3-Channel, Low Noise, Low Power,
A
www.BDTIC.com/ADI
16-/24-Bit, Σ-Δ ADC with On-Chip In-Amp
FEATURES
RMS noise:
27 nV at 4.17 Hz (AD7799)
65 nV at 16.7 Hz (AD7799)
40 nV at 4.17 Hz (AD7798)
85 nV at 16.7 Hz (AD7798)
Current: 380 μA typical
Power-down: 1 μA maximum
Low noise, programmable gain, instrumentation amp
Update rate: 4.17 Hz to 470 Hz
3 differential inputs
Internal clock oscillator
Simultaneous 50 Hz/60 Hz rejection
Reference detect
Low-side power switch
Programmable digital outputs
Burnout currents
Power supply: 2.7 V to 5.25 V
–40°C to +105°C temperature range
Independent interface power supply
16-lead TSSOP package
INTERFACE
3-wire serial
SPI®, QSPI™, MICROWIRE™, and DSP compatible
Schmitt trigger on SCLK
APPLICATIONS
Weigh scales
Pressure measurement
Strain gauge transducers
Gas analysis
Industrial process control
Instrumentation
Portable instrumentation
Blood analysis
Smart transmitters
Liquid/gas chromotography
6-digit DVM
AD7798/AD7799
FUNCTIONAL BLOCK DIAGRAM
AIN1(+)
AIN1(–)
AIN2(+)
AIN2(–)
IN3(+)/P1
AIN3(–)/P2
PSW
GNDAV
DD
AD7798/AD7799
AV
DD
MUX
GND
IN-AMP
GND
INTERNAL
CLOCK
GENERAL DESCRIPTION
The AD7798/AD7799 are low power, low noise, complete
analog front ends for high precision measurement applications.
The AD7798/AD7799 contains a low noise, 16-/24-bit ∑-Δ
ADC with three differential analog 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 27 nV for the AD7799 and 40 nV
for the AD7798 when the update rate equals 4.17 Hz.
On-chip features include a low-side power switch, reference
detect, programmable digital output pins, burnout currents,
and an internal clock oscillator. The output data rate from the
part is software-programmable and can be varied from 4.17 Hz
to 470 Hz.
The part operates with a power supply from 2.7 V to 5.25 V.
The AD7798 consumes a current of 300 μA typical, whereas the
AD7799 consumes 380 μA typical. Both devices are housed in a
16-lead TSSOP package.
REFIN(+)
Σ-Δ
ADC
Figure 1.
REFIN(–)
AD7798: 16-BIT
AD7799: 24-BIT
REFERENCE
DETECT
SERIAL
INTERFACE
AND
CONTROL
LOGIC
DOUT/RDY
DIN
SCLK
CS
DV
DD
04856-001
Rev. A
Information furnished by Analog Devices is believed to be accurate and reliable. However, no
responsibility is assumed by Anal og Devices for its use, nor for any infringements of patents or ot her
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 owners.
One Technology Way, P.O. Box 9106, Norwood, MA 02062-9106, U.S.A.
Tel: 781.329.4700 www.analog.com
Fax: 781.461.3113 ©2005–2007 Analog Devices, Inc. All rights reserved.
AD7798/AD7799
www.BDTIC.com/ADI
TABLE OF CONTENTS
Features.............................................................................................. 1
Interface ............................................................................................. 1
Applications....................................................................................... 1
Functional Block Diagram .............................................................. 1
General Description ......................................................................... 1
Revision History ............................................................................... 2
Specifications..................................................................................... 3
Timing Characteristics..................................................................... 6
Absolute Maximum Ratings............................................................ 8
ESD Caution.................................................................................. 8
Pin Configuration and Function Descriptions............................. 9
Output Noise and Resolution Specifications .............................. 10
AD7798........................................................................................ 10
AD7799........................................................................................ 11
Typical Performance Characteristics ........................................... 12
On-Chip Registers.......................................................................... 13
Communication Register ..........................................................13
Status Register............................................................................. 14
Mode Register ............................................................................. 14
Configuration Register ..............................................................16
Data Register............................................................................... 17
ID Register................................................................................... 17
IO Register................................................................................... 17
Offset Register ............................................................................ 17
Full-Scale Register...................................................................... 17
ADC Circuit Information.............................................................. 18
Overview ..................................................................................... 18
Digital Interface.......................................................................... 19
Circuit Description......................................................................... 22
Analog Input Channel............................................................... 22
Instrumentation Amplifier........................................................ 22
Bipolar/Unipolar Configuration .............................................. 22
Data Output Coding .................................................................. 23
Burnout Currents....................................................................... 23
Reference ..................................................................................... 23
Reference Detect......................................................................... 23
Reset ............................................................................................. 23
AV
Monitor ............................................................................. 24
DD
Calibration................................................................................... 24
Grounding and Layout.............................................................. 25
Applications Information.............................................................. 26
Weigh Scales................................................................................ 26
Outline Dimensions....................................................................... 27
Ordering Guide .......................................................................... 27
REVISION HISTORY
3/07—Rev. 0 to Rev. A
Updated Format..................................................................Universal
Changes to Specifications................................................................ 3
Changes to Table 5 and Table 6..................................................... 10
Changes to Table 7 and Table 8..................................................... 11
Changes to Table 14........................................................................ 15
Changes to Ordering Guide.......................................................... 27
1/05—Revision 0: Initial Version
Rev. A | Page 2 of 28
AD7798/AD7799
www.BDTIC.com/ADI
SPECIFICATIONS
AVDD = 2.7 V to 5.25 V; DVDD = 2.7 V to 5.25 V; GND = 0 V; REFIN(+) = AVDD; REFIN(−) = 0 V. All specifications T
otherwise noted.
Table 1.
Parameter AD7798B/AD7799B1Unit Test Conditions/Comments
ADC CHANNEL
Output Update Rate 4.17 − 470 Hz nom
No Missing Codes
2
24 Bits min AD7799: f
< 242 Hz
ADC
16 Bits min AD7798
Resolution See Tab le 5 to Tabl e 8
Output Noise and Update Rates See Table 5 to Tab le 8
Integral Nonlinearity ±15 ppm of FSR max
Offset Error
Offset Error Drift vs. Temperature
Full-Scale Error
Gain Drift vs. Temperature
3
3, 5
4
±1 μV typ
4
±10 nV/°C typ
±10 μV typ
±1 ppm/°C typ
Power Supply Rejection 100 dB min AIN = 1 V/gain, gain ≥ 4
ANALOG INPUTS
Differential Input Voltage Ranges ±V
Absolute AIN Voltage Limits
2
/gain V nom V
REF
= REFIN(+) – REFIN(–), gain = 1 to 128
REF
MIN
to T
MAX
, unless
Unbuffered Mode GND − 30 mV V min Gain = 1 or 2
AVDD + 30 mV V max
Buffered Mode GND + 100 mV V min Gain = 1 or 2
AVDD – 100 mV V max
In-Amp Active GND + 300 mV V min Gain = 4 to 128
AVDD − 1.1 V max
Common-Mode Voltage, VCM 0.5 V min VCM = (AIN(+) + AIN(−))/2, gain = 4 to 128
Analog Input Current
Buffered Mode or In-Amp Active
Average Input Current
2
±1 nA max Gain = 1 or 2, update rate < 100 Hz
±250 pA max Gain = 4 to 128, update rate < 100 Hz
±1 nA max AIN3(+)/AIN3(−), update rate < 100 Hz
Average Input Current Drift ±2 pA/°C typ
Unbuffered Mode Gain = 1 or 2
Average Input Current ±400 nA/V typ Input current varies with input voltage
Average Input Current Drift ±50 pA/V/°C typ
Normal Mode Rejection
2
@ 50 Hz, 60 Hz 65 dB min 80 dB typ, 50 ± 1 Hz, 60 ± 1 Hz (FS[3:0] = 1010)
@ 50 Hz 80 dB min 90 dB typ, 50 ± 1 Hz (FS[3:0] = 1001)
@ 60 Hz 90 dB min 100 dB typ, 60 ± 1 Hz (FS[3:0] = 1000)
6
6
Common-Mode Rejection
@ DC 100 dB min AIN = 1 V/gain, gain ≥ 4
@ 50 Hz, 60 Hz
2
100 dB min 50 ± 1 Hz, 60 ± 1 Hz (FS[3:0] = 1010)
@ 50 Hz, 60 Hz2 100 dB min
50 ± 1 Hz (FS[3:0] = 1001
(FS[3:0] = 1000
6
)
6
), 60 ± 1 Hz
6
6
Rev. A | Page 3 of 28
AD7798/AD7799
www.BDTIC.com/ADI
Parameter AD7798B/AD7799B1Unit Test Conditions/Comments
REFERENCE
External REFIN Voltage 2.5 V nom REFIN = REFIN(+) − REFIN(−)
Reference Voltage Range2 0.1 V min
AVDD V max
When V
limited to (0.9 x V
Absolute REFIN Voltage Limits
AV
2
GND − 30 mV V min
+ 30 mV V max
DD
Average Reference Input Current 400 nA/V typ
Average Reference Input Current Drift ±0.03 nA/V/°C typ
Normal Mode Rejection
Same as for analog
inputs
Common-Mode Rejection 100 dB typ
Reference Detect Levels 0.3 V min
0.65 V max NOXREF bit active if V
LOW-SIDE POWER SWITCH
RON 7 Ω max AVDD = 5 V
9 Ω max AVDD = 3 V
Allowable Current
2
30 mA max Continuous current
DIGITAL OUTPUTS (P1 and P2)
Output High Voltage, V
Output Low Voltage, V
Output High Voltage, V
Output Low Voltage, V
2
OH
2
OL
2
4 V min AVDD = 5 V, I
OH
2
OL
AVDD − 0.6 V min AVDD = 3 V, I
0.4 V max AVDD = 3 V, I
0.4 V max AVDD = 5 V, I
INTERNAL CLOCK
Frequenc y
2
64 ± 3% kHz min/max
LOGIC INPUTS
2
CS
Input Low Voltage, V
Input High Voltage, V
SCLK and DIN
(Schmitt-Triggered Input)
0.8 V max DVDD = 5 V
INL
INH
2
0.4
2.0
V max
V min
DV
DV
VT(+) 1.4/2 V min/max DVDD = 5 V
VT(–) 0.8/1.7 V min/max DVDD = 5 V
VT(+) – VT(–) 0.1/0.17 V min/max DVDD = 5 V
VT(+) 0.9/2 V min/max DVDD = 3 V
VT(–) 0.4/1.35 V min/max DVDD = 3 V
VT(+) − VT(–) 0.06/0.13 V min/max DVDD = 3 V
Input Currents ±10 μA max VIN = DVDD or GND
Input Capacitance 10 pF typ All digital inputs
LOGIC OUTPUTS
Output High Voltage, V
Output Low Voltage, V
Output High Voltage, V
Output Low Voltage, V
2
OH
2
OL
2
OH
2
OL
DVDD − 0.6 V min DVDD = 3 V, I
0.4 V max DVDD = 3 V, I
4 V min DVDD = 5 V, I
0.4 V max DVDD = 5 V, I
Floating-State Leakage Current ±10 μA max
Floating-State Output Capacitance 10 pF typ
Data Output Coding Offset binary
= AVDD, the differential input must be
REF
= 3 V
DD
= 3 V or 5 V
DD
/gain) if the in-amp is active.
REF
< 0.3 V
REF
= 100 μA
SOURCE
= 100 μA
SINK
= 200 μA
SOURCE
= 800 μA
SINK
= 100 μA
SOURCE
= 100 μA
SINK
= 200 μA
SOURCE
= 1.6 mA
SINK
Rev. A | Page 4 of 28
AD7798/AD7799
www.BDTIC.com/ADI
Parameter AD7798B/AD7799B1Unit Test Conditions/Comments
SYSTEM CALIBRATION
Full-Scale Calibration Limit 1.05 × FS V max
Zero-Scale Calibration Limit −1.05 × FS V min
Input Span 0.8 × FS V min
2.1 × FS V max
POWER REQUIREMENTS
Power Supply Voltage
AVDD – GND 2.7/5.25 V min/max
DVDD – GND 2.7/5.25 V min/max
Power Supply Currents
IDD Current 140 μA max
180 μA max
400 μA max
500 μA max
IDD (Power-Down Mode) 1 μA max
1
Temperature range is –40°C to +105°C. At the 19.6 Hz and 39.2 Hz update rates, the INL, power supply rejection (PSR), common-mode rejection (CMR), and normal
mode rejection (NMR) do not meet the data sheet specification if the voltage on the AIN(+) or AIN(−) pins exceeds AVDD − 1.6 V typically. When this voltage is
exceeded, the INL, for example, is reduced to 18 ppm of FS typically and the PSR is reduced to 69 dB typically. Therefore, for guaranteed performance at these update
rates, the absolute voltage on the analog input pins needs to be below AVDD − 1.6 V.
2
Specification is not production tested, but is supported by characterization data at initial product release.
3
Following a calibration, this error is in the order of the noise for the programmed gain and update rate selected.
4
Recalibration at any temperature removes these errors.
5
Full-scale error applies to both positive and negative full-scale and applies at the factory calibration conditions (AVDD = 4 V, gain = 1, TA = 25°C).
6
FS[3:0] are the four bits used in the mode register to select the output word rate.
7
Digital inputs equal to DVDD or GND.
2
FS = Full-scale analog input. When V
differential input must be limited to (0.9 × V
= AVDD, the
REF
/gain)
REF
if the in-amp is active.
7
Unbuffered mode, 110 μA typ @ AV
125 μA typ @ AV
DD
= 5 V
Buffered mode, gain = 1 or 2, 130 μA typ @ AV
165 μA typ @ AV
DD
= 5 V
AD7798: gain = 4 to 128, 300 μA typ @ AV
350 μA typ @ AV
DD
= 5 V
AD7799: gain = 4 to 128, 380 μA typ @ AV
440 μA typ @ AV
DD
= 5 V
= 3 V,
DD
= 3 V,
DD
= 3 V,
DD
DD
= 3 V,
Rev. A | Page 5 of 28
AD7798/AD7799
www.BDTIC.com/ADI
TIMING CHARACTERISTICS
AVDD = 2.7 V to 5.25 V, DVDD = 2.7 V to 5.25 V, GND = 0 V, Input Logic 0 = 0 V, Input Logic 1 = DVDD, unless otherwise noted.
Table 2.
Parameter
1, 2
Limit at T
MIN
, T
(B Version) Unit Conditions/Comments
MAX
t3 100 ns min SCLK high pulse width
t4 100 ns min SCLK low pulse width
Read Operation
t1 0 ns min
falling edge to DOUT/RDY active time
CS
60 ns max DVDD = 4.75 V to 5.25 V
80 ns max DVDD = 2.7 V to 3.6 V
3
t
2
0 ns min SCLK active edge to data valid delay
4
60 ns max DVDD = 4.75 V to 5.25 V
80 ns max DVDD = 2.7 V to 3.6 V
5, 6
t
5
10 ns min
Bus relinquish time after CS
inactive edge
80 ns max
t6 0 ns min
t7 10 ns min
SCLK inactive edge to CS
SCLK inactive edge to DOUT/RDY
inactive edge
high
Write Operation
t8 0 ns min
falling edge to SCLK active edge setup time
CS
4
t9 30 ns min Data valid to SCLK edge setup time
t10 25 ns min Data valid to SCLK edge hold time
t11 0 ns min
1
Sample tested during initial release to ensure compliance. All input signals are specified with tR = tF = 5 ns (10% to 90% of DVDD) and timed from a voltage level of 1.6 V.
2
See Figure 3 and Figure 4.
3
These times are measured with the load circuit of Figure 2 and defined as the time required for the output to cross the VOL or VOH limits.
4
SCLK active edge is the falling edge of SCLK.
5
These times are derived from the measured time taken by the data output to change 0.5 V when loaded with the circuit of Figure 2. The measured time is then
extrapolated back to remove the effects of charging or discharging the 50 pF capacitor. This means that the times quoted in the timing characteristics are the true bus
relinquish times of the part and, as such, are independent of external bus loading capacitances.
6
RDY
returns high after a read of the ADC. In single-conversion mode and continuous-conversion mode, data can be reread, if required, while
should be taken to ensure that subsequent reads do not occur close to the next output update. In continuous read mode, the digital word can be read only once.
rising edge to SCLK edge hold time
CS
RDY
is high, but care
I
(1.6mA WITH DVDD = 5V,
SINK
100µA WIT H DV
DD
= 3V)
TO
OUTPUT
PIN
50pF
I
SOURCE
100µA WIT H DV
Figure 2. Load Circuit for Timing Characterization
Rev. A | Page 6 of 28
1.6V
(200µA WIT H DVDD = 5V,
= 3V)
DD
4856-002
AD7798/AD7799
S
www.BDTIC.com/ADI
CS (I)
t
t
1
DOUT/RDY (O)
SCLK (I)
t
2
I = INPUT, O = OUTPUT
MSB LSB
t
3
t
4
Figure 3. Read Cycle Timing Diagram
CS (I)
6
t
5
t
7
04856-003
t
11
04856-004
CLK (I)
DIN (I)
I = INPUT, O = OUTPUT
t
8
t
9
t
10
MSB LSB
Figure 4. Write Cycle Timing Diagram
Rev. A | Page 7 of 28
AD7798/AD7799
www.BDTIC.com/ADI
ABSOLUTE MAXIMUM RATINGS
TA = 25°C, unless otherwise noted.
Table 3.
Parameter Rating
AVDD to GND −0.3 V to +7 V
DVDD to GND −0.3 V to +7 V
Analog Input Voltage to GND −0.3 V to AVDD + 0.3 V
Reference Input Voltage to GND −0.3 V to AVDD + 0.3 V
Digital Input Voltage to GND −0.3 V to DVDD + 0.3 V
Digital Output Voltage to GND −0.3 V to DVDD + 0.3 V
AIN/Digital Input Current 10 mA
Operating Temperature Range −40°C to +85°C
Storage Temperature Range −65°C to +150°C
Maximum Junction Temperature 150°C
TSSOP
θJA Thermal Impedance 128°C/W
θJC Thermal Impedance 14°C/W
Lead Temperature, Soldering
Vapor Phase (60 sec) 215°C
Infrared (15 sec) 220°C
Stresses above those listed under Absolute Maximum Ratings
may cause permanent damage to the device. This is a stress
rating only; functional operation of the device at these or any
other conditions above those listed in the operational sections
of this specification is not implied. Exposure to absolute
maximum rating conditions for extended periods may affect
device reliability.
ESD CAUTION
Rev. A | Page 8 of 28
AD7798/AD7799
www.BDTIC.com/ADI
PIN CONFIGURATION AND FUNCTION DESCRIPTIONS
1
SCLK
2
CS
AIN3(+)/P1
AIN3(–)/P2
3
AD7798/
4
AD7799
TOP VIEW
5
AIN1(+)
AIN1(–)
AIN2(+)
AIN2(–) REFI N(+)
(Not to Scale)
6
7
8
Figure 5. Pin Configuration
16
DIN
15
DOUT/RDY
14
DV
13
AV
12
GND
11
PSW
10
REFIN(–)
9
DD
DD
4856-005
Table 4. Pin Function Descriptions
Pin No. Mnemonic Description
1 SCLK
Serial Clock Input. This serial clock input is f
or data transfers to and from the ADC. The SCLK has a Schmitt-triggered
input, making the interface suitable for opto-isolated applications. The serial clock can be continuous, with all data
transmitted in a continuous train of pulses. Alternatively, it can be noncontinuous, with the information transmitted
to or from the ADC in smaller batches of data.
2
Chip Select Input. This is an active low logic input used to select the ADC. CS can be used to select the ADC in
CS
systems with more than one device on the serial bus, or it can be used as a frame synchronization signal when
communicating with the device. CS can be hardwired low, allowing the ADC to operate in 3-wire mode, with SCLK,
DIN, and DOUT/RDY
3 AIN3(+)/P1
Analog Input/Digital Output Pin. AIN3(+) is the positive termi
Alternatively, this pin can function as a general-purpose output bit referenced between AV
4 AIN3(−)/P2
Analog Input/Digital Output Pin. AIN3(−) is the negative terminal of the differential analog inp
Alternatively, this pin can function as a general-purpose output bit referenced between AV
used to interface with the device.
nal of the differential analog input pair AIN3(+)/AIN3(−).
and GND
DD
ut pair AIN3(+)/AIN3(−).
and GND
DD
5 AIN1(+) Analog Input. AIN1(+) is the positive terminal of the differential analog input pair AIN1(+)/AIN1(−).
6 AIN1(−) Analog Input. AIN1(−) is the negative terminal of the differential analog input pair AIN1(+)/AIN1(−).
7 AIN2(+) Analog Input. AIN2(+) is the positive terminal of the differential analog input pair AIN2(+)/AIN2(−).
8 AIN2(−) Analog Input. AIN2(−) is the negative terminal of the differential analog input pair AIN2(+)/AIN2(−).
9 REFIN(+)
10 REFIN(−)
Positive Reference Input. An external reference can be applied between REFIN(+) and REFIN(−). REFIN(+) can lie
nywhere between AV
a
can function with a reference from 0.1 V to AV
Negative Reference Input. REFIN(−) is the negative reference inp
between GND and AV
and GND + 0.1 V. The nominal reference voltage (REFIN(+) – REFIN(−)) is 2.5 V, but the part
DD
.
DD
ut for REFIN. This reference input can lie anywhere
− 0.1 V.
DD
11 PSW Low-Side Power Switch to GND.
12 GND Ground Reference Point.
13 AVDD Supply Voltage. 2.7 V to 5.25 V.
14 DVDD
15
DOUT/RDY
Digital Interface Supply Voltage. The logic levels for the serial interface pins are related to this supply, which is
ween 2.7 V and 5.25 V. The DV
bet
with DV
at 3 V, or vice versa.
DD
voltage is independent of the voltage on AVDD; therefore, AVDD can equal 5 V
DD
Serial Data Output/Data Ready Output. DOUT/RDY serves a dual purpose. It functions as a serial data output pin to
access the output shift register of the ADC. The output shift register can contain data from any of the on-chip data
or control registers. In addition, DOUT/RDY
operates as a data ready pin, going low to indicate the completion of a
conversion. If the data is not read after the conversion, the pin goes high before the next update occurs.
The DOUT/RDY
falling edge can be used as an interrupt to a processor, indicating that valid data is available. With
an external serial clock, the data can be read using the DOUT/RDY pin. With CS low, the data/control word
16 DIN
information is placed on the DOUT/RDY
Serial Data Input to the Input Shift Register on the ADC. Data in this
pin on the SCLK falling edge and is valid upon the SCLK rising edge.
shift register is transferred to the control registers
within the ADC, with the register selection bits of the communication register identifying the appropriate register.
Rev. A | Page 9 of 28
AD7798/AD7799
www.BDTIC.com/ADI
OUTPUT NOISE AND RESOLUTION SPECIFICATIONS
AD7798
Tabl e 5 shows the AD7798 output rms noise for some update
rates and gain settings. The numbers given are for the bipolar
input range with a 2.5 V reference. These numbers are typical
and are generated with a differential input voltage of 0 V.
hows the effective resolution, and the output peak-to-peak
s
resolution is shown in parentheses. It is important to note that
Tabl e 6
the effective resolution is calculated using the rms noise, whereas
the peak-to-peak resolution is based on the peak-to-peak noise.
The peak-to-peak resolution represents the resolution for which
there is no code flicker. These numbers are typical and are
rounded to the nearest LSB.
Table 5. Output RMS Noise (μV) vs. Gain and Output U
Update Rate Gain of 1 Gain of 2 Gain of 4 Gain of 8 Gain of 16 Gain of 32 Gain of 64 Gain of 128
4.17 Hz 0.64 0.6 0.29 0.22 0.1 0.065 0.039 0.041
8.33 Hz 1.04 0.96 0.38 0.26 0.13 0.078 0.057 0.055
16.7 Hz 1.55 1.45 0.54 0.36 0.18 0.11 0.087 0.086
33.2 Hz 2.3 2.13 0.74 0.5 0.23 0.17 0.124 0.118
62 Hz 2.95 2.85 0.92 0.58 0.29 0.2 0.153 0.144
123 Hz 4.89 4.74 1.49 1 0.48 0.32 0.265 0.283
242 Hz 11.76 9.5 4.02 1.96 0.88 0.45 0.379 0.397
470 Hz 11.33 9.44 3.07 1.79 0.99 0.63 0.568 0.593
Table 6. Typical Resolution (Bits) vs. Gain and Output Update Rate for the AD7798 Using a 2.5 V Reference
Update Rate Gain of 1 Gain of 2 Gain of 4 Gain of 8 Gain of 16 Gain of 32 Gain of 64 Gain of 128
4.17 Hz 16 (16) 16 (16) 16 (16) 16 (16) 16 (16) 16 (16) 16 (16) 16 (16)
8.33 Hz 16 (16) 16 (16) 16 (16) 16 (16) 16 (16) 16 (16) 16 (16) 16 (16)
16.7 Hz 16 (16) 16 (16) 16 (16) 16 (16) 16 (16) 16 (16) 16 (16) 16 (16)
33.2 Hz 16 (16) 16 (16) 16 (16) 16 (16) 16 (16) 16 (16) 16 (16) 16 (16)
62 Hz 16 (16) 16 (16) 16 (16) 16 (16) 16 (16) 16 (16) 16 (16) 16 (15.5)
123 Hz 16 (16) 16 (16) 16 (16) 16 (16) 16 (16) 16 (16) 16 (15.5) 16 (14.5)
242 Hz 16 (16) 16 (15.5) 16 (15.5) 16 (15.5) 16 (16) 16 (16) 16 (15) 16 (14)
470 Hz 16 (16) 16 (15.5) 16 (16) 16 (16) 16 (15.5) 16 (15.5) 16 (14.5) 15.5 (13.5)
pdate Rate for the AD7798 Using a 2.5 V Reference
Rev. A | Page 10 of 28
AD7798/AD7799
www.BDTIC.com/ADI
AD7799
Tabl e 7 shows the AD7799 output rms noise for some update
rates and gain settings. The numbers given are for the bipolar
input range with a 2.5 V reference. These numbers are typical
and are generated with a differential input voltage of 0 V.
hows the effective resolution, and the output peak-to-peak
s
resolution is given in parentheses. Note that the effective
Tabl e 8
resolution is calculated using the rms noise, whereas the
peak-to-peak resolution is based on peak-to-peak noise. The
peak-to-peak resolution represents the resolution for which
there is no code flicker. These numbers are typical and are
rounded to the nearest LSB.
Table 7. Output RMS Noise (μV) vs. Gain and Output U
Update Rate Gain of 1 Gain of 2 Gain of 4 Gain of 8 Gain of 16 Gain of 32 Gain of 64 Gain of 128
4.17 Hz 0.64 0.6 0.185 0.097 0.075 0.035 0.027 0.027
8.33 Hz 1.04 0.96 0.269 0.165 0.108 0.048 0.037 0.040
16.7 Hz 1.55 1.45 0.433 0.258 0.176 0.085 0.065 0.065
33.2 Hz 2.3 2.13 0.647 0.364 0.24 0.118 0.097 0.094
62 Hz 2.95 2.85 0.952 0.586 0.361 0.178 0.133 0.134
123 Hz 4.89 4.74 1.356 0.785 0.521 0.265 0.192 0.192
242 Hz 11.76 9.5 3.797 2.054 1.027 0.476 0.326 0.308
470 Hz 11.33 9.44 3.132 1.773 1.107 0.5 0.413 0.374
Table 8. Typical Resolution (Bits) vs. Gain and Output Update Rate for the AD7799 Using a 2.5 V Reference
Update Rate Gain of 1 Gain of 2 Gain of 4 Gain of 8 Gain of 16 Gain of 32 Gain of 64 Gain of 128
4.17 Hz 23 (20.5) 22 (19.5) 22.5 (20) 22.5 (20) 22 (19.5) 22 (19.5) 21.5 (19) 20.5 (18)
8.33 Hz 22 (19.5) 21.5 (19) 22 (19.5) 22 (19.5) 21.5 (19) 21.5 (19) 21 (18.5) 20 (17.5)
16.7 Hz 21.5 (19) 20.5 (18) 21.5 (19) 21 (18.5) 21 (18.5) 21 (18.5) 20 (17.5) 19 (16.5)
33.3 Hz 21 (18.5) 20 (17.5) 21 (18.5) 20.5 (18) 20.5 (18) 20.5 (18) 19.5 (17) 18.5 (16)
62 Hz 20.5 (18) 19.5 (17) 20.5 (18) 20 (17.5) 19.5 (17) 19.5 (17) 19 (16.5) 18 (15.5)
123 Hz 20 (17.5) 19 (16.5) 20 (17.5) 19.5 (17) 19 (16.5) 19 (16.5) 18.5 (16) 17.5 (15)
242 Hz 18.5 (16) 18 (15.5) 18.5 (16) 18 (15.5) 18 (15.5) 18.5 (16) 18 (15.5) 17 (14.5)
470 Hz 18.5 (16) 18 (15.5) 18.5 (16) 18.5 (16) 18 (15.5) 18.5 (16) 17.5 (15) 16.5 (14)
pdate Rate for the AD7799 Using a 2.5 V Reference
Rev. A | Page 11 of 28
AD7798/AD7799
www.BDTIC.com/ADI
TYPICAL PERFORMANCE CHARACTERISTICS
30
8388640
8388630
8388620
8388610
CODE
8388600
OCCURRENCE
25
20
15
10
5
8388590
8388580
0 200 400 600 800 999
Figure 6. AD7799 Noise (V
50
40
30
20
OCCURRENCE
10
0
8388581
8388590
SAMPLES
= AVDD/2, Gain = 64, Update Rate = 4.17 Hz)
REF
8388600
CODE
8388610
8388620
Figure 7. AD7799 Noise Distribution Histogram (V
Gain = 64, Update Rate = 4.17 Hz)
8388680
= AVDD/2,
REF
8388630
04856-006
04856-007
8388635
0
8388549
8388560
8388580
8388600
8388620
CODE
Figure 9. AD7799 Noise Distribution Histogram (V
8388640
= AVDD/2,
REF
04856-009
8388669
Gain = 64, Update Rate = 16.7 Hz)
3.0
VDD = 5V
UPDATE RATE = 16.6Hz
= 25°C
T
A
2.5
2.0
1.5
RMS NOISE (μV)
1.0
0.5
0
0 0.5 1.0 1.5 2.0 2.5 3.0 3.5 4.0 4.5 5.0
REFERENCE VOLTAGE (V)
04856-010
Figure 10. RMS Noise vs. Reference Voltage (Gain = 1)
8388660
8388640
8388620
CODE
8388600
8388580
8388560
8388540
Figure 8. AD7799 Noise (V
0 200 400 600 800 999
SAMPLES
= AVDD/2, Gain = 64, Update Rate = 16.7 Hz)
REF
04856-008
Rev. A | Page 12 of 28
AD7798/AD7799
www.BDTIC.com/ADI
ON-CHIP REGISTERS
The ADC is controlled and configured via a number of on-chip registers, which are described on the following pages. In the following
descriptions, set implies a Logic 1 state and cleared implies a Logic 0 state, unless otherwise stated.
COMMUNICATION REGISTER
RS2, RS1, RS0 = 0, 0, 0
The communication register is an 8-bit, write-only register. All communication to the part must start with a write operation to the
communication register. The data written to the communication register determines whether the next operation is a read or write
operation, and to which register this operation takes place. After the read or write operation is complete, the interface returns to its
default state, where it expects a write operation to the communication register. In situations where the interface sequence is lost, a write
operation of at least 32 serial clock cycles with DIN high returns the ADC to this default state by resetting the entire part.
e bit designations for the communication register. CR0 through CR7 indicate the bit location, with CR denoting that the bits are in the
th
communication register. CR7 denotes the first bit of the data stream. The number in parentheses indicates the power-on/reset default
status of that bit.
CR7 CR6 CR5 CR4 CR3 CR2 CR1 CR0
WEN(0) R/W(0)
Table 9. Communication Register Bit Designations
Bit Location Bit Name Description
CR7
CR6
CR5 to CR3 RS2 to RS0
CR2 CREAD
CR1 to CR0 0 These bits must be programmed to Logic 0 for correct operation.
WEN
R/W
RS2(0) RS1(0) RS0(0) CREAD(0) 0(0) 0(0)
Write Enable Bit. A 0 must be written to this bit so that the write to the communication register occurs.
If a 1 is the first bit written, the part does not clock subsequent bits into the register. It stays at this bit
location until a 0 is written to this bit. Once a 0 is written to the
to the communication register.
Read/Write Bit. A 0 in this bit location indicates that the next operation is a write to a specified register.
A 1 in this position indicates that the next operation is a read from the designated register.
Register Address Bits. These bits are used to select the register during the serial interface communication.
ee Tab le 10.
S
Continuous Read of the Data Register Bit. When this bit is set t
serial interface is configured so that the data register can be continuously read, that is, the contents of
the data register are placed on the DOUT pin automatically when the SCLK pulses are applied after the
pin goes low to indicate that a conversion is complete. The communication register does not have
RDY
to be written to for data reads. To enable continuous read mode, the instruction 01011100 must be
written to the communication register. To exit the continuous read mode, the instruction 01011000
must be written to the communication register while the
the ADC monitors activity on the DIN line for the instruction to exit continuous read mode. Additionally,
a reset occurs if 32 consecutive 1s are seen on DIN. Therefore, DIN should be held low in continuous read
mode until an instruction is to be written to the device.
RDY pin is low. While in continuous read mode,
bit, the next seven bits are loaded
WEN
o 1 and the data register is selected, the
Tabl e 9 outlines
Table 10. Register Selection
RS2 RS1 RS0 Register Register Size
0 0 0 Communication register during a write operation 8 bits
0 0 0 Status register during a read operation 8 bits
0 0 1 Mode register 16 bits
0 1 0 Configuration register 16 bits
0 1 1 Data register 16 bits (AD7798)/24 bits (AD7799)
1 0 0 ID register 8 bits
1 0 1 IO register 8 bits
1 1 0 Offset register 16 bits (AD7798)/24 bits (AD7799)
1 1 1 Full-scale register 16 bits (AD7798)/24 bits (AD7799)
Rev. A | Page 13 of 28
AD7798/AD7799
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STATUS REGISTER
RS2, RS1, RS0 = 0, 0, 0; Power-On/Reset = 0x80 (AD7798)/0x88 (AD7799)
The status register is an 8-bit, read-only register. To access the status register, the user must write to the communication register, select the
next operation to be a read, and load Bit RS2, Bit RS1, and Bit RS0 with 0. Tab l e 11 outlines the bit designations for the status register. SR0
rough SR7 indicate the bit locations, with SR denoting that the bits are in the status register. SR7 denotes the first bit of the data stream.
th
The number in parentheses indicates the power-on/reset default status of the bit.
SR7 SR6 SR5 SR4 SR3 SR2 SR1 SR0
RDY(1)
Table 11. Status Register Bit Designations
Bit Location Bit Name Description
SR7
SR6 ERR
SR5 NOREF
SR4 0 This bit is automatically cleared.
SR3 0/1 This bit is automatically cleared on the AD7798 and automatically set on the AD7799.
SR2 to SR0 CH2 to CH0 These bits indicate which channel is being converted by the ADC.
MODE REGISTER
RS2, RS1, RS0 = 0, 0, 1; Power-On/Reset = 0x000A
The mode register is a 16-bit register from which data can be read or to which data can be written. This register is used to select the
operating mode, update rate, and low-side power switch. Tab l e 12 outlines the bit designations for the mode register. MR0 through MR15
ndicate the bit locations, with MR denoting that the bits are in the mode register. MR15 denotes the first bit of the data stream. The
i
number in parentheses indicates the power-on/reset default status of that bit. A write to the mode register resets the modulator and filter
and sets the
ERR(0) NOREF(0) 0(0) 0/1 CH2(0) CH1(0) CH0(0)
Ready Bit. Cleared when data is written to the data register. Set after the data register is read or after a period
of time before the data register is updated with a new conversion result to indicate to the user not to read the
conversion data. It is also set when the part is placed in power-down mode. The end of a conversion is
indicated by the DOUT/RDY
the ADC for conversion data.
Error Bit. This bit is written to at the same time as the RDY
data register is clamped to all 0s or all 1s. Error sources include overrange and underrange. Cleared by a write
operation to start a conversion.
No Reference Bit. Set to indicate that the reference (REFIN) is at a voltage below a specified threshold. When
NOREF is set, c
the reference pins. The NOREF bit is enabled by setting the REF_DET bit in the configuration register to 1.
onversion results are clamped to all 1s. Cleared to indicate that a valid reference is applied to
pin. This pin can be used as an alternative to the status register for monitoring
bit. Set to indicate that the result written to the
RDY
RDY
bit.
MR15 MR14 MR13 MR12 MR11 MR10 MR9 MR8
MD2(0) MD1(0) MD0(0) PSW(0) 0(0) 0(0) 0(0) 0(0)
MR7 MR6 MR5 MR4 MR3 MR2 MR1 MR0
0(0) 0(0) 0(0) 0(0) FS3(1) FS2(0) FS1(1) FS0(0)
Table 12. Mode Register Bit Designations
Bit Location Bit Name Description
MR15 to MR13 MD2 to MD0 Mode Select Bits. These bits select the operational mode of the AD7798/AD7799 (see Table 13).
MR12 PSW
MR11 to MR4 0 These bits must be programmed with a Logic 0 for correct operation.
MR3 to MR0 FS3 to FS0 Filter Update Rate Select Bits (see Table 1 4).
Power Switch Control Bit. Set by user to close the pow
sink up to 30 mA. Cleared by user to open the power switch. When the ADC is placed in power-down
mode, the power switch is opened.
Rev. A | Page 14 of 28
er switch PSW to GND. The power switch can
AD7798/AD7799
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Table 13. Operating Modes
MD2 MD1 MD0 Mode
0 0 0
0 0 1
0 1 0
0 1 1 Power-Down Mode. In this mode, all AD7798/AD7799 circuitry is powered down, including the burnout currents.
1 0 0
1 0 1
1 1 0
1 1 1
Continuous-Conversion Mode (Default). In continuous-conversion mode, the ADC continuously performs conversions
and plac
change, or a write to the mode, configuration, or IO registers, the first conversion is available after a period of 2/f
and subsequent conversions are available at a frequency of f
Single-Conversion Mode. When single-conversion mode is selected, the ADC powers up and performs a single
c
a time of 2/f
down mode. The conversion remains in the data register and RDY
another conversion is performed.
Idle Mode. In idle mode, the ADC filter and modulator are held in a reset state, although the modulator clocks are
still p
Internal Zero-Scale Calibration. An internal short is automa
takes two conversion cycles to complete. RDY
calibration is complete. The ADC is placed in idle mode following a calibration. The measured offset coefficient is
placed in the offset register of the selected channel.
Internal Full-Scale Calibration. A full-scale input voltage is automatically connected to the selected analog input for
this cali
conversion cycles are required to perform the full-scale calibration. RDY
and returns low when the calibration is complete. The ADC is placed in idle mode following a calibration. The
measured full-scale coefficient is placed in the full-scale register of the selected channel. Internal full-scale
calibrations cannot be performed when the gain equals 128. The ADC is factory-calibrated at a gain of 128 and this
factory-generated value is placed in the full-scale register on power up and when the gain is set to 128. With this
gain setting, a system full-scale calibration can be performed. To minimize the full-scale error, a full-scale calibration
is required each time the gain of a channel is changed.
System Zero-Scale Calibration. Users should connect the system z
selected by the CH2 to CH0 bits. A system offset calibration takes two conversion cycles to complete. RDY
when the calibration is initiated and returns low when the calibration is complete. The ADC is placed in idle mode
following a calibration. The measured offset coefficient is placed in the offset register of the selected channel. A
zero-scale calibration is required each time the gain of a channel is changed.
System Full-Scale Calibration. Users should connect the system full-sca
by the CH2 to CH0 bits. A calibration takes two conversion cycles to complete. RDY
is initiated and returns low when the calibration is complete. The ADC is placed in idle mode following a calibration.
The measured full-scale coefficient is placed in the full-scale register of the selected channel. A full-scale calibration
is required each time the gain of a channel is changed.
es the result in the data register. RDY
onversion. The oscillator requires 1 ms to power up and settle. The ADC then performs the conversion, which takes
. The conversion result is placed in the data register, RDY goes low, and the ADC returns to power-
ADC
rovided.
bration. When the gain equals 1, a calibration takes two conversion cycles to complete. For higher gains, four
goes low when a conversion is complete. After power-on, a channel
.
ADC
remains active (low) until the data is read or
tically connected to the enabled channel. A calibration
goes high when the calibration is initiated and returns low when the
goes high when the calibration is initiated
ero-scale input to the channel input pins as
goes high
le input to the channel input pins, as selected
goes high when the calibration
ADC
,
Table 14. Update Rates Available
FS3 FS2 FS1 FS0 f
0 0 0 0 Reserved
0 0 0 1 470 4
0 0 1 0 242 8
0 0 1 1 123 16
0 1 0 0 62 32
0 1 0 1 50 40
0 1 1 0 39 48
0 1 1 1 33.2 60
1 0 0 0 19.6 101 90 dB (60 Hz only)
1 0 0 1 16.7 120 80 dB (50 Hz only)
1 0 1 0 16.7 120 65 dB
1 0 1 1 12.5 160 66 dB
1 1 0 0 10 200 69 dB
1 1 0 1 8.33 240 70 dB
1 1 1 0 6.25 320 72 dB
1 1 1 1 4.17 480 74 dB
(Hz) t
ADC
(ms) Rejection @ 50 Hz/60 Hz
SETTLE
Rev. A | Page 15 of 28
AD7798/AD7799
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CONFIGURATION REGISTER
RS2, RS1, RS0 = 0, 1, 0; Power-On/Reset = 0x0710
The configuration register is a 16-bit register from which data can be read or to which data can be written. This register is used to
configure the ADC for unipolar or bipolar mode, to enable or disable the buffer, to enable or disable the burnout currents, to select the
gain, and to select the analog input channel. Tab l e 1 5 outlines the bit designations for the filter register. CON0 through CON15 indicate
th
e bit locations, with CON denoting that the bits are in the configuration register. CON15 denotes the first bit of the data stream. The
number in parentheses indicates the power-on/reset default status of the bit.
CON15 CON14 CON13 CON12 CON11 CON10 CON9 CON8
0(0) 0(0) BO(0)
CON7 CON6 CON5 CON4 CON3 CON2 CON1 CON0
0(0) 0(0) REF_DET(0) BUF(1) 0(0) CH2(0) CH1(0) CH0(0)
U/B
(0)
Table 15. Configuration Register Bit Designations
Bit Location Bit Name Description
CON15 to CON14 0 These bits must be programmed with a Logic 0 for correct operation.
CON13 BO Burnout Current Enable Bit. When this bit is set to 1 by the user, the 100 nA current sources in the signal path
CON12
CON11 0 This bit must be programmed with a Logic 0 for correct operation.
CON10 to CON8 G2 to G0 Gain Select Bits. Written to by the user to select the ADC input range as follows:
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 to CON6 0 These bits must be programmed with a Logic 0 for correct operation.
CON5 REF_DET Enables the reference detect function. When REF_DET is set, the NOREF bit in the status register indicates
CON4 BUF Configures the ADC for buffered or unbuffered modes. If BUF is cleared, the ADC operates in unbuffered
CON3 0 This bit must be programmed with a Logic 0 for correct operation.
CON2 to CON0 CH2 to CH0 Channel Select Bits. Written to by the user to select the active analog input channel to the ADC as follows:
0 0 0 AIN1(+) – AIN1(–) 0
0 0 1 AIN2(+) – AIN2(–) 1
0 1 0 AIN3(+) – AIN3(–) 2
0 1 1 AIN1(–) – AIN1(–) 0
1 0 0 Reserved
1 0 1 Reserved
1 1 0 Reserved
1 1 1 AVDD monitor Automatically selects gain = 1/6 and internal
U/
B
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.
Unipolar/Bipolar Bit. Set by the user to enable unipolar coding, that is, zero differential input results in
0x000000 output, and a full-scale differential input results in 0xFFFFFF output. Cleared by the user to enable
bipolar coding. Negative full-scale differential input results in an output code of 0x000000, zero differential
input results in an output code of 0x800000, and a positive full-scale differential input results in an output
code of 0xFFFFFF.
G2 G1 G0 Gain ADC Input Range (2.5 V Reference)
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.
mode, lowering the power consumption of the device. If BUF is 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.
The buffer can be disabled when the gain equals 1 or 2. For higher gains, the buffer is automatically enabled.
With the buffer disabled, the voltage on the analog input pins can range from 30 mV below GND to 30 mV
above AV
must be limited to 100 mV within the power supply rails.
CH2 CH1 CH0 Channel Calibration Pair
. When the buffer is enabled, it requires some headroom; therefore, the voltage on any input pin
DD
0(0) G2(1) G1(1) G0(1)
reference = 1.17 V
Rev. A | Page 16 of 28
AD7798/AD7799
www.BDTIC.com/ADI
DATA REGISTER
RS2, RS1, RS0 = 0, 1, 1; Power-On/Reset = 0x0000(00)
The conversion result from the ADC is stored in the data register. This is a read-only register. Upon completion of a read operation from
RDY
this register, the
bit and DOUT/
ID REGISTER
RS2, RS1, RS0 = 1, 0, 0; Power-On/Reset = 0xX8 (AD7798)/0xX9 (AD7799)
The identification number for the AD7798/AD7799 is stored in the ID register. This is a read-only register.
IO REGISTER
RS2, RS1, RS0 = 1, 0, 1; Power-On/Reset = 0x00
The IO register is an 8-bit register from which data can be read or to which data can be written. This register is used to select the function
of the AIN3(+)/AIN3(−) pins. Table 16 outlines the bit designations for the IO register. IO0 through IO7 indicate the bit locations, with
IO denoting that the bits are in the IO register. IO7 denotes the first bit of the data stream. The number in parentheses indicates the
power-on/reset default status of that bit.
IO7 IO6 IO5 IO4 IO3 IO2 IO1 IO0
0(0) IOEN(0) IO2DAT(0) IO1DAT(0) 0(0) 0(0) 0(0) 0(0)
Table 16. IO Register Bit Designations
Bit Location Bit Name Description
IO7 0 This bit must be programmed with a Logic 0 for correct operation.
IO6 IOEN
IO5, IO4 IO2DAT, IO1DAT
IO3 to IO0 0 These bits must be programmed with a Logic 0 for correct operation.
OFFSET REGISTER
RS2, RS1, RS0 = 1, 1, 0; Power-On/Reset = 0x8000(AD7798)/0x800000 (AD7799)
Each analog input channel has a dedicated offset register that holds the offset calibration coefficient for the channel. This register is
16 bits wide on the AD7798 and 24 bits wide on the AD7799, and its power-on/reset value is 8000(00) hex. The offset 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
internal or system zero-scale calibration is initiated by the user. The offset register is a read/write register. However, the AD7798/AD7799
must be in idle mode or power-down mode when writing to the offset register.
RDY
pin are set.
Configures the pins AIN3(+)/P1 and AIN3(−
this bit is set, the pins are configured as Digital Output Pins P1 and P2. When this bit is cleared,
these pins are configured as analog input pins AIN3(+) and AIN3(−).
P1/P2 Data. When IOEN is set, the data for the Digital O
and Bit IO2DAT.
)/P2 as analog input pins or digital output pins. When
utput Pins P1 and P2 is written to Bit IO1DAT
FULL-SCALE REGISTER
RS2, RS1, RS0 = 1, 1, 1; Power-On/Reset = 0x5XXX (AD7798)/0x5XXX00 (AD7799)
The full-scale register is a 16-bit register on the AD7798 and a 24-bit register on the AD7799. The full-scale register holds the full-scale
calibration coefficient for the ADC. The AD7798/AD7799 has three full-scale registers, with each channel having a dedicated full-scale
register. The full-scale registers are read/write registers. However, when writing to the full-scale registers, users must place the ADC in
power-down mode or idle mode. Upon power-on, these registers are configured with factory-calibrated, full-scale calibration coefficients,
with the calibration performed at gain = 128, the default gain setting. The default value is automatically overwritten if an internal or
system full-scale calibration is initiated by the user, or the full-scale register is written to.
Rev. A | Page 17 of 28
AD7798/AD7799
www.BDTIC.com/ADI
ADC CIRCUIT INFORMATION
AV
DD
IN+
OUT– OUT+
IN–
GND AV
REFIN(+)
AIN1(+)
AIN1(–)
AIN2(+)
AIN2(–)
REFIN(–)
PSW
DD
AD7798/AD7799
AV
MUX
GND
DD
Figure 11. Basic Connection Diagram
OVERVIEW
The AD7798/AD7799 are low power ADCs that each incorporate
a ∑-Δ modulator, a buffer, an in-amp, and on-chip digital filtering
intended for the measurement of wide dynamic range, low
frequency signals, such as those in pressure transducers and
weigh scales.
Each part has three differential inputs that can be buffered or
nbuffered. The reference is provided by an external reference
u
source. Figure 11 shows the basic connections required to
o
perate the parts.
The output rate of the AD7798/AD7799 (f
mable. The allowable update rates, along with the corresponding
settling times, are listed in
he major function of the digital filter. Simultaneous 50 Hz and
t
Tabl e 14. Normal mode rejection is
60 Hz rejection is optimized when the update rate equals 16.7 Hz
or less, because notches are placed at both 50 Hz and 60 Hz with
these update rates (see
Figure 13).
The AD7798/AD7799 use slightly different filter types,
ending on the output update rate, so that the rejection of
dep
quantization noise and device noise is optimized. When the
update rate ranges from 4.17 Hz to 12.5 Hz, a sinc3 filter, along
with an averaging filter, is used. When the update rate ranges
from 16.7 Hz to 39 Hz, a modified sinc3 filter is used. This filter
gives simultaneous 50 Hz and 60 Hz rejection when the update
rate equals 16.7 Hz. A sinc4 filter is used when the update rate
ranges from 50 Hz to 242 Hz. Finally, an integrate-only filter is
used when the update rate equals 470 Hz.
Figure 15 show the frequency responses of the different filter
pes for a few of the update rates.
ty
) is user-program-
ADC
Figure 12 through
IN-AMP
INTERNAL
CLOCK
REFERENCE
DETECT
DOUT/RDY
DIN
SCLK
CS
DV
DD
FREQUENCY (Hz)
ADC
(dB)
–100
Σ-Δ
–20
–40
–60
–80
0
0 12010080604020
SERIAL
INTERFACE
AND
CONTROL
LOGIC
Figure 12. Filter Profile with Update Rate = 4.17 Hz
0
–20
–40
(dB)
–60
–80
–100
0 20018016014012010080604020
FREQUENCY (Hz)
Figure 13. Filter Profile with Update Rate = 16.7 Hz
04856-012
04856-013
04856-014
Rev. A | Page 18 of 28
AD7798/AD7799
www.BDTIC.com/ADI
0
–20
–40
(dB)
–60
–80
–100
0 30002500200015001000500
Figure 14. Filter Profile with Update Rate = 242 Hz
0
–10
–20
–30
(dB)
–40
–50
–60
0 10000900080007000600050004000300020001000
Figure 15. Filter Response with Upd
FREQUENCY (Hz)
FREQUENCY (Hz)
ate Rate = 470 Hz
04856-015
04856-016
DIGITAL INTERFACE
As previously outlined, the programmable functions of the
AD7798/AD7799 are controlled using a set of on-chip registers.
Data is written to these registers via the serial interface, which
also provides read access to the on-chip registers. All
communication with the part must start with a write to the
communication register. After power-on or reset, the device
expects a write to its communication register. The data written
to this register determines whether the next operation is a read
or write operation and to which register this operation occurs.
Therefore, write access to any register begins with a write
operation to the communication register, followed by a write to
the selected register. A read operation from any other register
(except when continuous-read mode is selected) starts with a
write to the communication register, followed by a read
operation from the selected register.
The serial interface of the AD7798/AD7799 consists of four
nals:
CS
, DIN, SCLK, and DOUT/
sig
to transfer data into the on-chip registers, and DOUT/
used for accessing data from the on-chip registers. SCLK is the
RDY
. The DIN line is used
RDY
is
serial clock input for the device and all data transfers (either on
DI
N or DOUT/
The DOUT/
RDY
) occur with respect to the SCLK signal.
RDY
pin operates as a data ready signal, with the
line going low when a new data-word is available in the output
register. It is reset high when a read operation from the data
register is complete. It also goes high prior to the updating of
the data register to indicate when not to read from the device to
ensure that a data read is not attempted while the register is
being updated.
CS
is used to select a device. It can be used to
decode the AD7798/AD7799 in systems where several
components are connected to the serial bus.
Figure 3 and Figure 4 show timing diagrams for interfacing to
th
e AD7798/AD7799, with
CS
being used to decode the part.
Figure 3 shows the timing for a read operation from the
AD7798/AD77
99 output shift register, and Figure 4 shows the
timing for a write operation to the input shift register. It is
possible to read the same word from the data register several
times, even though the DOUT/
RDY
line returns high after the
first read operation. However, care must be taken to ensure that
the read operations are complete before the next output update
occurs. In continuous-read mode, the data register can only be
read once.
The serial interface can operate in 3-wire mode by tying
RDY
In this case, the SCLK, DIN, and DOUT/
lines are used to
CS
low.
communicate with the AD7798/AD7799. The end of the con-
RDY
version can be monitored using the
bit in the status regis-
ter. This scheme is suitable for interfacing to microcontrollers.
CS
is required as a decoding signal, it can be generated from a
If
port pin. For microcontroller interfaces, it is recommended that
SCLK idles high between data transfers.
CS
The AD7798/AD7799 can be operated with
being used as a
frame-synchronization signal. This scheme is useful for DSP
interfaces. In this case, the first bit (MSB) is effectively clocked
CS
out by
, because CS normally occurs after the falling edge of
SCLK in DSPs. The SCLK can continue to run between data
transfers, provided that the timing numbers are obeyed.
The serial interface can be reset by writing a series of 1s on the
D
IN input. If a Logic 1 is written to the AD7798/AD7799 line
for at least 32 serial clock cycles, the serial interface is reset.
This ensures that the interface can be reset to a known state if
the interface is lost due to a software error or a glitch in the
system. Reset returns the interface to the state in which it is
expecting a write to the communication register. This operation
resets the contents of all registers to their power-on values.
Following a reset, the user should allow a period of 500 ms
before addressing the serial interface.
The AD7798/AD7799 can be configured to continuously
c
onvert or to perform a single conversion (See Figure 16
th
rough Figure 18).
Rev. A | Page 19 of 28
AD7798/AD7799
www.BDTIC.com/ADI
Single-Conversion Mode
In single-conversion mode, the AD7798/AD7799 is placed in
power-down mode after conversions. When a single conversion
is initiated by setting MD2, MD1, and MD0 to 0, 0, and 1 in the
mode register, the AD7798/AD7799 powers up, performs a
single conversion, and then returns to power-down mode. The
on-chip oscillator requires approximately 1 ms to power up. A
conversion requires a time period of 2 × t
. DOUT/
ADC
RDY
goes
low to indicate the completion of a conversion. When the dataword has been read from the data register, DOUT/
high. If
CS
is low, DOUT/
RDY
remains high until another
RDY
goes
conversion is initiated and completed. The data register can be
read several times if required, even when DOUT/
RDY
is high.
CS
Continuous-Conversion Mode
This is the default power-up mode. The AD7798/AD7799
continuously converts, with the
going low each time a conversion is complete. If
DOUT/
RDY
line also goes low when a conversion is complete.
RDY
bit in the status register
CS
is low, the
To read a conversion, the user can write to the communication
register, indicating that the next operation is a read of the data
register. The digital conversion is placed on the DOUT/
pin as soon as SCLK pulses are applied to the ADC. DOUT/
RDY
RDY
returns high when the conversion is read. The user can reread
this register if required. However, the user must ensure that the
data register is not accessed at the completion of the next
conversion, or the new conversion word is lost.
DIN
DOUT/RDY
SCLK
DIN
DOUT/RDY
CS
0x08 0x200A
0x58
DATA
Figure 16. Single Conversion
0x58 0x58
DATA DATA
04856-017
SCLK
Figure 17. Continuous Conversion
Rev. A | Page 20 of 28
04856-018
AD7798/AD7799
Y
www.BDTIC.com/ADI
Continuous Read
Rather than write to the communication register to access the
data each time a conversion is complete, the AD7798/AD7799
can be configured so that the conversions are placed on the
DOUT/
communication register, the user need only apply the
appropriate number of SCLK cycles to the ADC, and the
16-/24-bit word is automatically placed on the DOUT/
when a conversion is complete. The ADC should be configured
for continuous conversion mode.
When DOUT/
sufficient SCLK cycles must be applied to the ADC, and the
data conversion is placed on the DOUT/
conversion is read, DOUT/
conversion is available. In this mode, the data can only be read
once. In addition, the user must ensure that the data-word is
RDY
line automatically. By writing 01011100 to the
RDY
goes low to indicate the end of a conversion,
RDY
RDY
returns high until the next
CS
RDY
line. When the
line
read before the next conversion is complete. If the user does not
read the conversion before the completion of the next conversion,
or if insufficient serial clocks are applied to the AD7798/AD7799
to read the word, the serial output register is reset when the
next conversion is complete, and the new conversion is placed
in the output serial register.
To exit the continuous-read mode, the instruction 01011000
ust be written to the communication register while the
m
RDY
DOUT/
ADC monitors activity on the DIN line in case the instruction
to exit the continuous-read mode occurs. Additionally, a reset
occurs if 32 consecutive 1s are seen on DIN. Therefore, DIN
should be held low in continuous-read mode until an
instruction is written to the device.
pin is low. While in continuous-read mode, the
DOUT/RD
SCLK
DIN
0x5C
DATA
Figure 18. Continuous Read
DATA DATA
04856-019
Rev. A | Page 21 of 28
AD7798/AD7799
www.BDTIC.com/ADI
CIRCUIT DESCRIPTION
ANALOG INPUT CHANNEL
The AD7798/AD7799 each have three differential analog input
channels. These are connected to the on-chip buffer amplifier
when the devices are operated in buffered mode, and directly to
the modulator when the devices are operated in unbuffered mode.
In buffered mode (the BUF bit in the mode register is set to 1),
the input channel feeds into a high impedance input stage of the
buffer amplifier. Therefore, the input can tolerate significant
source impedances and is tailored for direct connection to
external resistive-type sensors, such as strain gages or resistance
temperature detectors (RTDs).
When BUF = 0, the parts are operated in unbuffered mode.
esults in a higher analog input current. Note that this
This r
unbuffered input path provides a dynamic load to the driving
source. Therefore, resistor/capacitor combinations on the input
pins can cause gain errors, depending on the output impedance
of the source that is driving the ADC input.
allowable external resistance/capacitance values for unbuffered
mode such that no gain error at the 20-bit level is introduced.
Table
17. External Resistance/Capacitance Combination for
Unbuffered Mode (Without 20-Bit Gain Error)
Capacitance (pF) Resistance (Ω)
50 9 k
100 6 k
500 1.5 k
1000 900
5000 200
The AD7798/AD7799 can be operated in unbuffered mode only
when the gain equals 1 or 2. At higher gains, the buffer is automatically enabled. The absolute input voltage range in buffered
mode is restricted to a range between GND + 100 mV and
AV
– 100 mV. When the gain is set to 4 or higher, the in-amp
DD
is enabled. The absolute input voltage range when the in-amp is
active is restricted to a range between GND + 300 mV and
AV
− 1.1 V. Care must be taken in setting up the common-
DD
mode voltage so that these limits are not exceeded; otherwise,
linearity and noise performance degrade.
The absolute input voltage in unbuffered mode includes the
ange between GND − 30 mV and AV
r
being unbuffered. The negative absolute input voltage limit
allows the possibility of monitoring small true bipolar signals
with respect to GND.
+ 30 mV as a result of
DD
Ta bl e
17 shows the
INSTRUMENTATION AMPLIFIER
When the gain equals 4 or higher, the output from the buffer is
applied to the input of the on-chip instrumentation amplifier.
This low noise in-amp means that signals of small amplitude
can be gained within the AD7798/AD7799 while still maintaining
excellent noise performance. For example, when the gain is set
to 64 and the update rate equals 4.17 Hz, the rms noise is 27 nV
typically for the AD7799, which is equivalent to 25.5 bits effective
resolution, or 20 bits peak-to-peak resolution when V
The AD7798/AD7799 can be programmed to have a gain of 1, 2,
, 16, 32, 64, or 128 using Bit G2 to Bit G0 in the configuration
4, 8
register. Therefore, with a 2.5 V reference, the unipolar ranges are
from (0 mV to 19.53 mV) to (0 V to 2.5 V), and the bipolar
ranges are from ±19.53 mV to ±2.5 V. When the in-amp is active
(gain ≥ 4), the common-mode voltage (AIN(+) + AIN(−))/2 must
be greater than or equal to 0.5 V.
If the AD7798/AD7799 operate with a reference that has a value
ual to AV
eq
V
/gain when the in-amp is active for correct operation.
REF
, the analog input signal must be limited to 90% of
DD
= 5 V.
REF
BIPOLAR/UNIPOLAR CONFIGURATION
The analog input to the AD7798/AD7799 can accept either
unipolar or bipolar input voltage ranges. A bipolar input range
does not imply that the parts can tolerate negative voltages with
respect to system GND. Unipolar and bipolar signals on the
AIN(+) input are referenced to the voltage on the AIN(–) input.
For example, if AIN(−) is 2.5 V and the ADC is configured for
unipolar mode and a gain of 1, the input voltage range on the
AIN(+) pin is 2.5 V to 5 V.
If the ADC is configured for bipolar mode, the analog input range
n the AIN(+) input is 0 V to 5 V. The bipolar/unipolar option is
o
chosen by programming the U/
B
bit in the configuration register.
Rev. A | Page 22 of 28
AD7798/AD7799
www.BDTIC.com/ADI
DATA OUTPUT CODING
When the ADC is configured for unipolar operation, the output
code is natural (straight) binary with a zero differential input
voltage resulting in a code of 00...00, a midscale voltage resulting
in a code of 100...000, and a full-scale input voltage resulting in
a code of 111...111. The output code for any analog input voltage
can be represented as
N
Code = (2
When the ADC is configured for bipolar operation, the output
code is offset binary, with a negative full-scale voltage resulting
in a code of 000...000, a zero differential input voltage resulting
in a code of 100...000, and a positive full-scale input voltage
resulting in a code of 111...111. The output code for any analog
input voltage can be represented as
Code = 2
where:
AIN is t
he analog input voltage.
N = 16 for the AD7798, and N = 24 for the AD7799.
× AIN × GAIN)/V
N – 1
× [(AIN × GAIN/V
REF
REF
) + 1]
BURNOUT CURRENTS
The AD7798/AD7799 each contain two 100 nA constant
current generators—one sourcing current from AV
AIN(+), and one sinking current from AIN(−) to GND. The
currents are switched to the selected analog input pair. Both
currents are either on or off, depending on the burnout current
enable (BO) bit in the configuration register. These currents can
be used to verify that an external transducer is still operational
before attempting to take measurements on that channel. Once
the burnout currents are turned on, they flow into the external
transducer circuit, and a measurement of the input voltage on
the analog input channel can be taken. If the resultant voltage
measured is full scale, the user must determine why this is the
case. A full-scale reading could mean that the front-end sensor
is open circuit, that the front-end sensor is overloaded and is
justified in outputting full scale, or that the reference is absent
and, thus, clamping the data to all 1s.
When reading all 1s from the output, the user should check
t
hese three cases before making a judgment. If the voltage
measured is 0 V, it might indicate that the transducer has shortcircuited. For normal operation, these burnout currents are
turned off by writing a 0 to the BO bit in the configuration
register. The current sources work over the normal absolute
input voltage range specifications with buffers on.
DD
to
REFERENCE
The common-mode range for these differential inputs is from
GND to AV
excessive resistance/capacitance source impedances introduce
gain errors. The reference voltage REFIN (REFIN(+) − REFIN(−))
is 2.5 V nominal, but the AD7798/AD7799 are functional with
reference voltages from 0.1 V to AV
excitation (voltage or current) for the transducer on the analog
input also drives the reference voltage for the part, the effect of
. The reference input is unbuffered; therefore,
DD
. In applications where the
DD
the low frequency noise in the excitation source is removed
because the application is ratiometric. If the AD7798/AD7799
are used in a nonratiometric application, a low noise reference
should be used.
Recommended 2.5 V reference voltage sources for the AD7798/
AD779
9 include the ADR381 and ADR391, which are low noise,
low power references. Also note that the reference inputs provide
a high impedance, dynamic load. Because the input impedance
of each reference input is dynamic, resistor/capacitor combinations on these inputs can cause dc gain errors, depending on the
output impedance of the source driving the reference inputs.
Reference voltage sources such as those recommended above
or example, ADR391) typically have low output impedances
(f
and are, therefore, tolerant to having decoupling capacitors on
REFIN(+) without introducing gain errors in the system.
Deriving the reference input voltage across an external resistor
means that the reference input sees a significant external source
impedance. External decoupling on the REFIN pins is not
recommended in this type of circuit configuration.
REFERENCE DETECT
The AD7798/AD7799 include on-chip circuitry to detect if
there is a valid reference for conversions or calibrations. This
feature is enabled when the REF_DET bit in the configuration
register is set to 1. If the voltage between the REFIN(+) and
REFIN(–) pins goes below 0.3 V, or either the REFIN(+) or
REFIN(–) inputs are open circuit, the AD7798/AD7799 detect
that there is no longer a valid reference. In this case, the NOREF
bit of the status register is set to 1. If the AD7798/AD7799 are
performing normal conversions and the NOREF bit becomes
active, the conversion results revert to all 1s. Therefore, it is not
necessary to continuously monitor the status of the NOREF bit
when performing conversions. It is only necessary to verify its
status if the conversion result read from the ADC data register
is all 1s. If the AD7798/AD7799 are performing an offset of fullscale calibration and the NOREF bit becomes active, the updating
of the respective calibration registers is inhibited to avoid loading
incorrect coefficients to these registers, and the ERR bit in the
status register is set. If the user is concerned about verifying that
a valid reference is in place every time a calibration is performed,
the status of the ERR bit should be checked at the end of the
calibration cycle.
RESET
The circuitry and serial interface of the AD7798/AD7799 can
be reset by writing 32 consecutive 1s to the device. This resets
the logic, the digital filter, and the analog modulator, and all
on-chip registers are reset to their default values. A reset is
automatically performed upon power-up. When a reset is
initiated, the user must allow a period of 500 μs before
accessing an on-chip register. A reset is useful if the serial
interface becomes asynchronous due to noise on the SCLK line.
Rev. A | Page 23 of 28
AD7798/AD7799
www.BDTIC.com/ADI
AVDD MONITOR
Along with converting external voltages, the ADC can be used
to monitor the voltage on the AV
equal 1, the voltage on the AV
and the resulting voltage is applied to the ∑-Δ modulator using
an internal 1.17 V reference for analog-to-digital conversion.
This is useful because variations in the power supply voltage
can be monitored.
pin. When Bits CH2 to CH0
DD
pin is internally attenuated by 6,
DD
CALIBRATION
The AD7798/AD7799 provide four calibration modes that can
be programmed via the mode bits in the mode register. These
are internal zero-scale calibration, internal full-scale calibration,
system zero-scale calibration, and system full-scale calibration,
which effectively reduce the offset error and full-scale error to
the order of the noise. After each conversion, the ADC conversion result is scaled using the ADC calibration registers
before being written to the data register. The offset calibration
coefficient is subtracted from the result prior to multiplication
by the full-scale coefficient.
To start a calibration, write the relevant value to the MD2 to
D0 bits in the mode register. After the calibration is complete,
M
the contents of the corresponding calibration registers are
updated, the
RDY
pin goes low (if CS is low), and the AD7798/AD7799
revert to idle mode.
During an internal zero-scale or full-scale calibration, the
re
spective zero-scale and full-scale input are automatically
connected internally to the ADC input pins. A system calibration,
however, expects the system zero-scale and system full-scale
voltages to be applied to the ADC pins before the calibration
mode is initiated. In this way, external ADC errors are removed.
From an operational point of view, a calibration should be
tr
eated like an ADC conversion. A zero-scale calibration (if
required) should always be performed before a full-scale
calibration. System software should monitor the
status register or the DOUT/
calibration via a polling sequence or an interrupt-driven routine.
RDY
bit in the status register is set, the DOUT/
RDY
RDY
pin to determine the end of
bit in the
Both an internal offset calibration and system offset calibration
t
ake two conversion cycles. An internal offset calibration is not
needed because the ADC itself removes the offset continuously.
To perform an internal full-scale calibration, a full-scale input
volt
age is automatically connected to the selected analog input
for this calibration. When the gain equals 1, a calibration takes
two conversion cycles to complete. For higher gains, four
conversion cycles are required to perform the full-scale
calibration. DOUT/
initiated and returns low when the calibration is complete. The
ADC is placed in idle mode following a calibration. The measured
full-scale coefficient is placed in the full-scale register of the
selected channel. Internal full-scale calibrations cannot be
performed when the gain equals 128. A factory calibration
is performed at this gain setting, and the factory value is
automatically loaded into the full-scale register when the gain is
set to 128. With this gain setting, a system full-scale calibration
can be performed. A full-scale calibration is required each time
the gain of a channel is changed to minimize the full-scale error.
An internal full-scale calibration can only be performed at
sp
ecified update rates. For gains of 1, 2, and 4, an internal fullscale calibration can be performed at any update rate. However,
for higher gains, internal full-scale calibrations must be performed
when the update rate is less than or equal to 16.7 Hz, 33.2 Hz,
or 50 Hz. Because the full-scale error does not vary with the
update rate, a calibration at one update rate is valid for all update
rates (assuming the gain or reference source is not changed).
A system full-scale calibration takes two conversion cycles to
co
mplete, irrespective of the gain setting. A system full-scale
calibration can be performed at all gains and update rates. If
system offset calibrations are performed along with system fullscale calibrations, the offset calibration should be performed
before the system full-scale calibration is initiated.
RDY
goes high when the calibration is
Rev. A | Page 24 of 28
AD7798/AD7799
www.BDTIC.com/ADI
GROUNDING AND LAYOUT
Because the analog inputs and reference inputs of the ADC are
differential, most of the voltages in the analog modulator are
common-mode voltages. The excellent common-mode rejection of the parts removes common-mode noise on these inputs.
The digital filter provides rejection of broadband noise on the
power supply, except at integer multiples of the modulator
sampling frequency. The digital filter also removes noise from
the analog and reference inputs, provided that these noise
sources do not saturate the analog modulator. As a result, the
AD7798/AD7799 are more immune to noise interference than
conventional high resolution converters. However, because the
resolution of the AD7798/AD7799 is so high and the noise
levels from the AD7798/AD7799 are so low, care must be taken
with regard to grounding and layout.
The printed circuit board that houses the AD7798/AD7799
s
hould be designed such that the analog and digital sections are
separated and confined to certain areas of the board. A minimum etch technique is generally best for ground planes because
it provides the best shielding.
It is recommended that the GND pin be tied to the AGND plane
f the system. In any layout, it is important that the user keep in
o
mind the flow of currents in the system, ensuring that the return
paths for all currents are as close as possible to the paths the
currents took to reach their destinations. Avoid forcing digital
currents to flow through the AGND sections of the layout.
The ground planes should be allowed to run under the AD7798/
AD7799 t
the AD7798/AD7799 should use as wide a trace as possible to
provide low impedance paths and reduce the effects of glitches
on the power supply line. Fast switching signals, such as clock
signals, should be shielded with digital ground to avoid
radiating noise to other sections of the board, and clock signals
should never be run near the analog inputs.
Avoid crossover of digital and analog signals. Traces on
o
pposite sides of the board should run at right angles to each
other. This reduces the effects of feedthrough through the
board. A microstrip technique works best, but it is not always
possible to use this method with a double-sided board. In this
technique, the component side of the board is dedicated to
ground planes, and signals are placed on the solder side.
Good decoupling is important when using high resolution
AD
parallel with 0.1 μF capacitors to GND. DV
decoupled with 10 μF tantalum in parallel with 0.1 μF
capacitors to the system’s DGND plane, with the system’s
AGND-to-DGND connection being close to the AD7798/
AD7799. To achieve the best from these decoupling components,
they should be placed as close as possible to the device, ideally
right up against the device. All logic chips should be decoupled
with 0.1 μF ceramic capacitors to DGND.
o prevent noise coupling. The power supply lines to
Cs. AV
should be decoupled with 10 μF tantalum in
DD
should be
DD
Rev. A | Page 25 of 28
AD7798/AD7799
www.BDTIC.com/ADI
APPLICATIONS INFORMATION
The AD7798/AD7799 provide a low cost, high resolution
analog-to-digital function. Because the analog-to-digital
function is provided by a ∑-Δ architecture, the parts are more
immune to noisy environments, making them ideal for use in
sensor measurement and industrial and process-control
applications.
WEIGH SCALES
Figure 19 shows the AD7798/AD7799 being used in a weigh
scale application. The load cell is arranged in a bridge network
and gives a differential output voltage between its OUT+ and
OUT– terminals. Assuming a 5 V excitation voltage, the fullscale output range from the transducer is 10 mV when the
sensitivity is 2 mV/V. The excitation voltage for the bridge can
be used to directly provide the reference for the ADC because
the reference input range includes the supply voltage.
A second advantage of using the AD77
based applications is that the low-side power switch can be fully
utilized in low power applications. The low-side power switch is
connected in series with the cold side of the bridge. In normal
AV
DD
98/AD7799 in transducer-
operation, the switch is closed and measurements can be taken.
In applications where power is of concern, the AD7798/AD7799
can be placed in standby mode, thus significantly reducing the
power consumed in the application. In addition, the low-side
power switch can be opened while in standby mode, thus
avoiding unnecessary power consumption by the front-end
transducer. When the part is taken out of standby mode and the
low-side power switch is closed, the user should ensure that the
front end circuitry is fully settled before attempting a read from
the AD7798/AD7799.
In Figure 19, temperature compensation is performed using a
hermistor. In addition, the reference voltage for the temperature
t
measurement is derived from a precision resistor in series with
the thermistor. This allows a ratiometric measurement—that is,
the ratio of the precision reference resistance to the thermistor
resistance is measured; therefore, variations of the reference
voltage do not affect the measurement.
IN+
OUT– OUT+
IN–
GND AV
REFIN(+)
AIN1(+)
AIN1(–)
AIN2(+)
AIN2(–)
REFIN(–)
PSW
Figure 19. Weigh Scales Using the AD7798/AD7799
DD
AD7798/AD7799
AV
MUX
GND
REFERENCE
DETECT
DD
DOUT/ RDY
DIN
SCLK
CS
DV
DD
04856-011
IN-AMP
INTERNAL
CLOCK
Σ-Δ
ADC
SERIAL
INTERFACE
AND
CONTRO L
LOGIC
Rev. A | Page 26 of 28
AD7798/AD7799
www.BDTIC.com/ADI
OUTLINE DIMENSIONS
5.10
5.00
4.90
16
4.50
4.40
4.30
PIN 1
0.15
0.05
0.65
BSC
COPLANARITY
COMPLIANT TO JEDEC STANDARDS MO-153-AB
Figure 20. 16-Lead Thin Shrink S
9
6.40
BSC
81
1.20
MAX
0.30
0.19
0.10
Dimensions shown in millimeters
0.20
0.09
SEATING
PLANE
mall Outline Package [TSSOP]
(RU-16)
8°
0°
0.75
0.60
0.45
ORDERING GUIDE
Model Temperature Range Package Description Package Option
AD7798BRUZ
AD7798BRUZ-REEL
EVAL-AD7798EB Evaluation Boar
AD7799BRU –40°C to +105°C 16-Lead TSSOP RU-16
AD7799BRU-REEL –40°C to +105°C 16-Lead TSSOP RU-16
AD7799BRUZ
AD7799BRUZ-REEL
EVAL-AD7799EB Evaluation Boar
1
Z = RoHS Compliant Part.
1
1
–40°C to +105°C 16-Lead TSSOP RU-16
–40°C to +105°C 16-Lead TSSOP RU-16
d
1
1
–40°C to +105°C 16-Lead TSSOP RU-16
–40°C to +105°C 16-Lead TSSOP RU-16
d
Rev. A | Page 27 of 28
AD7798/AD7799
www.BDTIC.com/ADI
NOTES
©2005–2007 Analog Devices, Inc. All rights reserved. Trademarks and
registered trademarks are the property of their respective owners.
D04856-0-3/07(A)
Rev. A | Page 28 of 28
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