comments
POWER REQUIREMENTS
7
Power Supply Voltage
V
DD
– GND
IOV
DD
– GND
Power Supply Currents
I
DD
Current
2.7/5.25
2.7/5.25
150
175
380
450
1
V min/max
V min/max
µA max
µA max
µA max
µA max
µA max
125 µA typ, Unbuffered Mode
150 µA typ, Buffered Mode, In-Amp Bypassed
330 µA typ, In-Amp used (AD7798)
400 µA typ, IN-AMP used (AD7799)
I
DD
(Power-Down Mode)
1
2
Temperature Range –40°C to +105°C.
Specification is not production tested but is supported by characterization data at initial product release.
3
A System calibration will reduce this error to the order of the noise for the programmed gain and update rate.
4
A calibration at any temperature will remove this error.
5
Full-scale error applies to both positive and negative full-scale and applies at the factory calibration conditions (AV
DD
= 4 V).
6
FS[3:0] are the four bits used in the mode register to select the output word rate.
7
Digital inputs equal to DV
DD
or GND.
REV. PrD. Page 5 of 17
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