AD AD7811YR 2.7 v to 5.5 v, 350 ksps, 10-bit 4-/8-channel sampling adc Datasheet

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2.7 V to 5.5 V, 350 kSPS, 10-Bit
4-/8-Channel Sampling ADCs
AD7811/AD7812
FEATURES
10-Bit ADC with 2.3 ␮s Conversion Time
The AD7811 has Four Single-Ended Inputs that
Can Be Configured as Three Pseudo Differential
Inputs with Respect to a Common, or as Two Independent Pseudo Differential Channels
The AD7812 has Eight Single-Ended Inputs that Can
Be Configured as Seven Pseudo Differential Inputs
with Respect to a Common, or as Four Independent
Pseudo Differential Channels
Onboard Track and Hold
Onboard Reference 2.5 V ⴞ 2.5%
Operating Supply Range: 2.7 V to 5.5 V
Specifications at 2.7 V–3.6 V and 5 V ⴞ 10%
DSP-/Microcontroller-Compatible Serial Interface
High Speed Sampling and Automatic Power-Down Modes
Package Address Pin on the AD7811 and AD7812 Allows
Sharing of the Serial Bus in Multipackage Applications
Input Signal Range: 0 V to VREF
Reference Input Range: 1.2 V to VDD
GENERAL DESCRIPTION
The AD7811 and AD7812 are high speed, low power, 10-bit
A/D converters that operate from a single 2.7 V to 5.5 V supply.
The devices contain a 2.3 µs successive approximation A/D
converter, an on-chip track/hold amplifier, a 2.5 V on-chip reference and a high speed serial interface that is compatible with the
serial interfaces of most DSPs (Digital Signal Processors) and
microcontrollers. The user also has the option of using an external reference by connecting it to the VREF pin and setting the
EXTREF bit in the control register. The VREF pin may be tied
to VDD. At slower throughput rates the power-down mode may
be used to automatically power down between conversions.
The control registers of the AD7811 and AD7812 allow the
input channels to be configured as single-ended or pseudo
differential. The control register also features a software convert
start and a software power-down. Two of these devices can
share the same serial bus and may be individually addressed in
a multipackage application by hardwiring the device address pin.
The AD7811 is available in a small, 16-lead 0.3" wide, plastic
dual-in-line package (mini-DIP), in a 16-lead 0.15" wide, Small
Outline IC (SOIC) and in a 16-lead, Thin Shrink Small Outline Package (TSSOP). The AD7812 is available in a small,
20-lead 0.3" wide, plastic dual-in-line package (mini-DIP), in a
20-lead, Small Outline IC (SOIC) and in a 20-lead, Thin Shrink
Small Outline Package (TSSOP).
PRODUCT HIGHLIGHTS
1. Low Power, Single Supply Operation
Both the AD7811 and AD7812 operate from a single 2.7 V
to 5.5 V supply and typically consume only 10 mW of power.
The power dissipation can be significantly reduced at
lower throughput rates by using the automatic powerdown mode e.g., 315 µW @ 10 kSPS, VDD = 3 V—see
Power vs. Throughput.
2. 4-/8-Channel, 10-Bit ADC
The AD7811 and AD7812 have four and eight single-ended
input channels respectively. These inputs can be configured
as pseudo differential inputs by using the Control Register.
3. On-chip 2.5 V (± 2.5%) reference circuit that is powered
down when using an external reference.
4. Hardware and Software Control
The AD7811 and AD7812 provide for both hardware and
software control of Convert Start and Power-Down.
FUNCTIONAL BLOCK DIAGRAMS
CREF
REFIN
VDD
1.23V
REF
BUF
CLOCK
OSC
MUX
VDD /3
CREF
DGND
BUF
DOUT
SERIAL
PORT
REFIN
VDD
1.23V
REF
AD7811
CHARGE
REDISTRIBUTION
DAC
VIN1
VIN2
VIN3
VIN4
AGND
DIN
RFS
TFS
SCLK
CONTROL
LOGIC
COMP
A0 CONVST
VIN1
VIN2
VIN3
VIN4
VIN5
VIN6
VIN7
VIN8
AGND DGND
AD7812
CLOCK
OSC
CHARGE
REDISTRIBUTION
DAC
DOUT
SERIAL
PORT
TFS
SCLK
MUX
VDD /3
COMP
DIN
RFS
CONTROL
LOGIC
A0 CONVST
REV. B
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
which may result from its use. No license is granted by implication or
otherwise under any patent or patent rights of Analog Devices.
One Technology Way, P.O. Box 9106, Norwood, MA 02062-9106, U.S.A.
Tel: 781/329-4700
World Wide Web Site: http://www.analog.com
Fax: 781/326-8703
© Analog Devices, Inc., 2000
(VDD = 2.7 V to 3.6 V, VDD = 5 V ⴞ 10%, GND = 0 V, VREF = VDD
AD7811/AD7812–SPECIFICATIONS [EXT]. All specifications –40ⴗC to +105ⴗC unless otherwise noted.)
Parameter
Y Version
Unit
58
–66
–80
dB min
dB max
dB typ
–67
–67
–80
dB max
dB max
dB typ
10
Bits
10
±1
±1
±2
± 0.75
±2
± 0.75
Bits
LSB max
LSB max
LSB max
LSB max
LSB max
LSB max
0
VREF
±1
20
V min
V max
µA max
pF max
Input Leakage Current
Input Capacitance
1.2
VDD
±3
20
V min
V max
µA max
pF max
ON-CHIP REFERENCE
Reference Error
Temperature Coefficient
± 2.5
50
% max
ppm/°C typ
LOGIC INPUTS2
VINH, Input High Voltage
VINL, Input Low Voltage
VINH, Input High Voltage
VINL, Input Low Voltage
Input Current, IIN
Input Capacitance, CIN
2.4
0.8
2
0.4
±1
8
V min
V max
V min
V max
µA max
pF max
4
2.4
V min
V min
0.4
±1
15
V max
µA max
pF max
2.3
200
µs max
ns max
DYNAMIC PERFORMANCE
Signal to (Noise + Distortion) Ratio1
Total Harmonic Distortion (THD)1
Peak Harmonic or Spurious Noise1
Intermodulation Distortion1, 2
Second Order Terms
Third Order Terms
Channel-to-Channel Isolation1, 2
DC ACCURACY
Resolution
Minimum Resolution for Which
No Missing Codes are Guaranteed
Relative Accuracy1
Differential Nonlinearity1
Gain Error1
Gain Error Match1
Offset Error1
Offset Error Match1
ANALOG INPUT
Input Voltage Range
Input Leakage Current2
Input Capacitance2
REFERENCE INPUTS2
VREF Input Voltage Range
fIN = 20 kHz
Any Channel
Nominal 2.5 V
Output Low Voltage, VOL
CONVERSION RATE
Conversion time
Track/Hold Acquisition Time1
fIN = 30 kHz Any Channel, fSAMPLE = 350 kHz
VREF Internal or External
fa = 29 kHz, fb = 30 kHz
LOGIC OUTPUTS
Output High Voltage, VOH
High Impedance Leakage Current
High Impedance Capacitance
Test Conditions/Comments
–2–
VDD = 5 V ± 10%
VDD = 5 V ± 10%
VDD = 3 V ± 10%
VDD = 3 V ± 10%
Typically 10 nA, VIN = 0 V to VDD
ISOURCE = 200 µA
VDD = 5 V ± 10%
VDD = 3 V ± 10%
ISINK = 200 µA
REV. B
AD7811/AD7812
Parameter
POWER SUPPLY
VDD
Y Version
Unit
Test Conditions/Comments
2.7
5.5
V min
V max
For Specified Performance
3.5
mA max
1
350
µA max
µA max
10.5
mW max
31.5
315
3.15
1.05
3
µW max
µW max
mW max
mW max
µW max
Digital Inputs = 0 V or VDD
IDD
Normal Operation
Power-Down
Full Power-Down
Partial Power-Down (Internal Ref)
Power Dissipation
Normal Operation
Auto Full Power-Down
Throughput 1 kSPS
Throughput 10 kSPS
Throughput 100 kSPS
Partial Power-Down (Internal Ref)
Full Power-Down
See Power-Up Times Section
VDD = 3 V
See Power vs. Throughput Section
NOTES
1
See Terminology.
2
Sample tested during initial release and after any redesign or process change that may affect this parameter.
Specifications subject to change without notice.
TIMING CHARACTERISTICS1, 2 (V
DD
= 2.7 V to 5.5 V, VREF = VDD [EXT] unless otherwise noted)
Parameter
Y Version
Unit
Conditions/Comments
tPOWER-UP
t1
t2
t3
t4
t5 3
t6 3
t7 3
t8
t9
t103, 4
t11
1.5
2.3
20
25
25
5
5
10
10
5
20
100
µs (max)
µs (max)
ns (min)
ns (min)
ns (min)
ns (min)
ns (min)
ns (max)
ns (min)
ns (min)
ns (max)
ns (min)
Power-Up Time of AD7811/AD7812 after Rising Edge of CONVST
Conversion Time
CONVST Pulsewidth
SCLK High Pulsewidth
SCLK Low Pulsewidth
RFS Rising Edge to SCLK Rising Edge Setup Time
TFS Falling Edge to SCLK Falling Edge Setup Time
SCLK Rising Edge to Data Out Valid
DIN Data Valid to SCLK Falling Edge Setup Time
DIN Data Valid after SCLK Falling Edge Hold Time
SCLK Rising Edge to DOUT High Impedance
DOUT High Impedance to CONVST Falling Edge
NOTES
1
Sample tested to ensure compliance.
2
See Figures 16, 17 and 18.
3
These numbers are measured with the load circuit of Figure 1. They are defined as the time required for the o/p to cross 0.8 V or 2.4 V for V DD = 5 V ± 10% and
0.4 V or 2 V for V DD = 3 V ± 10%.
4
Derived from the measured time taken by the data outputs to change 0.5 V when loaded with the circuit of Figure 1. The measured number is then extrapolated back
to remove the effects of charging or discharging the 50 pF capacitor. This means that the time, t 11, quoted in the Timing Characteristics is the true bus relinquish
time of the part and as such is independent of external bus loading capacitances.
Specifications subject to change without notice.
200␮A
TO
OUTPUT
PIN
IOL
2.1V
CL
50pF
200␮A
IOH
Figure 1. Load Circuit for Digital Output Timing Specifications
REV. B
–3–
AD7811/AD7812
ABSOLUTE MAXIMUM RATINGS*
SOIC Package, Power Dissipation . . . . . . . . . . . . . . . 450 mW
θJA Thermal Impedance . . . . . . . . . . . . . . . . . . . . . 75°C/W
Lead Temperature, Soldering
Vapor Phase (60 sec) . . . . . . . . . . . . . . . . . . . . . . . 215°C
Infrared (15 sec) . . . . . . . . . . . . . . . . . . . . . . . . . . 220°C
TSSOP Package, Power Dissipation . . . . . . . . . . . . . 450 mW
θJA Thermal Impedance . . . . . . . . . . . . . . . . . . . . 115°C/W
Lead Temperature, Soldering
Vapor Phase (60 sec) . . . . . . . . . . . . . . . . . . . . . . . 215°C
Infrared (15 sec) . . . . . . . . . . . . . . . . . . . . . . . . . . 220°C
VDD to DGND . . . . . . . . . . . . . . . . . . . . . . . . . –0.3 V to +7 V
Digital Input Voltage to DGND (CONVST, SCLK, RFS, TFS,
DIN, A0) . . . . . . . . . . . . . . . . . . . . . . . –0.3 V, VDD + 0.3 V
Digital Output Voltage to DGND (DOUT)
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . –0.3 V, VDD + 0.3 V
REFIN to AGND . . . . . . . . . . . . . . . . . . . –0.3 V, VDD + 0.3 V
Analog Inputs
VIN1–VIN4 (AD7811) . . . . . . . . . . . . . . –0.3 V, VDD + 0.3 V
VIN1–VIN8 (AD7812) . . . . . . . . . . . . . . –0.3 V, VDD + 0.3 V
Storage Temperature Range . . . . . . . . . . . . –65°C to +150°C
Junction Temperature . . . . . . . . . . . . . . . . . . . . . . . . . . 150°C
Plastic DIP Package, Power Dissipation . . . . . . . . . . 450 mW
θJA Thermal Impedance . . . . . . . . . . . . . . . . . . . . 105°C/W
Lead Temperature, (Soldering 10 sec) . . . . . . . . . . . . 260°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.
ORDERING GUIDE
Model
Linearity
Error
Package
Descriptions
Package
Options
AD7811YN
AD7811YR
AD7811YRU
± 1 LSB
± 1 LSB
± 1 LSB
16-Lead Plastic DIP
16-Lead Small Outline IC (SOIC)
16-Lead Thin Shrink Small Outline Package (TSSOP)
N-16
R-16A
RU-16
AD7812YN
AD7812YR
AD7812YRU
± 1 LSB
± 1 LSB
± 1 LSB
20-Lead Plastic DIP
20-Lead Small Outline IC (SOIC)
20-Lead Thin Shrink Small Outline Package (TSSOP)
N-20
R-20A
RU-20
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
the AD7811/AD7812 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.
–4–
WARNING!
ESD SENSITIVE DEVICE
REV. B
AD7811/AD7812
PIN CONFIGURATIONS
DIP/SOIC/TSSOP
VREF
1
16 VDD
VREF 1
20 VDD
CREF
2
15 CONVST
CREF
2
19 CONVST
VIN1
3
14 SCLK
VIN1
3
18 SCLK
AGND
4
VIN2
5
6
AGND 4
AD7811
13 DIN
17 DIN
VIN2
TOP VIEW
5 (Not to Scale) 12 DOUT
VIN3
6
11 RFS
VIN3
VIN4
7
10 TFS
VIN4
16 DOUT
TOP VIEW
15 RFS
(Not to Scale)
14 TFS
7
A0
8
VIN5
8
13 DGND
VIN6
9
12 A0
9 DGND
VIN7 10
AD7812
11 VIN8
PIN FUNCTION DESCRIPTIONS
Pin(s)
AD7811
Pin(s)
AD7812
Mnemonic
Description
1
1
VREF
2
2
CREF
3, 5–7
4
3, 5–11
4
VIN1–VIN4(8)
AGND
8
12
A0
9
10
13
14
DGND
TFS
11
15
RFS
12
16
DOUT
13
17
DIN
14
18
SCLK
15
19
CONVST
16
20
VDD
An external reference input can be applied here. When using an external precision
reference or VDD the EXTREF bit in the control register must be set to logic one. The
external reference input range is 1.2 V to VDD.
Reference Capacitor. A capacitor (10 nF) is connected here to improve the noise
performance of the on-chip reference.
Analog Inputs. The analog input range is 0 V to VREF.
Analog Ground. Ground reference for track/hold, comparator, on-chip reference and
DAC.
Package Address Pin. This Logic Input can be hardwired high or low. When used in
conjunction with the package address bit in the control register this input allows two
devices to share the same serial bus. For example a twelve channel solution can be
achieved by using the AD7811 and the AD7812 on the same serial bus.
Digital Ground. Ground reference for digital circuitry.
Transmit Frame Sync. The falling edge of this Logic Input tells the part that a new
control byte should be shifted in on the next 10 falling edges of SCLK.
Receive Frame Sync. The rising edge of this Logic Input is used to enable a counter in
the serial interface. It is used to provide compatibility with DSPs which use a continuous
serial clock and framing signal. In multipackage applications the RFS Pin can also be
used as a serial bus select pin. The serial interface will ignore the SCLK until it receives a
rising edge on this input. The counter is reset at the end of a serial read operation.
Serial Data Output. Serial data is shifted out on this pin on the rising edge of the serial
clock. The output enters a High impedance condition on the rising edge of the 11th
SCLK pulse.
Serial Data Input. The control byte is read in at this input. In order to complete a
serial write operation 13 SCLK pulses need to be provided. Only the first 10 bits are
shifted in—see Serial Interface section.
Serial Clock Input. An external serial clock is applied to this input to obtain serial data
from the AD7811/AD7812 and also to latch data into the AD7811/AD7812. Data is
clocked out on the rising edge of SCLK and latched in on the falling edge of SCLK.
Convert Start. This is an edge triggered logic input. The Track/Hold goes into its Hold
Mode on the falling edge of this signal and a conversion is initiated. The state of this
pin at the end of conversion also determines whether the part is powered down or not.
See operating modes section of this data sheet.
Positive Supply Voltage 2.7 V to 5.5 V.
REV. B
–5–
AD7811/AD7812
TERMINOLOGY
Signal to (Noise + Distortion) Ratio
usually distanced in frequency from the original sine waves
while the third order terms are usually at a frequency close to
the input frequencies. As a result, the second and third order
terms are specified separately. The calculation of the intermodulation distortion is as per the THD specification where it is
the ratio of the rms sum of the individual distortion products to
the rms amplitude of the fundamental expressed in dBs.
This is the measured ratio of signal to (noise + distortion) at the
output of the A/D converter. The signal is the rms amplitude of
the fundamental. Noise is the rms sum of all nonfundamental
signals up to half the sampling frequency (fS/2), excluding dc.
The ratio is dependent upon the number of quantization levels
in the digitization process; the more levels, the smaller the
quantization noise. The theoretical signal to (noise + distortion) ratio for an ideal N-bit converter with a sine wave input
is given by:
Channel-to-Channel Isolation
Channel-to-channel isolation is a measure of the level of
crosstalk between channels. It is measured by applying a fullscale 20 kHz sine wave signal to all nonselected input channels
and determining how much that signal is attenuated in the selected
channel. The figure given is the worst case across all four or
eight channels for the AD7811 and AD7812 respectively.
Signal to (Noise + Distortion) = (6.02N + 1.76) dB
Thus for a 10-bit converter, this is 62 dB.
Total Harmonic Distortion
Relative Accuracy
Total harmonic distortion (THD) is the ratio of the rms sum of
harmonics to the fundamental. For the AD7811 and AD7812
it is defined as:
THD (dB) = 20 log
Relative accuracy, or endpoint nonlinearity, is the maximum
deviation from a straight line passing through the endpoints of
the ADC transfer function.
V22 +V 32 +V 42 +V 52 +V 62
Differential Nonlinearity
This is the difference between the measured and the ideal
1 LSB change between any two adjacent codes in the ADC.
V1
where V1 is the rms amplitude of the fundamental and V2, V3,
V4, V5 and V6 are the rms amplitudes of the second through the
sixth harmonics.
Offset Error
Peak Harmonic or Spurious Noise
Offset Error Match
This is the deviation of the first code transition (0000 . . . 000)
to (0000 . . . 001) from the ideal, i.e., AGND + 1 LSB.
Peak harmonic or spurious noise is defined as the ratio of the
rms value of the next largest component in the ADC output
spectrum (up to fS/2 and excluding dc) to the rms value of the
fundamental. Normally, the value of this specification is
determined by the largest harmonic in the spectrum, but for
parts where the harmonics are buried in the noise floor, it will
be a noise peak.
This is the difference in Offset Error between any two channels.
Gain Error
This is the deviation of the last code transition (1111 . . . 110)
to (1111 . . . 111) from the ideal, i.e., VREF – 1 LSB, after the
offset error has been adjusted out.
Gain Error Match
Intermodulation Distortion
This is the difference in Gain Error between any two channels.
With inputs consisting of sine waves at two frequencies, fa and
fb, any active device with nonlinearities will create distortion
products at sum and difference frequencies of mfa ± nfb where
m, n = 0, 1, 2, 3, etc. Intermodulation terms are those for
which neither m nor n are equal to zero. For example, the
second order terms include (fa + fb) and (fa – fb), while the
third order terms include (2fa + fb), (2fa – fb), (fa + 2fb) and
(fa – 2fb).
Track/Hold Acquisition Time
Track/hold acquisition time is the time required for the output
of the track/hold amplifier to reach its final value, within
± 1/2 LSB, after the end of conversion (the point at which the
track/hold returns to track mode). It also applies to situations
where a change in the selected input channel takes place or
where there is a step input change on the input voltage applied
to the selected VIN input of the AD7811 or AD7812. It means
that the user must wait for the duration of the track/hold acquisition time after the end of conversion or after a channel change/
step input change to VIN before starting another conversion, to
ensure that the part operates to specification.
The AD7811 and AD7812 are tested using the CCIF standard
where two input frequencies near the top end of the input
bandwidth are used. In this case, the second and third order
terms are of different significance. The second order terms are
–6–
REV. B
AD7811/AD7812
Control Register (AD7811)
The Control Register is a 10-bit-wide, write only register. The Control Register is written to when the AD7811 receives a falling
edge on its TFS pin. The AD7811 will maintain the same configuration until a new control byte is written to the part. The control
register can be written to at the same time data is being read. This latter feature enhances throughput rates when software control is
being used or when the analog input channels are being changed frequently. The power-up default register contents are all zeros;
therefore, when the supplies are connected, the AD7811 is powered down by default.
Control Register AD7811
9
0
X*
A0
PD1
PD0
VIN4/AGND
DIFF/SGL
CH1
CH0
CONVST EXTREF
*This is a don’t care bit.
A0
This is the package address bit. It is used in conjunction with the package address pin to allow two AD7811s to
share the same serial bus. The AD7811 can also share the same serial bus with the AD7812. When a control word
is written to the control register of the AD7811 the control word is ignored if the package address bit in the control byte does not match how the package address pin is hardwired. Only the serial port of the device that received
the last valid control byte, i.e., the address bit matched the address pin, will attempt to drive the serial bus on the
next serial read. When the part powers up this bit is set to 0.
PD1, PD0
These bits allow the AD7811 to be fully powered down and powered up. Bit combinations PD1 = PD0 = 0 and
PD1 = PD0 = 1 override the automatic power-down decision at the end of conversion. These bits also decide the
power-down mode when the AD7811 enters a power-down at the end of a conversion. There are two power-down
modes—Full Power-Down and Partial Power-Down. See Power-Down Options section of this data sheet.
PD1
PD0
Description
0
0
1
1
0
1
0
1
Full Power-Down of the AD7811
Partial Power-Down at the End of Conversion
Full Power-Down at the End of Conversion
Power-Up the AD7811
VIN4/AGND
The DIF/SGL bit in the control register must be set to 0 to use this option otherwise this bit is ignored. Setting
VIN4/AGND to 0 configures the analog inputs of the AD7811 as four single-ended analog inputs referenced to
analog ground (AGND). By setting this bit to 1 the input channels VIN1 to VIN3 are configured as three pseudodifferential channels with respect to VIN4—see Table I.
DIF/SGL
This bit is used to configure the analog inputs as single ended or pseudo differential pairs. By setting this bit to 0
the analog inputs can be configured as single ended with respect to AGND, or pseudo differential with respect to
VIN4 as explained above. Setting this bit to 1 configures the analog input channels as two pseudo differential pairs
VIN1/VIN2 and VIN3/VIN4—see Table I.
CH1, CH0
These bits are used in conjunction with VIN4/AGND and DIF/SGL to select an analog input channel. The table
shows how the various channel selections are made—see Table I.
CONVST
Setting this bit to a logic one initiates a conversion. A conversion is initiated 400 ns after a write to the control
register has taken place. This allows a signal to be acquired even if the channel is changed and a conversion
initiated in the same serial write. The bit is reset after the end of a conversion.
EXTREF
This bit must be set to a logic one if the user wishes to use an external reference or use VDD as the reference.
When the external reference is selected the on chip reference circuitry powers down.
REV. B
–7–
AD7811/AD7812
Control Register (AD7812)
The Control Register is a 10-bit-wide, write only register. The Control Register is written to when the AD7812 receives a falling
edge on its TFS pin. The AD7812 will maintain the same configuration until a new control byte is written to the part. The control
register can be written to at the same time data is being read. This latter feature enhances throughput rates when software control is
being used or when the analog input channels are being changed frequently. The power-up default register contents are all zeros;
therefore, when the supplies are connected, the AD7812 is powered down by default.
Control Register AD7812
9
0
A0
PD1
PD0
VIN8/AGND
DIFF/SGL
CH2
CH1
CH0
CONVST EXTREF
A0
This is the package address bit. It is used in conjunction with the package address pin to allow two AD7812s to
share the same serial bus. The AD7812 can also share the same serial bus with the AD7811. When a control word
is written to the control register of the AD7812 the control word is ignored if the package address bit in the control byte does not match how the package address pin is hardwired. Only the serial port of the device which
received the last valid control byte, i.e., the address bit matched the address pin, will attempt to drive the serial bus
on the next serial read. When the part powers up this bit is set to 0.
PD1, PD0
These bits allow the AD7812 to be fully powered down and powered up. Bit combinations PD1 = PD0 = 0 and
PD1 = PD0 = 1 override the automatic power-down decision at the end of conversion. These bits also decide the
power-down mode when the AD7812 enters a power-down at the end of a conversion. There are two power-down
modes—Full Power-Down and Partial Power-Down. See Power-Down section of this data sheet.
PD1
PD0
Description
0
0
1
1
0
1
0
1
Full Power-Down of the AD7812
Partial Power-Down at the End of Conversion
Full Power-Down at the End of Conversion
Power-Up the AD7812
VIN8/AGND
The DIF/SGL bit in the control register must be set to 0 in order to use this option otherwise this bit is ignored.
Setting VIN8/AGND to 0 configures the analog inputs of the AD7812 as eight single-ended analog inputs
referenced to analog ground (AGND). By setting this bit to 1 the input channels VIN1 to VIN7 are configured
as seven pseudo differential channels with respect to VIN8—see Table II.
DIF/SGL
This bit is used to configure the analog inputs as single ended or pseudo differential pairs. By setting this bit to 0
the analog inputs can be configured as single ended with respect to AGND, or pseudo differential with respect to
VIN8 as explained above. Setting this bit to 1 configures the analog input channels as four pseudo differential pairs
VIN1/VIN2, VIN3/VIN4, VIN5/VIN6 and VIN7/VIN8—see Table II.
CH2, CH1, CH0 These bits are used in conjunction with VIN8/AGND and DIF/SGL to select an analog input channel. Table II
shows how the various channel selections are made.
CONVST
Setting this bit to a logic one initiates a conversion. A conversion is initiated 400 ns after a write to the control
register has taken place. This allows a signal to be acquired even if the channel is changed and a conversion initiated in the same write operation. The bit is reset after the end of a conversion.
EXTREF
This bit must be set to a logic one if the user wishes to use an external reference or use VDD as the reference.
When the external reference is selected the on-chip reference circuitry powers down and the current consumption
is reduced by about 1 mA.
–8–
REV. B
AD7811/AD7812
Table I. AD7811 Channel Configurations
VIN4/AGND
DIF/SGL
CH1
CH0
Description
0
0
0
0
1
1
1
X
X
X
X
0
0
0
0
0
0
0
1
1
1
1
0
0
1
1
0
0
1
0
0
1
1
0
1
0
1
0
1
0
0
1
0
1
VIN1 Single-Ended with Respect to AGND
VIN2 Single-Ended with Respect to AGND
VIN3 Single-Ended with Respect to AGND
VIN4 Single-Ended with Respect to AGND
VIN1 Pseudo Differential with Respect to VIN4
VIN2 Pseudo Differential with Respect to VIN4
VIN3 Pseudo Differential with Respect to VIN4
VIN1(+) Pseudo Differential with Respect to VIN2(–)
VIN3(+) Pseudo Differential with Respect to VIN4(–)
Internal Test. SAR Input Equal to VREF/2
Internal Test. SAR Input Equal to VREF
Table II. AD7812 Channel Configurations
VIN8/AGND
DIF/SGL
CH2
CH1
CH0
Description
0
0
0
0
0
0
0
0
1
1
1
1
1
1
1
X
X
X
X
X
X
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
1
1
1
1
1
1
0
0
0
0
1
1
1
1
0
0
0
0
1
1
1
0
0
0
0
1
1
0
0
1
1
0
0
1
1
0
0
1
1
0
0
1
0
0
1
1
0
0
0
1
0
1
0
1
0
1
0
1
0
1
0
1
0
0
1
0
1
0
1
VIN1 Single-Ended with Respect to AGND
VIN2 Single-Ended with Respect to AGND
VIN3 Single-Ended with Respect to AGND
VIN4 Single-Ended with Respect to AGND
VIN5 Single-Ended with Respect to AGND
VIN6 Single-Ended with Respect to AGND
VIN7 Single-Ended with Respect to AGND
VIN8 Single-Ended with Respect to AGND
VIN1 Pseudo Differential with Respect to VIN8
VIN2 Pseudo Differential with Respect to VIN8
VIN3 Pseudo Differential with Respect to VIN8
VIN4 Pseudo Differential with Respect to VIN8
VIN5 Pseudo Differential with Respect to VIN8
VIN6 Pseudo Differential with Respect to VIN8
VIN7 Pseudo Differential with Respect to VIN8
VIN1(+) Pseudo Differential with Respect to VIN2(–)
VIN3(+) Pseudo Differential with Respect to VIN4(–)
VIN5(+) Pseudo Differential with Respect to VIN6(–)
VIN7(+) Pseudo Differential with Respect to VIN8(–)
Internal Test. SAR Input Equal to VREF/2
Internal Test. SAR Input Equal to VREF
REV. B
–9–
AD7811/AD7812
CIRCUIT DESCRIPTION
Converter Operation
SUPPLY
2.7V TO 5.5V
The AD7811 and AD7812 are successive approximation analogto-digital converters based around a charge redistribution DAC.
The ADCs can convert analog input signals in the range 0 V to
VDD. Figures 2 and 3 show simplified schematics of the ADC.
Figure 2 shows the ADC during its acquisition phase. SW2 is
closed and SW1 is in position A, the comparator is held in a
balanced condition and the sampling capacitor acquires the
signal on VIN.
VDD VREF CREF
SW1
ACQUISITION
PHASE
SW2
VIN4(8)
DGND
When the ADC starts a conversion, see Figure 3, SW2 will
open and SW1 will move to position B causing the comparator
to become unbalanced. The Control Logic and the Charge
Redistribution DAC are used to add and subtract fixed amounts
of charge from the sampling capacitor to bring the comparator
back into a balanced condition. When the comparator is rebalanced, the conversion is complete. The Control Logic generates
the ADC output code. Figure 10 shows the ADC transfer
function.
CHARGE
REDISTRIBUTION
DAC
VIN
CONTROL
LOGIC
SW1
CONVERSION
PHASE
AGND
SW2
CONVST
TFS
A0
Figure 5 shows an equivalent circuit of the analog input structure of the AD7811 and AD7812. The two diodes D1 and D2
provide ESD protection for the analog inputs. Care must be
taken to ensure that the analog input signal never exceeds the
supply rails by more than 200 mV. This will cause these diodes
to become forward biased and start conducting current into
the substrate. 20 mA is the maximum current these diodes can
conduct without causing irreversible damage to the part. However, it is worth noting that a small amount of current (1 mA)
being conducted into the substrate due to an overvoltage on an
unselected channel can cause inaccurate conversions on a
selected channel. The capacitor C2 in Figure 5 is typically about
4 pF and can primarily be attributed to pin capacitance. The
resistor R1 is a lumped component made up of the on resistance
of a multiplexer and a switch. This resistor is typically about
125 Ω. The capacitor C1 is the ADC sampling capacitor and
has a capacitance of 3.5 pF.
COMPARATOR
VDD /3
µC/µP
Analog Input
Figure 2. ADC Acquisition Phase
SAMPLING
CAPACITOR
DIN
Figure 4. Typical Connection Diagram
CLOCK
OSC
VDD /3
B
AD7811/
AD7812
RFS
COMPARATOR
AGND
A
DOUT
VIN2
0V TO
VREF
INPUT
CONTROL
LOGIC
B
SCLK
VIN1
AGND
VIN
THREE-WIRE
SERIAL
INTERFACE
10nF
0.1␮F
CHARGE
REDISTRIBUTION
DAC
SAMPLING
CAPACITOR
A
10␮F
VDD
CLOCK
OSC
D1
R1
125⍀
Figure 3. ADC Conversion Phase
VDD /3
VIN
TYPICAL CONNECTION DIAGRAM
Figure 4 shows a typical connection diagram for the AD7811/
AD7812. The AGND and DGND are connected together at
the device for good noise suppression. The serial interface is
implemented using three wires with RFS/TFS connected to
CONVST see Serial Interface section for more details. VREF is
connected to a well decoupled VDD pin to provide an analog
input range of 0 V to VDD. If the AD7811 or AD7812 is not
sharing a serial bus with another AD7811 or AD7812 then A0
(package address pin) should be hardwired low. The default
power up value of the package address bit in the control register
is 0. For applications where power consumption is of concern,
the automatic power down at the end of a conversion should be
used to improve power performance. See Power-Down Options
section of the data sheet.
C1
3.5pF
C2
4pF
D2
CONVERSION PHASE – SWITCH OPEN
TRACK PHASE – SWITCH CLOSED
Figure 5. Equivalent Analog Input Circuit
The analog inputs on the AD7811 and AD7812 can be configured as single ended with respect to analog ground (AGND),
as pseudo differential with respect to a common, and also as
pseudo differential pairs—see Control Register section.
–10–
REV. B
AD7811/AD7812
An example of the pseudo differential scheme using the AD7811
is shown in Figure 6. The relevant bits in the AD7811 Control
Register are set as follows DIF/SGL = 1, CH1 = CH2 = 0, i.e.,
VIN1 pseudo differential with respect to VIN2. The signal is
applied to VIN1 but in the pseudo differential scheme the sampling capacitor is connected to VIN2 during conversion and not
AGND as described in the Converter Operation section. This
input scheme can be used to remove offsets that exist in a system. For example, if a system had an offset of 0.5 V the offset
could be applied to VIN2 and the signal applied to VIN1. This has
the effect of offsetting the input span by 0.5 V. It is only possible to offset the input span when the reference voltage is less
than VDD–OFFSET.
Figure 8 shows the equivalent charging circuit for the sampling
capacitor when the ADC is in its acquisition phase. R2 represents the source impedance of a buffer amplifier or resistive
network; R1 is an internal multiplexer resistance, and C1 is the
sampling capacitor. During the acquisition phase the sampling
capacitor must be charged to within a 1/2 LSB of its final value.
The time it takes to charge the sampling capacitor (TCHARGE) is
given by the following formula:
TCHARGE = 7.6 × (R2 + 125 Ω) × 3.5 pF
VIN1
VIN–
VOFFSET
For small values of source impedance, the settling time associated with the sampling circuit (100 ns) is, in effect, the acquisition time of the ADC. For example, with a source impedance
(R2) of 10 Ω the charge time for the sampling capacitor is
approximately 4 ns. The charge time becomes significant for
source impedances of 2 kΩ and greater.
CONTROL
LOGIC
VOFFSET
CONVERSION
PHASE
COMPARATOR
CLOCK
OSC
VIN2
VDD/3
AC Acquisition Time
In ac applications it is recommended to always buffer analog
input signals. The source impedance of the drive circuitry must
be kept as low as possible to minimize the acquisition time of
the ADC. Large values of source impedance will cause the THD
to degrade at high throughput rates. In addition, better performance can generally be achieved by using an External 1 nF
capacitor on VIN.
Figure 6. Pseudo Differential Input Scheme
When using the pseudo differential input scheme the signal on
VIN2 must not vary by more than a 1/2 LSB during the conversion process. If the signal on VIN2 varies during conversion, the
conversion result will be incorrect. In single-ended mode the
sampling capacitor is always connected to AGND during conversion. Figure 7 shows the AD7811/AD7812 pseudo differential input being used to make a unipolar dc current measurement.
A sense resistor is used to convert the current to a voltage and
the voltage is applied to the differential input as shown.
ON-CHIP REFERENCE
The AD7811 and AD7812 have an on-chip 2.5 V reference
circuit. The schematic in Figure 9 shows how the reference
circuit is implemented. A 1.23 V bandgap reference is gained up
to provide a 2.5 V ± 2% reference voltage. The on-chip reference is not available externally (SW2 is open). An external reference (1.2 V to VDD) can be applied at the VREF pin. However in
order to use an external reference the EXTREF bit in the control register (Bit 0) must first be set to a Logic 1. When EXTREF
is set to a Logic 1 SW2 will close, SW3 will open and the amplifier will power down. This will reduce the current consumption
of the part by about 1 mA. It is possible to use two different
reference voltages by selecting the on-chip reference or external
reference.
VDD
VIN+
RSENSE
SAMPLING
CAPACITOR
Figure 8. Equivalent Sampling Circuit
SAMPLING
CAPACITOR
VIN+
R1
125⍀
C1
3.5pF
CHARGE
REDISTRIBUTION
DAC
VIN1
VIN+
R2
AD7811/
AD7812
VIN–
RL
Figure 7. DC Current Measurement Scheme
DC Acquisition Time
The ADC starts a new acquisition phase at the end of a conversion and ends on the falling edge of the CONVST signal. At the
end of a conversion a settling time is associated with the sampling circuit. This settling time lasts approximately 100 ns. The
analog signal on VIN+ is also being acquired during this settling
time. Therefore, the minimum acquisition time needed is
approximately 100 ns.
CREF
EXTERNAL
CAPACITOR
VREF
SW1
SW2
1.23V
2.5V
7pF
SW3
AGND
Figure 9. On-Chip Reference Circuitry
REV. B
–11–
AD7811/AD7812
When using automatic power-down between conversions to
improve the power performance of the part (see Power vs.
Throughput) the switch SW1 will open when the part enters its
power-down mode if using the internal on-chip reference. This
provides a high impedance discharge path for the external
capacitor (see Figure 9). A typical value of external capacitance
is 10 nF. When the part is in Mode 2 Full Power-Down, because
the external capacitor holds its charge during power-down, the
internal bandgap reference will power up more quickly after
relatively short periods of full power-down. When operating the
part in Mode 2 Partial Power-Down the external capacitor is not
required as the on-chip reference stays powered up while the
rest of the circuitry powers down.
ADC TRANSFER FUNCTION
The output coding of the AD7811 and AD7812 is straight
binary. The designed code transitions occur at successive integer LSB values (i.e., 1 LSB, 2 LSBs, etc.). The LSB size is =
VREF/1024. The ideal transfer characteristic for the AD7811 and
AD7812 is shown in Figure 10.
ADC CODE
111...111
111...110
111...000
1LSB = VREF /1024
011...111
000...010
000...001
000...000
0V
1LSB
+VREF –1LSB
ANALOG INPUT
Figure 10. AD7811 and AD7812 Transfer Characteristic
POWER-DOWN OPTIONS
POWER-ON-RESET
If during normal operation, a power-save is performed by removing
power from the AD7811 and AD7812; the user must be wary
that a proper reset is done when power is applied to the part
again. To ensure proper power-on-reset, we recommend that
both PD bits are set to 0 and then set to 1. This procedure
causes an internal reset to occur.
POWER-UP TIMES
The AD7811 and AD7812 have a 1.5 µs power-up time when
using an external reference or when powering up from partial
power-down. When VDD is first connected, the AD7811 and
AD7812 are in a low current mode of operation. In order to
carry out a conversion the AD7811 and AD7812 must first be
powered up by writing to the control register of each ADC to
set the power-down bits (i.e., PD1 = 1, PD0 = 1) for a full
power-up. See the Quick Evaluation Setup section on the following page.
Mode 2 Full Power-Down (PD1 = 1, PD0 = 0)
The power-up time of the AD7811 and AD7812 after power is
first connected, or after a long period of Full Power-Down, is
the time it takes the on-chip 1.23 V reference to power up plus
the time it takes to charge the external capacitor CREF—see
Figure 9. The time taken to charge CREF to the 10-bit level is
given by the equation (7.6 × 2 kΩ × CREF). For CREF = 10 nF
the power-up time is approximately 152 µs. It takes 30 µs to
power up the on-chip reference so the total power-up time of
either ADC in either of these conditions is 182 µs. However,
when powering down fully between conversions to achieve a
better power performance this power-up time reduces to 1.5 µs
after a relatively short period of power-down as CREF holds its
charge (see On-Chip Reference section). The AD7811 and
AD7812 can therefore be used in Mode 2 with throughput
rates of 250 kSPS and under.
Mode 2 Partial Power-Down (PD1 = 0, PD0 = 1)
The AD7811 and AD7812 provide flexible power management
to allow the user to achieve the best power performance for a
given throughput rate.
The power management options are selected by programming
the power-down bits (i.e., PD1 and PD0) in the control register.
Table III below summarizes the options available. When the
power-down bits are programmed for Mode 2 Power Down (full
and partial), a rising edge on the CONVST pin will power up
the part. This feature is used when powering down between
conversions—see Power vs. Throughput. When the AD7811
and AD7812 are placed in partial power-down the on-chip
reference does not power down. However, the part will power
up more quickly after long periods of power-down when using
partial power-down—see Power-Up Times section.
Table III. AD7811/AD7812 Power-Down Options
PD1
PD0
CONVST*
Description
1
0
0
1
0
1
x
x
0
0
1
1
1
0
0
1
0
1
Full Power-Up
Full Power-Down
Mode 2 Partial Power-Down
(Reference Stays Powered-Up)
No Power-Down
Mode 2 Full Power-Down
No Power-Down
The power-up time of the AD7811 and AD7812 from a Partial
Power-Down is 1.5 µs maximum. When using a Partial PowerDown between conversions, there is no requirement to connect
an external capacitor to the CREF pin because the reference
remains powered up. This means that the AD7811 and AD7812
will power up in 30 µs after the supplies are first connected as
there is no requirement to charge an external capacitor.
POWER VS. THROUGHPUT
By using the Automatic Power-Down (Mode 2) at the end of a
conversion—see Operating Modes section of the data sheet,
superior power performance can be achieved.
Figure 11 shows how the Automatic Power-Down is implemented
using the CONVST signal to achieve the optimum power
performance for the AD7811 and AD7812. The AD7811 and
AD7812 are operated in Mode 2 and the control register Bits
PD1 and PD0 are set to 1 and 0 respectively for Full Power-Down,
or 0 and 1 for Partial Power-Down. The duration of the CONVST
pulse is set to be equal to or less than the power-up time of the
devices—see Operating Modes section. As the throughput rate
is reduced, the device remains in its power-down state longer
and the average power consumption over time drops accordingly.
*This refers to the state of the CONVST signal at the end of a conversion.
–12–
REV. B
AD7811/AD7812
t
tPOWER-UP CONVERT
2.3␮s
1.5␮s
QUICK EVALUATION SETUP
POWER-DOWN
CONVST
tCYCLE
100␮s @ 10kSPS
Figure 11. Automatic Power-Down
For example, if the AD7811 is operated in a continuous sampling mode with a throughput rate of 10 kSPS, PD1 = 1,
PD0 = 0 and using the on chip reference the power consumption is calculated as follows. The power dissipation during normal operation is 10.5 mW, VDD = 3 V. If the power-up time is
1.5 µs and the conversion time is 2.3 µs, the AD7811 can be
said to dissipate 10.5 mW for 3.8 µs (worst-case) during each
conversion cycle. If the throughput rate is 10 kSPS, the cycle
time is 100 µs and the average power dissipated during each
cycle is (3.8/100) × (10.5 mW) = 400 µW.
The schematic shown in Figure 14 shows a suggested configuration of the AD7812 for a first look evaluation of the part. No
external reference circuit is needed as the VREF pin can be
connected to VDD. The CONVST signal is connected to TFS
and RFS to enable the serial port. Also by selecting Mode 2
operation (see Operating Modes section) the power performance
of the AD7812 can be evaluated.
SUPPLY
VDD
10␮F
0.1␮F
10nF
VDD
0V TO VDD
INPUT
SCLK
VIN1
AD7812
VIN7
10
POWER – mV
1
CONVST
VIN8
RFS
AGND
TFS
DGND
A0
The setup uses a full duplex, 16-bit, serial interface protocol,
e.g., SPI. It is possible to use 8-bit transfers by carrying out two
consecutive read/write operations. The MSB of data is transferred first.
0
5
10
15
20
25
30
35
THROUGHPUT – kSPS
40
45
50
Figure 12. AD7811/AD7812 Power vs. Throughput
0
AD7811/12
2048 POINT FFT
SAMPLING 357.142kHz
fIN = 30.168kHz
–10
–20
–30
dBs
DIN
Figure 14. Evaluation Quick Setup
0.1
–40
1. When power is first connected to the device it is in a powered
down mode of operation and is consuming only 1 µA. The
AD7812 must first be configured by carrying out a serial
write operation.
2. The CONVST signal is first pulsed to enable the serial port
(rising and falling edge on RFS and TFS respectively—see
Serial Interface section).
3. Next, a 16-bit serial read/write operation is carried out. By
writing 6040 Hex to the AD7812 the part is powered up, set
up to use external reference (i.e., VDD) and the analog input
VIN1 is selected. The data read from the part during this read/
write operation is invalid.
4. It is necessary to wait approximately 1.5 µs before pulsing
CONVST again and initiating a conversion. The 1.5 µs is to
allow the AD7812 to power up correctly—see Power-Up
Times section.
–50
–60
–70
–80
–90
–100
DOUT
VIN2
Figure 12 shows the Power vs. Throughput Rate for automatic
full power-down.
0.01
VREF CREF
0
17
35
52
70
87
105 122
FREQUENCY – kHz
140
Figure 13. AD7811/AD7812 SNR
157 174
5. Approximately 2.3 µs after the falling edge of CONVST, i.e.,
after the end of the conversion, a serial read/write can take
place. This time 4040 Hex is written to the AD7812 and the
data read from the part is the result of the conversion. The
output code is in a straight binary format and will be left
justified in the 16-bit serial register (MSB clocked out first).
6. By idling the CONVST signal high or low it is possible to
operate the AD7812 in Mode 1 and Mode 2 respectively.
REV. B
–13–
AD7811/AD7812
OPERATING MODES
The mode of operation of the AD7811 and AD7812 is selected
when the (logic) state of the CONVST is checked at the end of
a conversion. If the CONVST signal is logic high at the end
of a conversion, the part does not power down and is operating in Mode 1. If, however, the CONVST signal is brought
logic low before the end of a conversion, the AD7811 and AD7812
will power down at the end of the conversion. This is Mode 2
operation.
Mode 1 Operation (High Speed Sampling)
When the AD7811 and AD7812 are operated in Mode 1 they
are not powered down between conversions. This mode of operation allows high throughput rates to be achieved. The timing
diagram in Figure 16 shows how this optimum throughput rate
is achieved by bringing the CONVST signal high before the end
of the conversion.
The sampling circuitry leaves its tracking mode and goes into
hold on the falling edge of CONVST. A conversion is also initiated at this time. The conversion takes 2.3 µs to complete. At
this point, the result of the current conversion is latched into the
serial shift register and the state of the CONVST signal checked.
The CONVST signal should be logic high at the end of the
conversion to prevent the part from powering down. The serial
port on the AD7811 and AD7812 is enabled on the rising edge
of the first SCLK after the rising edge of the RFS signal—see
Serial Interface section. As explained earlier, this rising edge
should occur before the end of the conversion process if the part
is not to be powered down. A serial read can take place at any
stage after the rising edge of CONVST. If a serial read is initiated before the end of the current conversion process (i.e., at
time “A”), the result of the previous conversion is shifted out on
the DOUT pin. It is possible to allow the serial read to extend
beyond the end of a conversion. In this case the new data will
not be latched into the output shift register until the read
has finished. The dynamic performance of the AD7811 and
AD7812 typically degrades by up to 3 dBs while reading during
a conversion. If the user waits until the end of the conversion
process, i.e., 2.3 µs after the falling edge of CONVST (Point
“B”) before initiating a read, the current conversion result is
shifted out. The serial read must finish at least 100 ns prior to
the next falling edge of CONVST to allow the part to accurately
acquire the input signal.
Mode 2 Operation (Automatic Power-Down)
When used in this mode of operation the part automatically
powers down at the end of a conversion. This is achieved by
leaving the CONVST signal low until the end of the conversion.
Because it takes approximately 1.5 µs for the part to power-up
after it has been powered down, this mode of operation is intended
to be used in applications where slower throughput rates are
required, i.e., in the order of 250 kSPS and improved power
performance is required—see Power vs. Throughput section.
There are two power-down modes the AD7811/AD7812 can
VDD
t POWER-UP
t CONVERT
t CONVERT
1.5␮s
2.3␮s
2.3␮s
CONVST
DIN
6040 HEX
4040 HEX
DOUT
NOT VALID
VALID DATA
4040 HEX
VALID DATA
Figure 15. Read/Write Sequence for AD7812
t1
CONVST
t2
A
B
t12
SCLK
DOUT
CURRENT CONVERSION
RESULT
Figure 16. Mode 1 Operation Timing Diagram
–14–
REV. B
AD7811/AD7812
enter during automatic power-down. These modes are discussed
in the Power-Up Times section of this data sheet. The timing
diagram in Figure 17 shows how to operate the part in Mode 2.
If the AD7811/AD7812 is powered down, the rising edge of the
CONVST pulse causes the part to power-up. Once the part
has powered up (~1.5 µs after the rising edge of CONVST)
the CONVST signal is brought low and a conversion is initiated
on this falling edge of the CONVST signal. The conversion
takes 2.3 µs and after this time the conversion result is latched
into the serial shift register and the part powers down. Therefore, when the part is operated in Mode 2 the effective conversion time is equal to the power-up time (1.5 µs) and the SAR
conversion time (2.3 µs).
NOTE: Although the AD7811 and AD7812 take 1.5 µs to
power up after the rising edge of CONVST, it is not necessary
to leave CONVST high for 1.5 µs after the rising edge before
bringing it low to initiate a conversion. If the CONVST signal
goes low before 1.5 µs in time has elapsed, then the power-up
time is timed out internally and a conversion is then initiated.
Hence the AD7811 and AD7812 are guaranteed to have always
powered-up before a conversion is initiated, even if the CONVST
pulsewidth is <1.5 µs. If the CONVST pulsewidth is > 1.5 µs,
then a conversion is initiated on the falling edge.
As in the case of Mode 1 operation, the rising edge of the first
SCLK after the rising edge of RFS enables the serial port of the
AD7811 and AD7812 (see Serial Interface section). If a serial
read is initiated soon after this rising edge (Point “A”), i.e.,
before the end of the conversion, the result of the previous conversion is shifted out on pin DOUT. In order to read the result
of the current conversion, the user must wait at least 2.3 µs after
power-up or at least 2.3 µs after the falling edge of CONVST,
t POWER-UP
(Point “B”), whichever occurs latest before initiating a serial
read. The serial port of the AD7811 and AD7812 is still functional even though the devices have been powered down.
Because it is possible to do a serial read from the part while it is
powered down, the AD7811 and AD7812 are powered up only
to do the conversion and are immediately powered down at the
end of a conversion. This significantly improves the power
consumption of the part at slower throughput rates—see Power
vs. Throughput section.
SERIAL INTERFACE
The serial interface of the AD7811 and AD7812 consists of five
wires, a serial clock input, SCLK, receive data to clock synchronization input RFS, transmit data to clock synchronization
input TFS, a serial data output, DOUT, and a serial data
input, DIN, (see Figure 18). The serial interface is designed to
allow easy interfacing to most microcontrollers and DSPs,
e.g., PIC16C, PIC17C, QSPI, SPI, DSP56000, TMS320
and ADSP-21xx, without the need for any gluing logic. When
interfacing to the 8051, the SCLK must be inverted. The
Microprocessor/Microcontroller Interface section explains
how to interface to some popular DSPs and microcontrollers.
Figure 18 shows the timing diagram for a serial read and write
to the AD7811 and AD7812. The serial interface works with
both a continuous and a noncontinuous serial clock. The rising
edge of RFS and falling edge of TFS resets a counter that
counts the number of serial clocks to ensure the correct number
of bits are shifted in and out of the serial shift registers. Once
the correct number of bits have been shifted in and out, the
SCLK is ignored. In order for another serial transfer to take
place the counter must be reset by the active edges of TFS and
t1
1.5␮s
CONVST
t2
SCLK
B
A
CURRENT CONVERSION
RESULT
DOUT
Figure 17. Mode 2 Operation Timing Diagram
t3
SCLK
1
A
2
4
3
5
6
7
8
9
10
11
t4
t5
RFS
t6
TFS
t7
DOUT
DB9
DB8
t10
DB7
DB5
DB6
t8
DIN
DB9
DB8
DB7
DB6
DB4
DB3
DB2
DB1
t9
DB5
DB4
DB3
DB2
DB1
Figure 18. Serial Interface Timing Diagram
REV. B
DB0
–15–
DB0
12
B
13
AD7811/AD7812
RFS. The first rising SCLK edge after the rising edge of the
RFS signal causes DOUT to leave its high impedance state and
data is clocked out onto the DOUT line and also on subsequent
SCLK rising edges. The DOUT pin goes back into a high
impedance state on the 11th SCLK rising edge—Point “A” on
Figure 18. A minimum of 11 SCLKs are therefore needed to
carry out a serial read. Data on the DIN line is latched in on
the first SCLK falling edge after the falling edge of the TFS
signal and on subsequent SCLK falling edges. The control
register is updated on the 13th SCLK rising edge—point “B” on
Figure 18. A minimum of 13 SCLK pulses are therefore needed
to complete a serial write operation. In multipackage applications
the RFS and TFS signals can be used as chip select signals. The
serial interface will not shift data in or out until it receives the
active edge of the RFS or TFS signal.
AD7811/AD7812 to MC68HC11
The Serial Peripheral Interface (SPI) on the MC68HC11 is
configured for Master Mode (MSTR = 0), Clock Polarity Bit
(CPOL) = 0 and the Clock Phase Bit (CPHA) = 1. The SPI is
configured by writing to the SPI Control Register (SPCR)—see
68HC11 user manual. A connection diagram is shown in
Figure 20.
AD7811/AD7812*
Simplifying the Serial Interface
SCLK
SCLK/PD4
DOUT
MISO/PD2
DIN
MOSI/PD3
CONVST
The five-wire interface is designed to support many different
serial interface standards. However, it is possible to reduce the
number of lines required to just three. By simply connecting the
TFS and RFS pins to the CONVST signal (see Figure 4), the
CONVST signal can be used to enable the serial port for reading and writing. This is only possible where a noncontinuous
serial clock is being used.
MC68HC11*
PA0
RFS
TFS
*ADDITIONAL PINS OMITTED FOR CLARITY
Figure 20. Interfacing to the MC68HC11
AD7811/AD7812 to 8051
MICROPROCESSOR INTERFACING
The serial interface on the AD7811 and AD7812 allows the
parts to be directly connected to a range of many different
microprocessors. This section explains how to interface the
AD7811 and AD7812 with some of the more common microcontroller and DSP serial interface protocols.
AD7811/AD7812 to PIC16C6x/7x
The PIC16C6x Synchronous Serial Port (SSP) is configured as
an SPI Master with the Clock Polarity bit = 0. This is done
by writing to the Synchronous Serial Port Control Register
(SSPCON). See user PIC16/17 Microcontroller User Manual.
Figure 19 shows the hardware connections needed to interface
to the PIC16/17. In this example I/O port RA1 is being used to
pulse CONVST and enable the serial port of the AD7811/
AD7812. This microcontroller transfers only eight bits of data
during each serial transfer operation; therefore, two consecutive
read/write operations are needed.
The AD7811/AD7812 requires a clock synchronized to the
serial data. The 8051 serial interface must therefore be operated
in Mode 0. In this mode serial data enters and exits through
RxD and a shift clock is output on TxD (half duplex). Figure 21
shows how the 8051 is connected to the AD7811/AD7812.
However, because the AD7811/AD7812 shifts data out on the
rising edge of the shift clock and latches data in on the falling
edge, the shift clock must be inverted.
8051*
AD7811/AD7812*
SCLK
TxD
DOUT
RxD
DIN
RFS
P1.1
TFS
AD7811/AD7812*
PIC16C6x/7x*
SCLK
SCK/RC3
DOUT
SDO/RC5
*ADDITIONAL PINS OMITTED FOR CLARITY
DIN
CONVST
Figure 21. Interfacing to the 8051 Serial Port
SDI/ RC4
RA1
RFS
TFS
*ADDITIONAL PINS OMITTED FOR CLARITY
Figure 19. Interfacing to the PIC16/17
–16–
REV. B
AD7811/AD7812
It is possible to implement a serial interface using the data ports
on the 8051. This would also allow a full duplex serial transfer
to be implemented. The technique involves “bit banging” an
I/O port (e.g., P1.0) to generate a serial clock and using two
other I/O ports (e.g., P1.1 and P1.2) to shift data in and out—
see Figure 22.
AD7811/AD7812*
8051*
SCLK
P1.0
DOUT
P1.1
DIN
P1.2
RFS
P1.3
TFS
AD7811/AD7812 to ADSP-21xx
The ADSP-21xx family of DSPs are easily interfaced to the
AD7811/AD7812 without the need for extra gluing logic. The
SPORT is operated in normal framing mode. The SPORT
control register should be set up as follows:
TFSW
INVRFS
DTYPE
SLEN
ISCLK
TFSR
IRFS
ITFS
AD7811/AD7812*
Figure 22. Interfacing to the 8051 Using I/O Ports
AD7811/AD7812 to TMS320C5x
The serial interface on the TMS320C5x uses a continuous
serial clock and frame synchronization signals to synchronize
the data transfer operations with peripheral devices like the
AD7811. Frame synchronization inputs have been supplied on
the AD7811/AD7812 to allow easy interfacing with no extra
gluing logic. The serial port of the TMS320C5x is set up to
operate in Burst Mode with internal CLKX (Tx serial clock)
and FSX (Tx frame sync). The Serial Port Control register
(SPC) must have the following setup: F0 = 0, FSM = 1,
MCM = 1 and TXM = 1. The connection diagram is shown
in Figure 23.
SCLK
TMS320C5x*
CLKX
CLKR
DOUT
DR
DIN
DT
RFS
TFS
RFSW = 0, Normal Framing
INVTFS = 0, Active High Frame Signal
00, Right Justify Data
1001, 10-Bit Data Words
1, Internal Serial Clock
RFSR = 1, Frame Every Word
0, External Framing Signal
1, Internal Framing Signal
The 10-bit data words will be right justified in the 16-bit serial
data registers when using this configuration. Figure 24 shows
the connection diagram.
*ADDITIONAL PINS OMITTED FOR CLARITY
AD7811/AD7812*
=
=
=
=
=
=
=
=
ADSP-21xx*
SCLK
SCLK
DOUT
DR
DIN
DT
RFS
RFS
TFS
TFS
*ADDITIONAL PINS OMITTED FOR CLARITY
Figure 24. Interfacing to the ADSP-21xx
AD7811/AD7812 to DSP56xxx
The connection diagram in Figure 25 shows how the AD7811
and AD7812 can be connected to the SSI (Synchronous Serial
Interface) of the DSP56xxx family of DSPs from Motorola. The
SSI is operated in Synchronous Mode (SYN bit in CRB =1)
with internally generated 1-bit clock period frame sync for both
Tx and Rx (FSL1 and FSL0 bits in CRB = 1 and 0 respectively).
AD7811/AD7812*
FSX
FSR
*ADDITIONAL PINS OMITTED FOR CLARITY
DSP56xxx*
SCLK
SCK
DOUT
SRD
DIN
STD
RFS
SC2
Figure 23. Interfacing to the TMS320C5x
TFS
*ADDITIONAL PINS OMITTED FOR CLARITY
Figure 25. Interfacing to the DSP56xxx
REV. B
–17–
AD7811/AD7812
OUTLINE DIMENSIONS
Dimensions shown in inches and (mm).
16-Lead Plastic DIP
(N-16)
0.840 (21.33)
0.745 (18.93)
16
9
1
8
PIN 1
0.280 (7.11)
0.240 (6.10)
0.325 (8.25)
0.300 (7.62) 0.195 (4.95)
0.115 (2.93)
0.060 (1.52)
0.015 (0.38)
0.210 (5.33)
MAX
0.130
(3.30)
MIN
0.160 (4.06)
0.115 (2.93)
0.015 (0.381)
0.008 (0.204)
0.070 (1.77) SEATING
0.045 (1.15) PLANE
0.100
(2.54)
BSC
0.022 (0.558)
0.014 (0.356)
16-Lead Small Outline Package (SOIC)
(R-16A)
0.3937 (10.00)
0.3859 (9.80)
0.1574 (4.00)
0.1497 (5.80)
16
9
1
8
0.0688 (1.75)
0.0532 (1.35)
PIN 1
0.0098 (0.25)
0.0040 (0.10)
0.0500
(1.27)
BSC
SEATING
PLANE
0.2550 (6.20)
0.2284 (5.80)
0.0192 (0.49)
0.0138 (0.35)
0.0196 (0.50)
x 45°
0.0099 (0.25)
0.0099 (0.25)
0.0075 (0.19)
8°
0°
0.0500 (1.27)
0.0160 (0.41)
16-Lead Thin Shrink Outline Package (TSSOP)
(RU-16)
0.201 (5.10)
0.193 (4.90)
9
0.256 (6.50)
0.246 (6.25)
0.177 (4.50)
0.169 (4.30)
16
1
8
PIN 1
0.006 (0.15)
0.002 (0.05)
SEATING
PLANE
0.0433
(1.10)
MAX
0.0256
(0.65)
BSC
0.0118 (0.30)
0.0075 (0.19)
0.0079 (0.20)
0.0035 (0.090)
–18–
8°
0°
0.028 (0.70)
0.020 (0.50)
REV. B
AD7811/AD7812
OUTLINE DIMENSIONS
Dimensions shown in inches and (mm).
1.060 (26.90)
0.925 (23.50)
20
11
1
10
PIN 1
0.280 (7.11)
0.240 (6.10)
0.325 (8.25)
0.300 (7.62) 0.195 (4.95)
0.115 (2.93)
0.060 (1.52)
0.015 (0.38)
0.210 (5.33)
MAX
0.130
(3.30)
MIN
0.160 (4.06)
0.115 (2.93)
0.022 (0.558)
0.014 (0.356)
0.100
(2.54)
BSC
C01312a–0–10/00 (rev. B)
20-Lead Plastic DIP
(N-20)
0.015 (0.381)
0.008 (0.204)
0.070 (1.77) SEATING
0.045 (1.15) PLANE
20-Lead Small Outline Package (SOIC)
(R-20A)
11
1
10
PIN 1
0.4193 (10.65)
0.3937 (10.00)
20
0.2992 (7.60)
0.2914 (7.40)
0.5118 (13.00)
0.4961 (12.60)
0.1043 (2.65)
0.0926 (2.35)
0.0291 (0.74)
x 45°
0.0098 (0.25)
8°
0.0500 0.0192 (0.49)
0°
(1.27) 0.0138 (0.35) SEATING 0.0125 (0.32)
PLANE
BSC
0.0091 (0.23)
0.0118 (0.30)
0.0040 (0.10)
0.0500 (1.27)
0.0157 (0.40)
20-Lead Thin Shrink Outline Package (TSSOP)
(RU-20)
0.260 (6.60)
0.252 (6.40)
11
1
0.006 (0.15)
0.002 (0.05)
SEATING
PLANE
REV. B
PRINTED IN U.S.A.
0.256 (6.50)
0.246 (6.25)
0.177 (4.50)
0.169 (4.30)
20
10
PIN 1
0.0433
(1.10)
MAX
0.0256 (0.65)
BSC
0.0118 (0.30)
0.0075 (0.19)
0.0079 (0.20)
0.0035 (0.090)
–19–
8°
0°
0.028 (0.70)
0.020 (0.50)