Page 1
A
A
Low Power, 16-Bit and 24-Bit,
FEATURES
AD7788: 16-bit resolution
AD7789: 24-bit resolution
Power
Supply: 2.5 V to 5.25 V operation
Normal: 75 µA maximum
Power-down: 1 µA maximum
RMS noise: 1.5 µV
AD7788: 16-bit p-p resolution
AD7789: 19-bit p-p resolution (21.5 bits effective)
Integral nonlinearity: 3.5 ppm typical
Simultaneous 50 Hz and 60 Hz rejection
Internal clock oscillator
monitor channel
V
DD
10-lead MSOP
INTERFACE
3-wire serial
SPI®-, QSPI™-, MICROWIRE™-, and DSP-compatible
Schmitt trigger on SCLK
APPLICATIONS
Smart transmitters
Battery applications
Portable instrumentation
Sensor measurement
Temperature measurement
Pressure measurement
Weigh scales
4 to 20 mA loops
Sigma-Delta ADCs
AD7788/AD7789
FUNCTIONAL BLOCK DIAGRAM
REFIN(+) REFIN(–) V
AD7788/
AD7789
IN(+)
IN(–)
*AD7788: 16-BIT ADC
AD7789: 24-BIT ADC
Σ-∆
ADC*
Figure 1.
GENERAL DESCRIPTION
The AD7788/AD7789 are low power, low noise, analog front
ends for low frequency measurement applications. The AD7789
contains a low noise, 24-bit, ∑-∆ ADC with one differential
input. The AD7788 is a 16-bit version of the AD7789.
The devices operate from an internal clock. Therefore, the
user does not have to supply a clock source to the devices.
The output data rate is 16.6 Hz, which gives simultaneous
50 Hz/60 Hz rejection.
The parts operate with a single power supply from 2.5 V to
5.25 V. When operating from a 3 V supply, the power dissipation for the part is 225 µW maximum. The AD7788/AD7789
are housed in 10-lead MSOPs.
GND
CLOCK
SERIAL
INTERFACE
AND
CONTROL
LOGIC
DD
DOUT/RDY
DIN
SCLK
CS
03539-001
Rev. A
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 owners.
One Technology Way, P.O. Box 9106, Norwood, MA 02062-9106, U.S.A.
Tel: 781.329.4700
Fax: 781.326.8703 © 2004 Analog Devices, Inc. All rights reserved.
www.analog.com
Page 2
AD7788/AD7789
TABLE OF CONTENTS
AD7789 Specifications..................................................................... 3
AD7788 Specifications..................................................................... 4
AD7788/AD7789 Specifications..................................................... 5
Timing Characteristics..................................................................... 6
Absolute Maximum Ratings............................................................ 8
ESD Caution.................................................................................. 8
Pin Configuration and Function Descriptions............................. 9
Typical Performance Characteristics ...........................................10
On-Chip Registers.......................................................................... 11
Communications Register......................................................... 11
Status Register ............................................................................. 12
Mode Register ............................................................................. 13
Data Register ............................................................................... 13
REVISION HISTORY
ADC Circuit Information.............................................................. 14
Noise Performance ..................................................................... 14
Digital Interface.......................................................................... 14
Circuit Description......................................................................... 17
Analog Input Channel ............................................................... 17
Bipolar/Unipolar Configuration .............................................. 17
Data Output Coding .................................................................. 17
Reference Input........................................................................... 17
V
Monitor ................................................................................ 18
DD
Grounding and Layout .............................................................. 18
Outline Dimensions ....................................................................... 19
Ordering Guide .......................................................................... 19
11/04—Rev. 0 to Rev. A
Updated Format..................................................................Universal
Added Footnote 2 to Integral Nonlinearity A Grade................... 4
Changes to Figure 5.......................................................................... 9
Updated Outline Dimensions....................................................... 19
Changes to Ordering Guide.......................................................... 19
8/03—Revision 0: Initial Version
Rev. A | Page 2 of 20
Page 3
AD7788/AD7789
AD7789 SPECIFICATIONS
VDD = 2.5 V to 5.25 V; REFIN(+) = 2.5 V; REFIN(−) = GND; GND = 0 V; all specifications T
Table 1.
Parameter1 AD7789B Unit Test Conditions/Comments
ADC CHANNEL SPECIFICATION
Output Update Rate 16.6 Hz nom
ADC CHANNEL
No Missing Codes
2
24 Bits min
Resolution 19 Bits p-p
Output Noise 1.5 µV rms typ
Integral Nonlinearity ±15 ppm of FSR max
Offset Error ±3 µV typ
Offset Error Drift vs. Temperature ±10 nV/°C typ
Full-Scale Error
3
±10 µV typ
Gain Drift vs. Temperature ±0.5 ppm/°C typ
Power-Supply Rejection 90 dB min 100 dB typ, AIN = 1 V
ANALOG INPUTS
Differential Input Voltage Ranges ±REFIN V nom REFIN = REFIN(+) – REFIN(−)
Absolute AIN Voltage Limits2 GND − 30 mV V min
V
+ 30 mV V max
DD
Analog Input Current Input current varies with input voltage
Average Input Current2 ±400 nA/V typ
Average Input Current Drift ±50 pA/V/°C typ
Normal-Mode Rejection2
@ 50 Hz, 60 Hz 65 dB min 50 ± 1 Hz, 60 ± 1 Hz
Common-Mode Rejection AIN = 1 V
@ DC 90 dB min 100 dB typ
@ 50 Hz, 60 Hz2 100 dB min 50 ± 1 Hz, 60 ± 1 Hz
REFERENCE INPUT
REFIN Voltage 2.5 V nom REFIN = REFIN(+)− REFIN(−)
Reference Voltage Range
V
Absolute REFIN Voltage Limits
V
2
2
0.1 V min
DD
V max
GND − 30 mV V min
+ 30 mV V max
DD
Average Reference Input Current 0.5 µA/V typ
Average Reference Input Current Drift ±0.03 nA/V/°C typ
Normal-Mode Rejection2
@ 50 Hz, 60 Hz 65 dB min 50 ± 1 Hz, 60 ± 1 Hz
Common-Mode Rejection AIN = 1 V
@ DC 110 dB typ
@ 50 Hz, 60 Hz 110 dB typ 50 ± 1 Hz, 60 ± 1 Hz
1
Temperature range −40°C to +105°C.
2
Specification is not production tested but is supported by characterization data at initial product release.
3
Full-scale error applies to both positive and negative full scale and applies at the factory calibration conditions (VDD = 4 V).
MIN
to T
, unless otherwise noted.
MAX
Rev. A | Page 3 of 20
Page 4
AD7788/AD7789
AD7788 SPECIFICATIONS
VDD = 2.5 V to 5.25 V (B grade); VDD = 2.7 V to 5.25 V (A grade); REFIN(+) = 2.5 V; REFIN(−) = GND; GND = 0 V; all specifications
to T
T
MIN
Table 2.
Parameter1 AD7788 A, AD7788B Unit Test Conditions/Comments
ADC CHANNEL SPECIFICATION
Output Update Rate 16.6 Hz nom
ADC CHANNEL
No Missing Codes
Resolution 16 Bits p-p
Output Noise 1.5 µV rms typ
Integral Nonlinearity ±15 ppm of FSR max B grade
±50 ppm of FSR max A grade2
Offset Error ±3 µV typ
Offset Error Drift vs. Temperature ±10 nV/°C typ
Full-Scale Error
Gain Drift vs. Temperature ±0.5 ppm/°C typ
Power-Supply Rejection 90 dB min B grade
90 dB typ A grade
ANALOG INPUTS
Differential Input Voltage Ranges ±REFIN V nom REFIN = REFIN(+) − REFIN(−)
Absolute AIN Voltage Limits2 GND – 30 mV V min
V
Analog Input Current
Average Input Current2 ±400 nA/V typ
Average Input Current Drift ±50 pA/V/°C typ
Normal-Mode Rejection2
@ 50 Hz, 60 Hz 65 dB min B grade, 50 ± 1 Hz, 60 ± 1 Hz
60 dB min A grade, 50 ± 1 Hz, 60 ± 1 Hz
Common-Mode Rejection AIN = 1 V
@ DC 90 dB min B grade, 100 dB typ
90 dB typ A grade
@ 50 Hz, 60 Hz2 100 dB min B grade, 50 ± 1 Hz, 60 ± 1 Hz
100 dB typ A grade, 50 ± 1 Hz, 60 ± 1 Hz
REFERENCE INPUT
REFIN Voltage 2.5 V nom REFIN = REFIN(+) − REFIN(−)
Reference Voltage Range2 0.1 V min
V
Absolute REFIN Voltage Limits2 GND − 30 mV V min
V
Average Reference Input Current 0.5 µA/V typ
Average Reference Input Current Drift ±0.03 nA/V/°C typ
Normal-Mode Rejection2
@ 50 Hz, 60 Hz 65 dB min B grade, 50 ± 1 Hz, 60 ± 1 Hz
60 dB min A grade
Common-Mode Rejection AIN = 1 V
@ DC 100 dB typ
@ 50 Hz, 60 Hz 110 dB typ 50 ± 1 Hz, 60 ± 1 Hz
1
Temperature range: B grade, −40°C to +105°C; A grade, −40°C to +85°C.
2
Specification is not production tested but is supported by characterization data at initial product release.
3
Full-scale error applies to both positive and negative full scale and applies at the factory calibration conditions (VDD = 4 V).
, unless otherwise noted.
MAX
2
3
16 Bits min
±10 µV typ
+ 30 mV V max
DD
DD
+ 30 mV V max
DD
V max
Input current varies with input
voltage
Rev. A | Page 4 of 20
Page 5
AD7788/AD7789
AD7788/AD7789 SPECIFICATIONS
Table 3.
Parameter AD7788A, AD7788B/AD7789B Unit Test Conditions/Comments
LOGIC INPUTS
All Inputs Except SCLK
V
, Input Low Voltage 0.8 V max VDD = 5 V
INL
0.4 V max VDD = 3 V
V
, Input High Voltage 2.0 V min VDD = 3 V or 5 V
INH
SCLK Only (Schmitt-Triggered Input)
VT(+) 1.4/2 V min/V max VDD = 5 V
VT(−) 0.8/1.4 V min/V max VDD = 5 V
VT(+) − VT(−) 0.3/0.85 V min/V max VDD = 5 V
VT(+) 0.9/2 V min/V max VDD = 3 V
VT(−) 0.4/1.1 V min/V max VDD = 3 V
VT(+) − VT(−) 0.3/0.85 V min/V max VDD = 3 V
Input Currents ±1 µA max VIN = V
Input Capacitance 10 pF typ All digital inputs
LOGIC OUTPUTS
VOH, Output High Voltage1 V
VOL, Output Low Voltage1 0.4 V max VDD = 3 V, I
VOH, Output High Voltage1 4 V min VDD = 5 V, I
VOL, Output Low Voltage1 0.4 V max VDD = 5 V, I
Floating-State Leakage Current ±1 µA max
Floating-State Output Capacitance 10 pF typ
Data Output Coding Offset binary
POWER REQUIREMENTS
Power-Supply Voltage
VDD − GND 2.5/5.25 V min/max AD7789, AD7788B grade
2.7/5.25 V min/max AD7788 A grade
Power-Supply Currents
IDD Current 75 µA max 65 µA typ, VDD = 3.6 V
80 µA max 73 µA typ, VDD = 5.25 V
IDD (Power-Down Mode) 1 µA max
1
Specification is not production tested but is supported by characterization data at initial product release.
2
Digital inputs equal to VDD or GND.
1
2
1
DD
− 0.6 V min VDD = 3 V, I
DD
= 100 µA
SOURCE
= 100 µA
SINK
= 200 µA
SOURCE
= 1.6 mA
SINK
Rev. A | Page 5 of 20
Page 6
AD7788/AD7789
TIMING CHARACTERISTICS
VDD = 2.5 V to 5.25 V (AD7788B and AD7789); VDD = 2.7 V to 5.25 V (AD7788A); GND = 0 V; REFIN(+) = 2.5 V; REFIN(−) = GND;
Input logic 0 = 0 V; Input logic 1 = V
Table 4.
Parameter
t
3
t
4
1, 2
Limit at T
100 ns min SCLK high pulse width
100 ns min SCLK low pulse width
Read Operation
t
1
0 ns min
60 ns max VDD = 4.75 V to 5.25 V
80 ns max VDD = 2.7 V to 3.6 V
3
t
2
0 ns min SCLK active edge to data valid delay
60 ns max VDD = 4.75 V to 5.25 V
80 ns max VDD = 2.7 V to 3.6 V
5, 6
t
5
10 ns min
80 ns max
t
6
t
7
0 ns min
10 ns min
Write Operation
t
8
t
9
t
10
t
11
0 ns min
30 ns min Data valid to SCLK edge setup time
25 ns min Data valid to SCLK edge hold time
0 ns min
, unless otherwise noted.
DD
, T
MIN
(B Version) Unit Conditions/Comments
MAX
CS falling edge to DOUT/RDY active time
4
Bus relinquish time after
SCLK inactive edge to
SCLK inactive edge to DOUT/
CS inactive edge
CS inactive edge
RDY high
CS falling edge to SCLK active edge setup time
CS rising edge to SCLK edge hold time
4
1
Sample tested during initial release to ensure compliance. All input signals are specified with tR = tF = 5 ns (10% to 90% of VDD) and timed from a voltage level of 1.6 V.
2
See Figure 3 and Figure 4.
3
These numbers are measured with the load circuit of, and defined as, the time required for the output to cross the VOL or VOH limits.
4
SCLK active edge is falling edge of SCLK.
5
These numbers 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 number 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, the same data can be read again, if required, while
although care 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.
RDY
is high,
Rev. A | Page 6 of 20
Page 7
AD7788/AD7789
T
I
(1.6mA WITH VDD = 5V,
SINK
100µA WITH V
DD
= 3V)
CS (I)
DOUT/RDY (O)
SCLK (I)
O OUTPUT
PIN
50pF
I
SOURCE
100µA WITH V
1.6V
(200µA WITH VDD = 5V,
= 3V)
DD
Figure 2. Load Circuit for Timing Characterization
t
1
MSB LSB
t
2
t
3
t
I = INPUT, O = OUTPUT
4
Figure 3. Read Cycle Timing Diagram
03539-002
t
6
t
5
t
7
03539-003
CS (I)
t
11
03539-004
SCLK (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 20
Page 8
AD7788/AD7789
ABSOLUTE MAXIMUM RATINGS
TA= 25°C, unless otherwise noted.
Table 5.
Parameter Rating
VDD to GND −0.3 V to +7 V
Analog Input Voltage to GND −0.3 V to VDD + 0.3 V
Reference Input Voltage to GND −0.3 V to VDD + 0.3 V
Total AIN/REFIN Current (Indefinite) 30 mA
Digital Input Voltage to GND −0.3 V to VDD + 0.3 V
Digital Output Voltage to GND −0.3 V to VDD + 0.3 V
Operating Temperature Range
B Grade −40°C to +105°C
A Grade −40°C to +85°C
Storage Temperature Range −65°C to +150°C
Maximum Junction Temperature 150°C
MSOP
θJA Thermal Impedance 206°C/W
θJC Thermal Impedance 44°C/W
Lead Temperature, Soldering (10 sec) 300°C
IR Reflow, Peak Temperature 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
ESD (electrostatic discharge) sensitive device. Electrostatic charges as high as 4000 V readily accumulate on
the human body and test equipment and can discharge without detection. Although this product features
proprietary ESD protection circuitry, permanent damage may occur on devices subjected to high energy
electrostatic discharges. Therefore, proper ESD precautions are recommended to avoid performance
degradation or loss of functionality.
Rev. A | Page 8 of 20
Page 9
AD7788/AD7789
PIN CONFIGURATION AND FUNCTION DESCRIPTIONS
SCLK
CS
AIN(+)
AIN(–)
REFIN(+)
1
AD7788/
2
AD7789
3
TOP VIEW
(Not to Scale)
4
5
10
DIN
9
DOUT/RDY
8
V
DD
GND
7
REFIN(–)
6
03539-005
Figure 5. Pin Configuration
Table 6. Pin Function Descriptions
Pin No. Mnemonic Function
1 SCLK
Serial Clock Input for 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 a noncontinuous clock with the information being transmitted to or from the ADC in smaller batches of data.
2
CS Chip Select Input. This is an active low logic input used to select the ADC. CS can be used to select the ADC in
systems with more than one device on the serial bus or as a frame synchronization signal in communicating
with the device.
RDY used to interface with the device.
DOUT/
CS can be hardwired low, allowing the ADC to operate in 3-wire mode with SCLK, DIN, and
3 AIN(+) Analog Input. AIN(+) is the positive terminal of the fully differential analog input.
4 AIN(−) Analog Input. AIN(–) is the negative terminal of the fully differential analog input.
5 REFIN(+)
Positive Reference Input. REFIN(+) can lie anywhere between V
voltage (REFIN(+)
− REFIN(−)) is 2.5 V, but the part functions with a reference from 0.1 V to V
and GND + 0.1 V. The nominal reference
DD
.
DD
6 REFIN(−) Negative Reference Input. This reference input can lie anywhere between GND and VDD − 0.1 V.
7 GND Ground Reference Point.
8 V
9
DD
DOUT/
10 DIN
Supply Voltage, 3 V or 5 V nominal.
RDY 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/
information is placed on the DOUT/
The end of a conversion is also indicated by the
pin is three-stated, but the
RDY bit remains active.
RDY pin on the SCLK falling edge and is valid on the SCLK rising edge.
RDY bit in the status register. When CS is high, the DOUT/RDY
RDY pin. With CS low, the data/control word
Serial Data Input to the Input Shift Register on the ADC. Data in this shift register is transferred to the control
registers within the ADC, the register selection bits of the communications register identifying the appropriate
register.
Rev. A | Page 9 of 20
Page 10
AD7788/AD7789
TYPICAL PERFORMANCE CHARACTERISTICS
0
–10
–20
–30
–40
–50
–60
dB
–70
–80
–90
–100
–110
–120
0408020 60 100 120 140
FREQUENCY (Hz)
Figure 6. Frequency Response with 16.6 Hz Update Rate
VDD = 3V
V
= 2.048V
REF
70
T
= 25°C
A
RMS NOISE = 1.25µV
60
160
03539-007
8388625
CODE
VDD = 3V, V
T
= 25°C, RMS NOISE = 1.25µV
8388591
0 200 400 600 800
A
REF
= 2.048V,
READ NO.
Figure 8. AD7789 Noise Plot
3.0
VDD = 5V
UPDATE RATE = 16.6Hz
T
= 25°C
A
2.5
1000
03539-009
50
40
30
OCCURENCE
20
10
0
8388591
CODE
Figure 7. AD7789 Noise Histogram
8388625
03539-008
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
V
(V)
REF
Figure 9. AD7788/AD7789 Noise vs. V
03539-013
5.0
REF
Rev. A | Page 10 of 20
Page 11
AD7788/AD7789
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.
COMMUNICATIONS REGISTER
(RS1, RS0 = 0, 0)
The communications register is an 8-bit, write-only register. All communications to the part must start with a write operation to the
communications register. The data written to the communications register determines whether the next operation is a read or write
operation, and to which register this operation takes place. For read or write operations, once the subsequent read or write operation to
the selected register is complete, the interface returns to where it expects a write operation to the communications register. This is the
default state of the interface and, on power-up or after a reset, the ADC is in this default state waiting for a write operation to the
communications 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. Table 7 outlines the bit designations for the communications
register. CR0 through CR7 indicate the bit location, CR denoting the bits are in the communications register. CR7 denotes the first bit of
the data stream. The number in brackets indicates the power-on/reset default status of that bit.
CR7 CR6 CR5 CR4 CR3 CR2 CR1 CR0
WEN(0)
0(0) RS1(0) RS0(0)
R/W(0)
Table 7. Communications Register Bit Designations
Bit Location Bit Name Description
CR7
CR6 0 This bit must be programmed with a Logic 0 for correct operation.
CR5 to CR4 RS1 to RS0
CR3
CR2 CREAD
CR1 to CR0 CH1 to CH0
WEN Write Enable Bit. A 0 must be written to this bit so that the write to the communications register actually
occurs. If a 1 is the first bit written, the part does not clock on to subsequent bits in the register. It stays at
this bit location until a 0 is written to this bit. Once a 0 is written to the
loaded to the communications register.
Register Address Bits. These address bits are used to select which of the ADC’s registers are being selected
during this serial interface communication. See Table 8.
W A 0 in this bit location indicates that the next operation is a write to a specified register. A 1 in this position
R/
indicates that the next operation is a read from the designated register.
Continuous Read of the Data Register. When this bit is set to 1 (and the data register is selected), the serial
interface is configured so that the data register can be continuously read, i.e., the contents of the data
register are placed on the DOUT/
communications register does not have to be written to for data reads. To enable continuous-read mode,
the instruction 001111XX must be written to the communications register. To exit the continuous-read
mode, the instruction 001110XX must be written to the communications register while the DOUT/
is low. While in continuous-read mode, the ADC monitors activity on the DIN line so that it can receive 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.
These bits are used to select the analog input channel. The differential channel can be selected
AIN(+)/AIN(−) or an internal short AIN(−)/AIN(−) can be selected. Alternatively, the power supply can be
selected—that is, the ADC can measure the voltage on the power supply, which is useful for monitoring
power supply variation. The power-supply voltage is divided by 5 and then applied to the modulator for
conversion. The ADC uses a 1.17 V ± 5% on-chip reference as the reference source for the analog-to-digital
conversion. Any change in channel resets the filter and a new conversion is started.
RDY pin automatically when the SCLK pulses are applied. The
CREAD(0) CH1(0) CH0(0)
WEN bit, the next seven bits are
RDY pin
Rev. A | Page 11 of 20
Page 12
AD7788/AD7789
Table 8. Register Selection
RS1 RS0 Register Register Size
0 0 Communications register during a write operation 8-Bit
0 0 Status register during a read operation 8-Bit
0 1 Mode register 8-Bit
1 0 Reserved 8-Bit
1 1 Data register 16-Bit (AD7788)
24-Bit (AD7789)
Table 9. Channel Selection
CH1 CH0 Channel
0 0 AIN(+) − AIN(−)
0 1 Reserved
1 0 AIN(−) − AIN(−)
1 1 VDD monitor
STATUS REGISTER
(RS1, RS0 = 0, 0; Power-On/Reset = 0x88 [AD7788] and 0x8C [AD7789])
The status register is an 8-bit, read-only register. To access the ADC status register, the user must write to the communications register,
select the next operation to be a read, and load Bits RS1 and RS0 with 0. Table 10 outlines the bit designations for the status register. SR0
through SR7 indicate the bit locations, SR denoting the bits are in the status register. SR7 denotes the first bit of the data stream. The
number in brackets indicates the power-on/reset default status of that bit.
SR7 SR6 SR5 SR4 SR3 SR2 SR1 SR0
RDY(1)
ERR(0) 0(0) 0(0) 1(1) WL(1/0) CH1(0) CH0(0)
Table 10. Status Register Bit Designations
Bit Location Bit Name Description
SR7
SR6 ERR
SR5 0 This bit is automatically cleared.
SR4 0 This bit is automatically cleared.
SR3 1 This bit is automatically set.
SR2 WL
SR1 to SR0 CH1 to CH0 These bits indicate which channel is being converted by the ADC.
RDY Ready bit for ADC. Cleared when data is written to the ADC data register. The RDY bit is set automatically
after the ADC data register has been read or a period of time before the data register is updated with a
new conversion result to tell 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/
can be used as an alternative to the status register for monitoring the ADC for conversion data.
ADC Error Bit. This bit is written to at the same time as the
to the ADC data register has been clamped to all 0s or all 1s. Error sources include overrange, underrange. Cleared by a write operation to start a conversion.
AD7788/AD7789 Identifier. This bit is automatically cleared if the device is an AD7788 and it is
automatically set if the device is an AD7789. This bit can be used to distinguish between the AD7788
and AD7789.
RDY bit. Set to indicate that the result written
RDY pin also. This pin
Rev. A | Page 12 of 20
Page 13
AD7788/AD7789
MODE REGISTER
(RS1, RS0 = 0, 1; Power-On/Reset = 0x02)
The mode register is an 8-bit register from which data can be read or to which data can be written. This register is used to configure the
ADC for range, to set unipolar or bipolar mode, to enable or disable the buffer, or to place the device into power-down mode. Table 11
outlines the bit designations for the mode register. MR0 through MR7 indicate the bit locations, MR denoting the bits are in the mode
register. MR7 denotes the first bit of the data stream. The number in brackets indicates the power-on/reset default status of that bit. Any
write to the setup register resets the modulator and filter, and sets the
MR7 MR6 MR5 MR4 MR3 MR2 MR1 MR0
MD1(0) MD0(0) 0(0) 0(0) 0(0)
Table 11. Mode Register Bit Designations
Bit Location Bit Name Description
MR7 to MR6 MD1 to MD0
MR5 to MR3 0 This bit must be programmed with a Logic 0 for correct operation.
MR2
MR1 1 This bit must be programmed with a Logic 1 for correct operation.
MR0 0 This bit must be programmed with a Logic 0 for correct operation.
B Unipolar/Bipolar Bit. Set by user to enable unipolar coding; that is, zero differential input results in
U/
Mode Select Bits. These bits select between continuous-conversion mode, single-conversion mode, and
standby mode. In continuous-conversion mode, the ADC continuously performs conversions and places
the result in the data register. DOUT/
conversions by placing the device in continuous-read mode whereby the conversions are automatically
placed on the DOUT/
output the conversion by writing to the communications register. After power-on, the first conversion is
available after a period 2/ f
conversion mode, the ADC is placed in power-down mode when conversions are not being performed.
When single-conversion mode is selected, the ADC powers up (which takes 1 ms) and performs a single
conversion, which requires a duration of 2/f
RDY goes low, and the ADC returns to power-down mode. The conversion remains in the data register and
RDY remains active (low) until the data is read or another conversion is performed. See Table 12.
DOUT/
000…000 output, and a full-scale differential input results in 111…111 output. Cleared by the user to
enable bipolar coding. Negative full-scale differential input results in an output code of 000…000, zero
differential input results in an output code of 100…000, and a positive full-scale differential input results in
an output code of 111…111.
RDY line when SCLK pulses are applied. Alternatively, the user can instruct the ADC to
while subsequent conversions are available at a frequency of f
ADC
Table 12. Operating Modes
MD1 MD0 Mode
0 0 Continuous-conversion mode (default)
0 1 Reserved
1 0 Single-conversion mode
1 1 Power-down mode
bit.
RDY
B(0)
U/
RDY goes low when a conversion is complete. The user can read these
. The conversion result is placed in the data register, DOUT/
ADC
1(1) 0(0)
. In single-
ADC
DATA REGISTER
(RS1, RS0 = 1, 1; Power-On/Reset = 0x0000 [AD7788] and 0x000000 [AD7789])
The conversion result from the ADC is stored in this data register. This is a read-only register. On completion of a read operation from
this register, the
bit/pin is set.
RDY
Rev. A | Page 13 of 20
Page 14
AD7788/AD7789
ADC CIRCUIT INFORMATION
The AD7788/AD7789 are low power ADCs that incorporate a
Σ-Δ modulator and on-chip digital filtering intended for the
measurement of wide dynamic range, low frequency signals,
such as those in pressure transducers, weigh scales, and temperature measurement applications. The part has one unbuffered
differential input. The device requires an external reference
voltage between 0.1 V and V
connections required to operate the part.
POWER
SUPPLY
10µF0.1µF
IN+
OUT–
IN–
REFIN(+)
OUT+
AIN(+)
AIN(–)
REFIN(–)
Figure 10. Basic Connection Diagram
The output rate of the AD7788/AD7789 (f
the settling time equal to 2 × t
rejection is the major function of the digital filter. Simultaneous
50 Hz and 60 Hz rejection is optimized as notches are placed at
both 50 Hz and 60 Hz with this update rate (see Figure 6).
NOISE PERFORMANCE
Typically, the devices have an rms noise of 1.5 µV rms, which
corresponds to a peak-to-peak resolution of 16 bits for the
AD7788 and 19 bits (equivalent to an effective resolution of
21.5 bits) for the AD7789. These numbers are for the bipolar
input range with a reference of 2.5 V. The noise was measured
with a differential input voltage of 0 V. The peak-to-peak
resolution figures represent the resolution for which there is no
code flicker within a six-sigma limit. The output noise comes
from two sources. The first is the electrical noise in the semiconductor devices (device noise) used in the implementation of
the modulator. The second is quantization noise, added when
the analog input is converted into the digital domain.
DIGITAL INTERFACE
As previously outlined, the AD7788/AD7789 programmable
functions are controlled using a set of on-chip registers. Data is
written to these registers via the serial interface and read access
to the on-chip registers is also provided by this interface. All
communications with the devices must start with a write to the
communications register. After power-on or reset, the devices
expect a write to the communications register. The data written
to this register determines whether the next operation is a read
. Figure 10 shows the basic
DD
V
DD
AD7788/
AD7789
CS
DOUT/RDY
SCLK
GND
ADC
(120.4 ms). Normal-mode
ADC
MICROCONTROLLER
03539-006
) is 16.6 Hz with
operation or a write operation, and also determines to which
register this read or write operation occurs. Therefore, write
access to any of the other registers on the devices begins with a
write operation to the communications 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 communications register followed by a read
operation from the selected register.
The AD7788/AD7789 serial interface consists of four signals:
, DIN, SCLK, and DOUT/
CS
transfer data into the on-chip registers while DOUT/
. The DIN line is used to
RDY
RDY
is
used for accessing from the on-chip registers. SCLK is the serial
clock input for the device, and all data transfers (either on DIN
or DOUT/
DOUT/
) occur with respect to the SCLK signal. The
RDY
pin operates as a data-ready signal also, the line
RDY
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 data register
update to indicate when not to read from the device; this
ensures that a data read is not attempted while the register is
being updated.
is used to select a device. It can be used to
CS
decode the AD7788/AD7789 in systems where several components are connected to the serial bus.
Figure 3 and Figure 4 show timing diagrams for interfacing to
the AD7788/AD7789 with
being used to decode the devices.
CS
Figure 3 shows the timing for a read operation from the output
shift register, while Figure 4 shows the timing for a write operation to the input shift register. In all modes except continuousread mode, it is possible to read the same word from the data
register several times even though the DOUT/
line returns
RDY
high after the first read operation. However, care must be taken
to ensure that the read operations have been completed before
the next output update occurs. In continuous-read mode, the
data register can be read only once.
The serial interface can operate in 3-wire mode by tying CS low.
In this case, the SCLK, DIN, and DOUT/
lines are used to
RDY
communicate with the AD7788/AD7789. The end of conversion
can be monitored using the
scheme is suitable for interfacing to microcontrollers. If
bit in the status register. This
RDY
CS
is
required as a decoding signal, it can be generated from a port
pin. For microcontroller interfaces, it is recommended that
SCLK idles high between data transfers.
The AD7788/AD7789 can operate with
being used as a
CS
frame synchronization signal. This scheme is useful for DSP
interfaces. In this case, the first bit (MSB) is effectively clocked
out by
, because CS normally occurs after the falling edge of
CS
SCLK in DSPs. The SCLK can continue to run between data
transfers, provided the timing numbers are obeyed.
Rev. A | Page 14 of 20
Page 15
AD7788/AD7789
The serial interface can be reset by writing a series of 1s on the
DIN input. If a Logic 1 is written to the AD7788/AD7789 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
inter-face gets 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 communications register. This operation resets the contents of all registers to their power-on values.
The AD7788/AD7789 can be configured to continuously
convert or to perform a single conversion. See Figure 11
through Figure 13.
Single-Conversion Mode
In single-conversion mode, the AD7788/AD7789 are placed in
power-down mode between conversions. When a single conversion is initiated by setting MD1 to 1 and MD0 to 0 in the mode
register, the AD7788/AD7789 power up, perform a single conversion, and then return to power-down mode. The devices
require 1 ms to power up and settle. The AD7788/AD7789
then perform a conversion, which requires a time period of
2 × t
. DOUT/
ADC
goes low to indicate the completion of
RDY
a conversion. When the data-word has been read from the data
register, DOUT/
goes high. If CS is low, DOUT/
RDY
RDY
remains high until another conversion is initiated and
completed. The data register can be read several times, if
required, even when DOUT/
has gone high.
RDY
Continuous-Conversion Mode
This is the default power-up mode. The AD7788/AD7789
continuously convert, the
low each time a conversion is complete. If
DOUT/
line also goes low when a conversion is complete.
RDY
pin in the status register going
RDY
is low, the
CS
To read a conversion, the user can write to the communications
register, indicating that the next operation is a read of the data
register. The digital conversion is placed on the DOUT/
RDY
pin as soon as SCLK pulses are applied to the ADC. DOUT/
returns high when the conversion is read. The user can
RDY
read this register additional times, if required. However, the
user must ensure that the data register is not being accessed
at the completion of the next conversion or else the new
conversion word is lost.
DIN
DOUT/RDY
SCLK
DIN
CS
0x10 0x82
Figure 11. Single Conversion
CS
0x38 0x38
0x38
DATA
03539-010
DOUT/RDY
SCLK
Figure 12. Continuous-Conversion Mode
DATA DATA
03539-012
Rev. A | Page 15 of 20
Page 16
AD7788/AD7789
Y
Continuous-Read Mode
Rather than write to the communications register each time a
conversion is complete to access the data, the AD7788/AD7789
can be placed in continuous-read mode. By writing 001111XX
to the communications register, the user needs only to apply the
appropriate number of SCLK cycles to the ADC and the dataword is automatically placed on the DOUT/
conversion is complete.
When DOUT/
goes low to indicate the end of a conver-
RDY
sion, sufficient SCLK cycles must be applied to the ADC and
the data conversion is placed on the DOUT/
the conversion is read, DOUT/
returns high until the next
RDY
conversion is available. In this mode, the data can be read only
once. Also, the user must ensure that the data-word is read
before the next conversion is complete. If the user has not read
CS
line when a
RDY
line. When
RDY
the conversion before the completion of the next conversion, or
if insufficient serial clocks are applied to the AD7788/AD7789
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 continuous-read mode, the instruction 001110XX must
be written to the communications register while the DOUT/
pin is low. While in continuous-read mode, the ADC
RDY
monitors activity on the DIN line so that it can receive 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.
DOUT/RD
SCLK
DIN
0x3C
DATA DATA DATA
Figure 13. Continuous-Read Mode
03539-011
Rev. A | Page 16 of 20
Page 17
AD7788/AD7789
CIRCUIT DESCRIPTION
ANALOG INPUT CHANNEL
The AD7788/AD7789 have one differential analog input
channel that is connected to the modulator, which means the
input is unbuffered. Note that this unbuffered input path
provides a dynamic load to the driving source. Therefore,
resistor/capacitor combinations on the input pins can cause dc
gain errors, depending on the output impedance of the source
that is driving the ADC input. Table 13 shows the allowable
external resistance/capacitance values such that no gain error at
the 16-bit level is introduced (AD7788), while Table 14 shows
the allowable external resistance/capacitance values such that
no gain error at the 20-bit level is introduced (AD7789).
Table 13. External R-C Combination for No 16-Bit
Gain Error (AD7788)
C (pF) R (Ω)
50 22.8 k
100 13.1 k
500 3.3 k
1000 1.8 k
5000 360
Table 14. External R-C Combination for No 20-Bit
Gain Error (AD7789)
C (pF) R (Ω)
50 16.7 k
100 9.6 k
500 2.2 k
1000 1.1 k
5000 160
The absolute input voltage includes the range between GND −
30 mV and V
+ 30 mV. The negative absolute input voltage
DD
limit does allow the possibility of monitoring small true bipolar
signals with respect to GND.
The absolute input voltage includes the range between GND −
30 mV and V
+ 30 mV. The negative absolute input voltage
DD
limit does allow the possibility of monitoring small true bipolar
signals with respect to GND.
BIPOLAR/UNIPOLAR CONFIGURATION
The analog input to the devices can accept either unipolar or
bipolar input voltage ranges. A bipolar input range does not
imply that the parts can tolerate large 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, 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 on the AIN(+) input is 0 V to 5 V. The
bipolar/unipolar option is chosen by programming the U/
in the mode register.
B
bit
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
Code = 2
× (AIN/V
REF
)
N
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
× [(AIN/V
REF
) + 1]
N – 1
where AIN is the analog input voltage and N = 16 for the
AD7788 and N = 24 for the AD7789.
REFERENCE INPUT
The AD7788/AD7789 have a fully differential input capability
for the channel. The common-mode range for these differential
inputs is from GND to V
and, therefore, excessive R-C source impedances introduce gain
errors. The reference voltage REFIN [REFIN(+) – REFIN(−)] is
2.5 V nominal, but the AD7788/AD7789 are functional with
reference voltages from 0.1 V to V
excitation (voltage or current) for the transducer on the analog
input also drives the reference voltage for the parts, the effect of
the low frequency noise in the excitation source is removed
because the application is ratiometric. If the AD7788/AD7789
are used in a nonratiometric application, a low noise reference
should be used.
Recommended 2.5 V reference voltage sources for the AD7788/
AD7789 include the ADR381 and ADR391, which are low
noise, low power references. If the analog circuitry uses a 2.5 V
power supply, the reference voltage source requires some headroom. In this case, a 2.048 V reference such as the ADR380 or
ADR390 can be used. Again, these are low power, low noise
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 that is driving the reference inputs.
. The reference input is unbuffered
DD
. In applications where the
DD
Rev. A | Page 17 of 20
Page 18
AD7788/AD7789
Reference voltage sources like those recommended in the previous section (such as the ADR391) typically have low output
impedances 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.
VDD MONITOR
Along with converting external voltages, the analog input
channel can be used to monitor the voltage on the V
When the CH1 and CH0 bits in the communications register
are set to 1, the voltage on the V
pin is internally attenuated
DD
by 5 and the resultant voltage is applied to the Σ-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.
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 part 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 AD7788/AD7789 are more immune to noise interference
than conventional high resolution converters. However, because
the resolution of the AD7788/AD7789 is so high, and the noise
levels from the AD7788/AD7789 are so low, care must be taken
with regard to grounding and layout.
DD
pin.
The printed circuit board that houses the AD7788/AD7789
should 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 gives the best shielding.
It is recommended that the AD7788/AD7789’s GND pins be
tied to the AGND plane of the system. In any layout, it is
important that the user consider 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 AD7788/AD7789’s ground plane should be allowed to run
under the devices to prevent noise coupling. The power supply
lines to the AD7788/AD7789 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 clocks 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 opposite sides of the board
should run at right angles to each other. This reduces the effects
of feedthrough through the board. A microstrip technique is by
far the best, but it is not always possible with a double-sided
board. In this technique, the component side of the board is
dedicated to ground planes, while signals are placed on the
solder side.
Good decoupling is important when using high resolution
ADCs. V
should be decoupled with a 10 µF tantalum in
DD
parallel with 0.1 µF capacitors to GND. 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.
Rev. A | Page 18 of 20
Page 19
AD7788/AD7789
OUTLINE DIMENSIONS
3.00 BSC
6
10
3.00 BSC
1
PIN 1
0.50 BSC
0.95
0.85
0.75
0.15
0.00
0.27
0.17
COPLANARITY
0.10
COMPLIANT TO JEDEC STANDARDS MO-187BA
Figure 14. 10-Lead Mini Small Outline Package [MSOP]
ORDERING GUIDE
Model Temperature Range Package Description Package Option Branding
AD7788BRM −40°C to +105°C 10-Lead Mini Small Outline Package (MSOP) RM-10 COX
AD7788BRM-REEL −40°C to +105°C 10-Lead Mini Small Outline Package (MSOP) RM-10 COX
AD7788ARM −40°C to +85°C 10-Lead Mini Small Outline Package (MSOP) RM-10 COZ
AD7788ARM-REEL −40°C to +85°C 10-Lead Mini Small Outline Package (MSOP) RM-10 COZ
AD7789BRM −40°C to +105°C 10-Lead Mini Small Outline Package (MSOP) RM-10 COY
AD7789BRM-REEL −40°C to +105°C 10-Lead Mini Small Outline Package (MSOP) RM-10 COY
EVAL-AD7788EB Evaluation Board
EVAL-AD7789EB Evaluation Board
4.90 BSC
5
1.10 MAX
SEATING
PLANE
0.23
0.08
8°
0°
(RM-10)
Dimensions shown in millimeters
0.80
0.60
0.40
Rev. A | Page 19 of 20
Page 20
AD7788/AD7789
NOTES
© 2004 Analog Devices, Inc. All rights reserved. Trademarks and
registered trademarks are the property of their respective owners.
C03539-0-11/04(A)
Rev. A | Page 20 of 20
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