Monolithic 12-Bit
2 MHz A/D Converter
AD671
a
FEATURES
12-Bit Resolution
24-Pin “Skinny DIP” Package
Conversion Time: 500 ns max—AD671J/K/S-500
Conversion Time: 750 ns max—AD671J/K/S-750
Low Power: 475 mW
Unipolar (0 V to +5 V, 0 V to +10 V) and Bipolar Input
Ranges (65 V)
Twos Complement or Offset Binary Output Data
Out-of-Range Indicator
MIL-STD-883 Compliant Versions Available
OBS
PRODUCT DESCRIPTION
FUNCTIONAL BLOCK DIAGRAM
AIN BPO/UPO
20
21
ENCODE REF IN VCC ACOM VEE
23
19
16
RANGE
SELECT
3-BIT
FLASH
22
V LOGIC
DCOM
17
18
24
X4
DAC
3-BIT
FLASH
3
8-BIT
LADDER
MATRIX
COARSE
4-BIT
FLASH
DAC
3
FINE
4-BIT
FLASH
4
CORRECTION LOGIC
4
8
OLE
LATCHES
The AD671 is a high speed monolithic 12-bit A/D converter
offering conversion rates of up to 2 MHz (500 ns conversion
time). The combination of a merged high speed bipolar/CMOS
process and a novel architecture results in a combination of
speed and power consumption far superior to previously available hybrid implementations. Additionally, the greater reliability
of monolithic construction offers improved system reliability
and lower costs than hybrid designs.
The AD671 uses a subranging flash conversion technique, with
digital error correction for possible errors introduced in the first
part of the conversion cycle. An on-chip timing generator provides strobe pulses for each of the four internal flash cycles and
assures adequate settling time for the interflash residue amplifier. A single ENCODE pulse is used to control the converter.
The performance of the AD671 is made possible by using high
speed, low noise bipolar circuitry in the linear sections and low
power CMOS for the logic sections. Analog Devices’ ABCMOS-1
process provides both high speed bipolar and 2-micron CMOS
devices on a single chip. Laser trimmed thin-film resistors are
used to provide accuracy and temperature stability.
AD671
12
14
13
OTR MSB
12 1
15
BIT1-12
DAV
TE
PRODUCT HIGHLIGHTS
1. The AD671 offers a single chip 2 MHz analog-to-digital
conversion function in a space saving 24-pin DIP.
2. Input signal ranges are 0 V to +5 V and 0 V to +10 V unipolar, and –5 V to +5 V bipolar, selected by pin strapping. Input resistance is 1.5 kΩ. Power supplies are +5 V and –5 V,
and typical power consumption is less than 500 mW.
3. The external +5 V reference can be chosen to suit the dc accuracy and temperature drift requirements of the application.
4. Output data is available in unipolar, bipolar offset or bipolar
twos complement binary format.
5. An OUT OF RANGE output bit indicates when the input
signal is beyond the AD671’s input range.
6. The AD671 is available in versions compliant with the MILSTD-883. Refer to the Analog Devices Military Products
Databook or current AD671/883B data sheet for detailed
specifications.
The AD671 is available in two conversion speeds and performance grades. The AD671J and K grades are specified for operation over the 0°C to +70°C temperature range. The AD671S
grades are specified for operation over the –55°C to +125°C
temperature range. All grades are available in a 0.300 inch wide
24-pin ceramic DIP. The J and K grades are also available in a
24-pin plastic DIP.
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: 617/329-4700
Fax: 617/326-8703
AD671–SPECIFICATIONS
(T to T with V
MIN
MAX
= +5 V 6 5%, VLOGIC = +5 V 610%, VEE = –5 V 6 5%, VREF = +5.000 V,
CC
DC SPECIFICATIONS unless otherwise noted)
Parameter
Min
RESOLUTION
12
ACCURACY (+25°C)
Integral Nonlinearity (INL)
TMIN to TMAX
Differential Nonlinearity (DNL)
TMIN to TMAX
No Missing Codes
Unipolar Offsetl
Bipolar Zerol
Gain Error2
AD671J/S-500
Typ
10
11
POWER SUPPLIES
Power Supply Rejection4
VCC (+5 V ± 0.25 V)
VLOGIC (+5 V ± 0.5 V)
VEE (–5 V ± 0.25 V)
Operating Voltages
VCC
VLOGIC
VEE
Operating Current
ICC
ILOGIC5
IEE
1.0
0.5
2.4
0.1
OLE
+5
+5
+10
–5
0
0
2.0
1.0
1.0
0.5
4.7
2.4
1.5
0.75
10
3.5
61
61
61
+4.75
+4.5
–5.25
POWER CONSUMPTION
TEMPERATURE RANGE
Specified (J/K)
Specified (S)
Bits
610
615
620
1.5
0.75
10
3.5
LSB
11 Bits Guaranteed
64
610
0.25
0.1
Units
Bits
62
10 Bits Guaranteed
–5
0
0
Max
12
OBS
Input Resistance
10 Volt Range
5 Volt Range
Input Capacitance
Reference Input Resistance
AD671K-500
Typ
Min
64
TEMPERATURE COEFFICIENTS3
Unipolar Offset
Bipolar Zero
Gain Error
ANALOG INPUT
Input Ranges
Bipolar
Unipolar
Max
+5.25
+5.5
–4.75
+4.75
+4.5
–5.25
64
610
0.25
LSB
LSB
% FSR
610
615
620
ppm/°C
ppm/°C
ppm/°C
+5
+5
+10
Volts
Volts
Volts
2.0
1.0
4.7
kΩ
kΩ
pF
kΩ
61
61
61
LSB
LSB
LSB
+5.25
+5.5
–4.75
Volts
Volts
Volts
TE
46
3
46
56
6
56
46
3
46
56
6
56
mA
mA
mA
475
621
475
621
mW
+70
°C
°C
0
–55
+70
+125
0
NOTES
1
Adjustable to zero with external potentiometers. See Offset/Gain Calibration section for additional information.
2
Full-scale range (FSR) is 5 V for the 0 V to 5 V range and 10 V for the 0 V to 10 V and –5 V to +5 V ranges.
3
25°C to TMIN and 25°C to TMAX.
4
Change in gain error as a function of the dc supply voltage.
5
Tested under static conditions. See Figure 12 for typical curves of ILOGIC vs. Conversion Rate and Output Loading.
Specifications subject to change without notice.
Specifications shown in boldface are tested on all devices at final electrical test with worst case supply voltages at 0, +25 °C and +70°C. Results from those tests are
used to calculate outgoing quality levels. All min and max specifications are guaranteed, although only those shown in boldface are tested.
–2–
REV. B
AD671
(TMIN to TMAX with VCC = +5 V 6 5%, VLOGIC = +5 V 6 10%, VEE = –5 V 6 5%, VREF = +5.000 V,
DC SPECIFICATIONS unless otherwise noted)
Parameter
Min
RESOLUTION
12
ACCURACY (+25°C)
Integral Nonlinearity (INL)
TMIN to TMAX (J)
TMIN to TMAX (S)
Differential Nonlinearity (DNL)
TMIN to TMAX
No Missing Codes
Unipolar Offsetl
Bipolar Zerol
Gain Error2
ANALOG INPUT
Input Ranges
Bipolar
Unipolar
Input Resistance
10 Volt Range
5 Volt Range
Input Capacitance
Reference Input Resistance
POWER SUPPLIES
Power Supply Rejection4
VCC (+5 V ± 0.25 V)
VLOGIC (+5 V ± 0.5 V)
VEE (–5 V ± 0.25 V)
Operating Voltages
Vcc
VLOGIC
VEE
Operating Current
ICC
ILOGIC5
IEE
AD671K-750
Typ
Min
Max
12
11
12
Bits
0.1
610
615
620
OLE
–5
0
0
1.0
0.5
2.4
1.5
0.75
10
3.5
+5
+5
+10
–5
0
0
2.0
1.0
1.0
0.5
4.7
2.4
+5.25
+5.5
–4.75
64
610
0.25
LSB
LSB
% FSR
610
615
620
ppm/°C
ppm/°C
ppm/°C
TE
1.5
0.75
10
3.5
61
61
61
+4.75
+4.5
–5.25
LSB
LSB
12 Bits Guaranteed
64
610
0.25
0.1
Units
Bits
61.5
11 Bits Guaranteed
POWER CONSUMPTION
TEMPERATURE RANGE
Specified (J/K)
Specified (S)
Max
62
62.5
OBS
TEMPERATURE COEFFICIENTS3
Unipolar Offset
Bipolar Zero
Gain Error
AD671J/S-750
Typ
+4.75
+4.5
–5.25
+5
+5
+10
Volts
Volts
Volts
2.0
1.0
4.7
kΩ
kΩ
pF
kΩ
61
61
61
LSB
LSB
LSB
+5.25
+5.5
–4.75
Volts
Volts
Volts
46
3
46
56
6
56
46
3
46
56
6
56
mA
mA
mA
475
621
475
621
mW
+70
°C
°C
0
–55
+70
+125
0
NOTES
1
Adjustable to zero with external potentiometers. See Offset/Gain Calibration section for additional information.
2
Full-scale range (FSR) is 5 V for the 0 V to 5 V range and 10 V for the 0 V to 10 V and –5 V to +5 V ranges.
3
25°C to TMIN and 25°C to TMAX.
4
Change in gain error as a function of the dc supply voltage.
5
Tested under static conditions. See Figure 12 for typical curves of I LOGIC vs. Conversion Rate and Output Loading.
Specifications subject to change without notice.
Specifications shown in boldface are tested on all devices at final electrical test with worst case supply voltages at 0, +25 °C and +70°C. Results from those tests are
used to calculate outgoing quality levels. All min and max specifications are guaranteed, although only those shown in boldface are tested.
REV. B
–3–
AD671–SPECIFICATIONS
(For all grades T
DIGITAL SPECIFICATIONS 6 5%, V
REF
MIN to TMAX, with VCC = +5 V 6 5%, VLOGIC = +5 V 6 10%, VEE = –5 V
= +5.000 V, unless otherwise noted)
Parameter
Symbol
Min
LOGIC INPUT
High Level Input Voltage
Low Level Input Voltage
High Level Input Current (VIN = VLOGIC)
Low Level Input Current (VIN = 0 V)
Input Capacitance
VIH
VIL
IIH
IIL
CIN
+2.0
LOGIC OUTPUTS
High Level Output Voltage (IOH = 0.5 mA)
Low Level Output Voltage (IOL = 1.6 mA)
Output Capacitance
VOH
VOL
COUT
+2.4
OBS
Typ
–10
–10
Max
Units
+0.8
+10
+10
V
V
µA
µA
pF
+0.4
V
V
pF
5
5
Specifications shown in boldface are tested on all devices at final electrical test. Results from those tests are used to calculate outgoing quality levels. All min and max
specifications are guaranteed, although only those shown in boldface are tested.
Specifications subject to change without notice.
OLE
(For all grades TMIN to TMAX with VCC = +5 V 6 5%, VLOGIC = +5 V 6 10%, VEE = –5 V
IL = 0.8 V, VIH = 2.0 V, VOL = 0.4 V and VOH = 2.4 V)
SWITCHING SPECIFICATIONS 6 5%, V
Parameter
Conversion Time
(AD671-500)
(AD671-750)
ENCODE Pulse Width High
(AD671-500)
(AD671-750)
ENCODE Pulse Width Low
DAV Pulse Width
(AD671-500)
(AD671-750)
ENCODE Falling Edge Delay
Start New Conversion Delay
Data and OTR Delay from DAV Falling Edge
Data and OTR Valid before DAV Rising Edge
Symbol
Min
tC
tC
Typ
475
725
tENC
tENC
tENCL
20
20
20
tDAV
tDAV
tF
tR
tDD1
tSS2
75
75
0
0
20
20
Max
Units
TE
500
750
ns
ns
30
50
ns
ns
ns
200
300
ns
ns
ns
ns
ns
ns
75
75
NOTES
1
tDD is measured from when the falling edge of DAV crosses 0.8 V to when the output crosses 0.4 V or 2.4 V with a 25 pF load capacitor on each output pin.
2
tSS is measured from when the outputs cross 0.4 V or 2.4 V to when the rising edge of DAV crosses 2.4 V with a 25 pF load capacitor on each output pin.
b. Encode Pulse LOW
a. Encode Pulse HIGH
Figure 1. AD671 Timing Diagrams
–4–
REV. B
AD671
ABSOLUTE MAXIMUM RATINGS*
Parameter
VCC
VEE
VLOGIC
ACOM
VCC
ENCODE
REF IN
AIN, BPO/UPO
Junction Temperature
Storage Temperature
Lead Temperature (10 sec)
Power Dissipation
With
Respect
to
Min
ACOM
ACOM
DCOM
DCOM
VLOGIC
DCOM
ACOM
ACOM
Max
ORDERING GUIDE
Units
–0.5
–6.5
–0.5
–1.0
–6.5
–0.5
–0.5
–6.5
+6.5
Volts
+0.5
Volts
+6.5
Volts
+1.0
Volts
+6.5
Volts
VLOGIC +0.5 Volts
VCC +0.5 Volts
11.0
Volts
+175
°C
–65 +150
°C
+300
°C
1000
mW
Modell
Linearity
Temperature
Range
Package
Options2
AD671JD-500
AD671KD-500
AD671JD-750
AD671KD-750
AD671SD-500
AD671SD-750
± 4 LSB
± 2 LSB
± 2 LSB
± 1.5 LSB
± 4 LSB
± 2.5 LSB
0°C to +70°C
0°C to +70°C
0°C to +70°C
0°C to +70°C
–55°C to +125°C
–55°C to +125°C
D-24A
D-24A
D-24A
D-24A
D-24A
D-24A
NOTES
1
For details on grade and package offerings screened in accordance with
MIL-STD-883, refer to the Analog Devices Military Products Databook or
current AD671/883 data sheet.
2
D = Ceramic DIP.
OBS
*Stresses above those listed under “Absolute Maximum Ratings” may cause
permanent damage to the device. This is a stress rating only and functional
operation of the device at these or any other conditions above those indicated in the
operational sections of this specification is not implied. Exposure to absolute
maximum ratings for extended periods may effect device reliability.
OLE
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 AD671 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. B
–5–
TE
WARNING!
ESD SENSITIVE DEVICE
AD671
AD671 PIN DESCRIPTION
Symbol
Pin
Type Name and Function
ACOM
AIN
BIT1 (MSB)
BIT2–BIT11
BIT12 (LSB)
BPO/UPO
22
20
12
11–2
1
21
P
AI
DO
DO
DO
AI
CONNECTION DIAGRAM
PINOUT
Analog Ground.
Analog Input Signal.
Most Significant Bit.
Data Bits 2–11.
Least Significant Bit.
Bipolar or Unipolar
Configuration Pin. Connect to
AIN for 0 V to +5 V Span, to
ACOM for 0 V to +10 V Span
and to REF IN for –5 V to
+5 V Span.
Data Available Output. The
Rising Edge of DAV Indicates
an End of Conversion and Can
Be Used to Latch Current
Data into an External
Register. The Falling Edge of
DAV Can Be Used to Latch
Previous Data into an External
Register.
Digital Ground.
The AD671 Starts a
Conversion on the Rising
Edge of the ENCODE Pulse.
Inverted Most Significant Bit.
Provides Twos Complement
Output Data Format.
Out of Range Is Active HIGH
when the analog input is
beyond the input range of the
converter.
+5 V Reference Input.
+5 V Analog Power.
–5 V Analog Power.
+5 V Digital Power.
OBS
DAV
15
DO
DCOM
ENCODE
18
16
P
DI
MSB
13
DO
OTR
14
DO
REF IN
VCC
VEE
VLOGIC
19
23
24
17
AI
P
P
P
1
24 VEE
BIT11 2
23 VCC
BIT10
3
22 ACOM
BIT9
4
21 BPO/UPO
BIT12 (LSB)
20 AIN
BIT8
5
BIT7
6
AD671
19 REF IN
BIT6
7
TOP VIEW
(Not to Scale)
18 DCOM
BIT5
8
17 VLOGIC
BIT4
9
16 ENCODE
BIT3 10
15 DAV
BIT2 11
14 OTR
BIT1 (MSB) 12
13 MSB
OLE
TE
TYPE:
AI = Analog Input
DI = Digital Input
DO = Digital Output
P = Power
–6–
REV. B
AD671
DEFINITIONS OF SPECIFICATIONS
GAIN ERROR
INTEGRAL NONLINEARITY (INL)
The last transition (from 1111 1111 1110 to 1111 1111 1111)
should occur for an analog value 1 1/2 LSB below the nominal
full scale (9.9963 volts for 10.000 volts full scale). The gain error is the deviation of the actual level at the last transition from
the ideal level. The gain error can be adjusted to zero as shown
in Figures 7, 8 and 9.
Integral nonlinearity refers to the deviation of each individual
code from a line drawn from “zero” through “full scale.” The
point used as “zero” occurs 1/2 LSB (1.22 mV for a 10 V span)
before the first code transition (all zeros to only the LSB on).
“Full scale” is defined as a level 1 1/2 LSB beyond the last code
transition (to all ones). The deviation is measured from the low
side transition of each particular code to the true straight line.
TEMPERATURE COEFFICIENTS
The temperature coefficients for unipolar offset, bipolar zero
and gain error specify the maximum change from the initial
(+25°C) value to the value at TMIN or TMAX.
DIFFERENTIAL NONLINEARITY (DNL, NO MISSING
CODES)
An ideal ADC exhibits code transitions that are exactly 1 LSB
apart. DNL is the deviation from this ideal value. Thus every
code must have a finite width. Guaranteed no missing codes to
10-bit resolution indicates that all 1024 codes represented by
Bits 1–10 must be present over all operating ranges. Guaranteed
no missing codes to 11- or 12-bit resolution indicates that all
2048 and 4096 codes, respectively, must be present over all operating ranges.
POWER SUPPLY REJECTION
OBS
UNIPOLAR OFFSET
The only effect of power supply error on the performance of the
device will be a small change in gain. The specifications show
the maximum full-scale change from the initial value with the
supplies at the various limits.
OLE
SIGNAL-TO-NOISE AND DISTORTION (S/N+D) RATIO
S/N+D is the ratio of the rms value of the measured input signal
to the rms sum of all other spectral components, including harmonics but excluding dc. The value for S/N+D is expressed in
decibels.
The first transition should occur at a level 1/2 LSB above analog
common. Unipolar offset is defined as the deviation of the actual from that point. This offset can be adjusted as discussed
later. The unipolar offset temperature coefficient specifies the
maximum change of the transition point over temperature, with
or without external adjustments.
BIPOLAR ZERO
TE
EFFECTIVE NUMBER OF BITS (ENOB)
ENOB is calculated from the expression SNR = 6.02N +
1.8 dB, where N is equal to the effective number of bits.
TOTAL HARMONIC DISTORTION (THD)
THD is the ratio of the rms sum of the first six harmonic components to the rms value of the measured input signal and is expressed as a percentage or in decibels.
In the bipolar mode the major carry transition (0111 1111 1111
to 1000 0000 0000) should occur for an analog value 1/2 LSB
below analog common. The bipolar offset error and temperature
coefficient specify the initial deviation and maximum change in
the error over temperature.
PEAK SPURIOUS OR PEAK HARMONIC COMPONENT
The peak spurious or peak harmonic component is the largest
spectral component excluding the input signal and dc. This
value is expressed in decibels relative to the rms value of a fullscale input signal.
Theory of Operation
The AD671 uses a successive subranging architecture. The analog to digital conversion takes place in four independent steps or
flashes. The analog input signal is subranged to an intermediate
residue voltage for the final 12-bit result by utilizing multiple
flashes with subtraction DACs (see the AD671 functional block
diagram).
(AD671-500) and less than 50 ns after the falling edge of
ENCODE (AD671–750) or after the falling edge of DAV. The
time window prevents digitally coupled noise from being introduced during the final stages of conversion. An internal timing
generator circuit accurately controls all internal timing.
ACOM 22
The AD671 can be configured to operate with unipolar (0 V to
+5 V, 0 V to +10 V) or bipolar (± 5 V) inputs by connecting
AIN (Pin 20), REFIN (Pin 19) and BPO/UPO (Pin 21) as
shown in Figure 2.
BPO/UPO 21
AIN 20
The AD671 conversion cycle begins by simply providing an active HIGH pulse on the ENCODE pin (Pin 16). The rising
edge of the ENCODE pulse starts the conversion. The falling
edge of the ENCODE pulse is specified to operate within a window of time: less than 30 ns after the rising edge of ENCODE
REV. B
BPO/UPO 21
REF IN 19
AIN
+ 5V REF
0 TO + 5V
BPO/UPO 21
AIN 20
REF IN 19
AIN
+ 5V REF
0 TO +10V
AIN 20
REF IN 19
+ 5V REF
– 5V TO + 5V
Figure 2. Input Range Connections
–7–
AIN
AD671
Upon receipt of an ENCODE command, the first 3-bit flash
converts the analog input voltage. The 3-bit result is passed to a
correction logic register and a segmented current output DAC.
The DAC output is connected through a resistor (within the
Range/Span Select Block) to AIN. A residue voltage is created
by subtracting the DAC output from AIN, which is less than
one eighth of the full-scale analog input. The second flash has
an input range that is configured with one bit of overlap with the
previous DAC. The overlap allows for errors during the flash
conversion. The first residue voltage is connected to the second
3-bit flash and to the noninverting input of a high speed, differential, gain-of-four amplifier. The second flash result is passed
to the correction logic register and to the second segmented current output DAC. The output of the second DAC is connected
to the inverting input of the differential amplifier. The differential amplifier output is connected to a two step backend 8-bit
flash. This 8-bit flash consists of coarse and fine flash converters. The result of the coarse 4-bit flash converter, also configured to overlap one bit of DAC 2, is connected to the correction
logic register and selects one of 16 resistors from which the fine
4-bit flash will establish its span voltage. The fine 4-bit flash is
connected directly to the output latches.
OBS
INPUT BUFFER AMPLIFIER
The closed-loop output impedance of an op amp is equal to the
open loop output impedance (usually a few hundred ohms) divided by the loop gain at the frequency of interest. It is often
assumed that loop gain of a follower-connected op amp is sufficiently high to reduce the closed-loop output impedance to a
negligibly small value, particularly if the input signal is low
frequency. At higher frequencies the open-loop gain is lower,
increasing the output impedance which decreases the instantaneous analog input voltage and produces an error.
The recommended wideband, fast settling input amplifiers for
use with the AD671 are the AD841, AD843, AD845 or the
AD847. The AD841 is unity gain stable and recommended as a
follower connected op amp. The AD843 and AD845 FET inputs make them ideal for high speed sample-and-hold amplifiers
and the AD847 can be used as a low power, high speed buffer.
Figure 4 shows the AD841 driving the AD671. As shown in the
figure the analog input voltage should be produced with respect
to the ACOM pin.
OLE
The AD671 will flag an out-of-range condition when the input
voltage exceeds the analog input range. OTR (Pin 14) is active
HIGH when an out of range high or low condition exists. Bits
1–12 are HIGH when the analog input voltage is greater than
the selected input range and LOW when the analog input is less
than the selected input range.
APPLYING THE AD671
4
+
±5V
–
DRIVING THE AD671 ANALOG INPUT
+ 5V REF
The AD671 uses a very high speed current output DAC to subtract a known voltage from the analog input. This results in very
fast steps of current at the analog input. It is important to recognize that the signal source driving the analog input of the
AD671 must be capable of maintaining the input voltage under
dynamically-changing load conditions. When the AD671 starts
its conversion cycle, the subtraction DAC will sink up to 5 mA
(see Figure 3) from the source driving the analog input. The
source must respond to this current step by settling the input
voltage back to a fraction of an LSB before the AD671 makes its
final 12-bit decision.
+
IIN
R
–
A/D
IA/D
11
AD841 10
5
6
DAC
IDAC
AD671
Figure 3. Driving the Analog Input
Unlike successive approximation A/Ds, where the input voltage
must settle to a fraction of a 12-bit LSB before each successive
bit decision is made, the AD671 requires the analog input voltage settle to within 12 bits before the third flash conversion,
approximately 200 ns. This “free” 200 ns is useful in applications requiring a sample-and-hold amplifier (SHA), overlapping
the SHA’s hold mode settling time within the 200 ns window
will increase total system throughput. See the “Discrete Sampleand-Hold” section for a high speed SHA application.
17
24
VEE VLOGIC
23
VCC
TE
BIT1 1
20 AIN
BIT12 12
22 ACOM
ENCODE 16
18 DCOM
DAV 15
19 REF IN
OTR 14
21 BPO/UPO
MSB 13
AD671
Figure 4. Input Buffer Amplifier
REFERENCE INPUT
The AD671 uses a standard +5 volt reference. The initial accuracy and temperature stability of the reference can be selected to
meet specific system requirements. Like the analog input, fast
switching input-dependent currents are modulated at the reference input pin (REF IN–Pin 19). However, unlike the analog
input the reference input is held at a constant +5 volts with the
use of capacitor. The recommended reference is the AD586, a
+5 V precision reference with an output buffer amplifier. Figure 5 shows the AD671 configured in the ± 5 V input range.
The 6.8 µF capacitor maintains a constant +5 volts under the
dynamically changing load conditions. An optional 1 µF noise
reduction capacitor can be connected to the AD586, further reducing broadband output noise. To minimize ground voltage
drops the AD586’s ground pin should be tied as close as possible to the AD671’s ACOM pin. See Figures 20, 21 and 22 for
PCB layout recommendations.
–8–
REV. B
AD671
23
VCC
20 AIN
+15V
8
1µF
C14
AD586
U4
± 5V
2
+VIN
VOUT
NOISE
6
REDUCTION
6.8µF
C15
GND
4
Table I. Grounding and Decoupling Guidelines
24
17
VEE VLOGIC
Power Supply
Decoupling
BIT1 1
U3
BIT12 12
22 ACOM
ENCODE 16
18 DCOM
DAV 15
19 REF IN
OTR 14
21 BPO/UPO
MSB 13
Comment
0.1 µF (Ceramic) and 10 µF (Tantalum).
(Surface Mount Chip Capacitors Recommended to Reduce Lead Inductance).
Capacitor Values
Capacitor Locations Directly at Positive and Negative
Supply Pins to Respective Ground Plane.
AD671
Grounding
OBS
Analog Ground
Ground Plane or Wide Ground Return
Connected to the Analog Power Supply.
Digital Ground
Ground Plane or Wide Ground Return
Connected to the Digital Power Supply.
Figure 5. AD586 as Reference Input for AD671
GROUNDING AND DECOUPLING RULES
Proper grounding and decoupling should be a primary design
objective in any high speed, high resolution system. The AD671
separates analog and digital grounds to optimize the management of analog and digital ground currents in a system. The
AD671 is designed to minimize the current flowing from
ACOM (Pin 22) by directing the majority of the current from
VCC (+5 V–Pin 23) to VEE (–5 V–Pin 24). Minimizing analog
ground currents hence reduces the potential for large ground
voltage drops. This can be especially true in systems that do not
utilize ground planes or wide ground runs. ACOM is also configured to be code independent, therefore reducing input dependent analog ground voltage drops and errors. The input current
supplied by the external reference (REFIN–Pin 19) and the majority of the full-scale input signal (AIN–Pin 20) are also directed to VÉE. Also critical in any high speed digital design are
the use of proper digital grounding techniques to avoid potential
CMOS “ground bounce.” Figure 6 is provided to assist in the
proper layout, grounding and decoupling techniques.
Analog and Digital
Ground
OLE
Connected Together Once at the AD671.
UNIPOLAR (0 V TO +10 V) CALIBRATION
The AD671 is factory trimmed to minimize offset, gain and linearity errors. In some applications the offset and gain errors of
the AD671 need to be externally adjusted to zero. This is accomplished by trimming the voltage at BPO/UPO (Pin 21) and
REFIN (Pin 19). In those applications the AD588, a high precision pin programmable voltage reference, is an ideal choice. The
AD588 includes a reference cell and three additional amplifiers
which can be configured to provide offset and gain trims for the
AD671. The circuit in Figure 7 is recommended for calibrating
offset and gain errors of the AD671 when configured in the 0 V
to +10 V input range.
TE
+ 5V
10µF
Table I is a list of grounding and decoupling guidelines that
should be reviewed before laying out a printed circuit board.
+ 5V
10µF
– 5V
10µF
0.1µF
0 TO +10V
+ 5V
0.1µF
23
VCC
10µF
0.1µF
24
VEE
1µF
7
6
4
3
1
VLOGIC
14
BIT1 12
BIT12 1
16
5
–
22 ACOM
10
ENCODE 16
+ 5V REF
18 DCOM
DAV 15
19 REF IN
OTR 14
21 BPO/UPO
MSB 13
8
12
BIT1 12
11
22 ACOM
ENCODE 16
18 DCOM
DAV 15
19 REF IN
OTR 14
21 BPO/UPO
MSB 13
0.1µF
10k
AD671
150 10µF
+15
0.1µF
–15
13
R2
100k
DGP*
5k
50
Figure 7. Unipolar (0 V to +10 V) Calibration
The AD671 is intended to have a nominal 1/2 LSB offset so
that the exact analog input for a given code will be in the middle
of that code (halfway between the transitions to the codes above
it and below it). Thus, the first transition ( from 0000 0000 0000
to 0000 0000 0001) will occur for an input level of +1/2 LSB
(1.22 mV for 10 V range). If the offset trim resistor R2 is used,
AD671
*GROUND PLANE RECOMMENDED
Figure 6. AD671 Grounding and Decoupling
REV. B
9
17
VLOGIC
BIT12 1
10µF
1µF
15
2
AGP*
VEE
50
17
AD588
VIN ± 5V
0.1µF
24
R1
100
150pF
+
20 AIN
0.1µF
23
VCC
+ 5V
10µF
20 AIN
+15V
39k
0.1µF
–5V
10µF
–9–
AD671
it should be trimmed as above, although a different offset can be
set for a particular system requirement. This circuit will give approximately ± 50 mV of offset trim range.
The gain trim is done by applying a signal 1 1/2 LSBs below the
nominal full scale (9.9963 for a 10 V range). Trim R1 to give
the last transition (1111 1111 1110 to 11111111 1111).
Bipolar calibration is similar to unipolar calibration. First, a signal 1/2 LSB above negative full scale (–4.9988 V) is applied and
R1 is trimmed to give the first transition (0000 0000 0000 to
0000 0000 0001). Then a signal 1 1/2 LSB below positive full
scale (+4.9963) is applied, and R2 is trimmed to give the last
transition (1111 1111 1110 to 1111 1111 1111).
UNIPOLAR (0 V TO +5 V) CALIBRATION
OUTPUT LATCHES
The connections for the 0 V to +5 V input range calibration is
shown in Figure 8. The AD586, a +5 V precision voltage reference, is an excellent choice for this mode of operation because
of its performance, stability and optional fine trim. The AD845
(16 MHz, low power, low cost op amp) is used to maintain the
+5 volts under the dynamically changing load conditions of the
reference input.
OBS
+15V
0.1µF
7
2
AD845
0 TO +5V
3
1
4
8
23
VCC
20 AIN
6
24
17
VEE VLOGIC
21 BPO/UPO
1kΩ
+15V
390
–15V
0.1µF
22 ACOM
+15V
+15V
7
2
2
+VIN
AD845
VOUT 6
3
TRIM 5
8 NOISE
REDUCTION
0.1µF
18 DCOM
6
19 REFIN
4
10kΩ
–15V
0.1µF
4
OLE
BIT1 12
BIT12 1
BIT1
BIT2
BIT3
BIT4
BIT5
BIT6
BIT7
BIT8
DAV
ENCODE 16
DAV 15
OTR 14
MSB 13
AD671
AD586
1µF
Figure 10 shows the AD671 connected to the 74HC574 Octal
D-type edge triggered latches with 3-state outputs. The latch
can drive highly capacitive loads (i.e., bus lines, I/O ports) while
maintaining the data signal integrity. The maximum set-up and
hold times of the 574 type latch must be less than 20 ns (tDD
and tSS minimum). To satisfy the requirements of the 574 type
latch the recommended logic families are HC, S, AS, ALS, F or
BCT. New data from the AD671 is latched on the rising edge of
the DAV (Pin 24) output pulse. Previous data can be latched by
inverting the DAV output with a 7404 type inverter. See Figures 20, 21 and 22 for PCB layout recommendations.
GND
The AD671 offset error must be trimmed within the analog input path, either directly in front of the AD671 or within the signal conditioning chain, eliminating offset errors induced by the
signal conditioning circuitry. Figure 8 shows an example of how
the offset error can be trimmed in front of the AD671. The
AD586 is configured in the optional fine trim mode to provide
+6%/–2% (+240 LSBs/–80 LSBs) of gain trim. The procedure
for trimming the offset and gain errors is similar to that used for
the unipolar 10 V range with the analog input values set to onehalf the 10 V range values.
BIPOLAR (65 V) CALIBRATION
The connections for the bipolar input range is shown in Figure
9. The AD588 is configured to provide dual +5 V outputs. Providing a +5 V reference voltage for the AD671 gain trim and the
+5 V BPO/UPO input for the bipolar offset trim.
23
VCC
±5V
24
VEE
20 AIN
6.2kΩ
+15V
BIT12 1
150pF
7
6
4
3
10µF
14
150pF
AD588
15
2
16
9
10
8
12
11
22 ACOM
ENCODE 16
18 DCOM
DAV 15
19 REF IN
OTR 14
21 BPO/UPO
MSB 13
50
+15
10µF
1Q
2Q
3Q
4Q
5Q
6Q
7Q
8Q
OC
3-STATE
CONTROL
Figure 10. AD671 to Output Latches
OUT OF RANGE
An Out of Range condition exists when the analog input voltage
is beyond the input range (0 V to +5 V, 0 V to +10 V, ± 5 V) of
the converter. OTR (Pin 14) is set low when the analog input
voltage is within the analog input range. OTR is set HIGH and
will remain HIGH when the analog input voltage exceeds the
input range by typically 1/2 LSB (OTR transition is tested to
± 6 LSBs of accuracy) from the center of the ± full-scale output
codes. OTR will remain HIGH until the analog input is within
the input range and another conversion is completed. By logical
ANDing OTR with the MSB and its complement overrange
high or underrange low conditions can be detected. Table II is a
truth table for the over/under range circuit in Figure 11. Systems requiring programmable gain conditioning prior to the
AD671 can immediately detect an out of range condition, thus
eliminating gain selection iterations.
Table II. Out of Range Truth Table
0.1µF
R2
100
OC
74HC574
1D
2D
3D
4D
5D U5
6D
7D
8D
CLK
AD671
50
1
5
BIT1 12
R1
100
39k
1µF
17
VLOGIC
DATA BUS
1Q
2Q
3Q
4Q
5Q
6Q
7Q
8Q
TE
BIT9
BIT10
BIT11
BIT12
Figure 8. Unipolar (0 V to +5 V) Calibration
74HC574
1D
2D
3D
4D
5D U6
6D
7D
8D
CLK
OTR
MSB
Analog Input Is
0
0
1
1
0
1
0
1
In Range
In Range
Underrange
Overrange
AD671
0.1µF
–15
13
Figure 9. Bipolar (± 5 V) Calibration
–10–
REV. B
AD671
MSB
OVER = "1"
OTR
UNDER = "1"
MSB
Figure 11. Overrange or Underrange Logic
OUTPUT DATA FORMAT
The AD671 provides both MSB and MSB outputs, delivering
data in positive true straight binary for unipolar input ranges
and positive true offset binary or twos complement for bipolar
OBS
input ranges. Straight binary coding is used for systems that accept positive-only signals. If straight binary coding is used with
bipolar input signals a 0 V input would result in a binary output
of 2048. The application software would have to subtract 2048
to determine the true input voltage. Most processors typically
perform math on signed integers and assume data is in that format. Twos complement format minimizes software overhead
which is especially important in high speed data transfers, such
as a DMA operation. The CPU is not bogged down performing
data conversion steps, hence increasing the total system
throughput.
Table III. Output Data Format
Input
Range
Coding
Analog
Input1
Digital
Output
OTR2
0 to +5 V
Straight Binary
≤ –0.00061 V
0V
+5 V
>+5.00061 V
0000 0000 0000
0000 0000 0000
1111 1111 1111
1111 1111 1111
1
0
0
1
0 to +10 V
Straight Binary
≤ –0.00122 V
0V
+10 V
≥ +10.00122 V
0000 0000 0000
0000 0000 0000
1111 1111 1111
1111 1111 1111
1
0
0
1
–5 V to +5 V
Offset Binary
≤ –5.00122 V
–5 V
0V
+4.99756 V
≥ +4.99878 V
0000 0000 0000
0000 0000 0000
1000 0000 0000
1111 1111 1111
1111 1111 1111
1
0
0
0
1
–5 V to +5 V
2s Complement
(Using MSB)
≤ –5.00122 V
–5 V
0V
+4.99756 V
≥ +4.99878 V
1000 0000 0000
1000 0000 0000
0000 0000 0000
0111 1111 1111
0111 1111 1111
1
0
0
0
1
OLE
TE
NOTES
1
Voltages listed are with offset and gain errors adjusted to zero.
2
Typical performance.
ILOGIC vs. CONVERSION RATE
Figure 12 shows the typical logic supply current vs. conversion
rate for various capacitive loads on the digital outputs.
6.5
6.0
5.5
CL = 50pF
5.0
4.5
mA
4.0
CL = 30pF
3.5
3.0
2.5
2.0
CL = 0pF
1.5
1.0
0.5
1k
10k
100k
CONVERSION RATE – Hz
1M
10M
Figure 12. ILOGIC vs. Conversion Rate for Various
Capacitive Loads on the Digital Outputs
REV. B
–11–
AD671
HIGH PERFORMANCE SAMPLE-AND-HOLD
AMPLIFIER (SHA)
CROSS COUPLED LATCH
In order to take full advantage of the AD671’s high speed capabilities, a sample-and-hold amplifier (SHA) with fast acquisition
capabilities and rigid accuracy requirements is essential. One
possibility is a hybrid SHA such as the HTC-0300A, but often a
cost effective alternative like the one shown in Figure 13 may be
a better solution. This discrete SHA requires very few components and is able to acquire signals to 0.01% accuracy in less
than 350 nanoseconds. Combined with the AD671, signals with
bandwidths up to 500 kHz can be converted with 12-bit accuracy.
As noted in the Theory of Operation, the ENCODE pulse is
specified to operate within a window of time. The circuit in Figure 14 can be used to generate a valid ENCODE pulse if a clock
pulse width of greater than 30 ns is available.
1/4
7402
tw
1/4
7402
1k
–15V
OBS
1k
+15V
VIN
(5Vp–p)
R6
4
2k
R10
10k
C24
0.1µF
11
U8
10
AD841
6 C25
5
R8
250
2
4 IN1
SD5001
OUT1 1
5 IN2
OUT2 8
U10
0.1µF
–15V
D1
1N4148
13 IN3
R11
250
1k
VR2 100k
S/H
0.1µF
6
TIMING DESCRIPTION
Figure 15 shows the timing requirements for the discrete SHA.
The complementary S/H inputs are HCMOS-compatible although larger gate voltages will improve performance by lowering the on resistances of the DMOS switches. It should be noted
that a conversion is started before the SHA has settled to 0.01%
accuracy. The discrete SHA takes advantage of the fact that the
AD671 does not require a 12-bit accurate input until it is 150 ns
into its conversion cycle. See Figures 21, 22 and 23 for PCB
layout recommendations.
OLE
R13
S/H
Figure 14. Cross Coupled Latch
C26
7
U9
AD845
4
3
C29
20pF
DAV
C28
20pF
+15V
2
OUT3 16
OUT4 9
12 IN4
G1 G2 G3 G4
3 6 14 11
ENCODE
1/4
7402
R9
R7
AD671
C27
–15V 0.1µF
C34
5pF
R14
226
PEDESTAL ADJ
Figure 13. Discrete High Speed Sample-and-Hold Amplifier
TE
t SAMPLE = 1µs
CIRCUIT DESCRIPTION
ENCODE
The discrete SHA shown in Figure 13 is a closed-loop, noninverting architecture which accepts 5 V p-p inputs. The overall
gain of the SHA is +2 in order to accommodate the 10 V input
span of the AD671. The AD841, with a 0.01% settling time of
110 ns, is the suggested input buffer to the SHA. The circuit
also employs a SD5001 which contains four ultrahigh speed
DMOS switches (Q1–Q4). The high CMRR, low input offset
current, and fast settling time of the AD845 op amp are all critical features necessary for optimal performance of the discrete
SHA.
In sample mode, Q1 and Q3 of the SD5001 are closed (Q2 and
Q4 are open). C28 is charged to the input voltage level at a rate
primarily determined by the time constant, R9 • C28. Simultaneously, C29 is connected to ground through a 250 ohm resistor. If C28 is equal to C29, charge injection from Q1 will be
approximately equal to charge injection from Q3 based on the
symmetry of the circuit and the inherent matching of the switch
capacitances. The resultant pedestal errors appear as a commonmode signal to the AD845. VR2, R13, R14, and C34 may be included if further reduction of pedestal error is required.
t CONVERSION = 500ns
DAV
t ACQUIRE
≈ 350ns
t SETTLE
350ns
S/H
Figure 15. AD671 to Discrete SHA Timing Diagram
DYNAMIC PERFORMANCE
In most sampling applications the dynamic performance of the
system is limited by the performance of the SHA. The SHA’s
dynamic performance can be selected to meet the system sampling requirements. Figures 16 and 17 are typical FFT plots
using the discrete SHA in Figure 13.
In hold mode, Q2 and Q4 are closed (Q1 and Q3 are open) to
reduce feedthrough. The input signal is attenuated –78 dB
relative to the input signal at frequencies up to 500 kHz. The
AD845 buffers the voltage on C28 and also provides the wideband, low-impedance output necessary to drive the input of the
AD671.
Droop, which occurs as a result of leakage currents, will appear
on C28 and will similarly appear on C29. Like pedestal errors,
droop appears as a common-mode signal to the AD845 and is
greatly reduced by the differential nature of the circuit. Voltage
droop is typically 5 µV/µs.
Figure 16. Typical FFT Plot of AD671 and Discrete SHA
FIN = 100 kHz
–12–
REV. B
AD671
DYNAMIC CHARACTERISTICS
(@ +25°C, tested using the discrete SHA in Figure 15 with V CC = +5 V,
VLOGIC = +5 V, VEE = –5 V, f SAMPLE = 1 MSPS)1
Model
AD671JD-500
Typ
Units
Effective Number of Bits (ENOB)
FIN = 100 kHz
FIN = 490 kHz
Figure 17. Typical FFT Plot of AD671 and Discrete SHA
FIN = 500 kHz
OBS
MULTICHANNNEL DATA ACQUISITION SYSTEM
The AD684, a quad high speed sample-and-hold amplifier is
ideally suited for multichannel data acquisition applications.
Figure 18 shows a typical data acquisition circuit using the
AD684 (SHA), ADG201HS (Multiplexer), AD588 (Reference)
and the AD671. The AD684 is configured to simultaneously
sample four analog inputs. Each held analog input voltage can
be selected by the multiplexer and buffered by the AD841. The
AD671 is connected in the bipolar input range (± 5 V).
Bits
Bits
Signal-to-Noise and Distortion (S/N+D) Ratio
FIN = 100 kHz
70
FIN = 490 kHz
68
dB
dB
Total Harmonic Distortion (THD)
FIN = 100 kHz
FIN = 490 kHz
–80
–75
dB
dB
Peak Spurious (dc to 490 kHz)
–79
dB
–76
dB
OLE
Peak Harmonic Component (dc to 490 kHz)
NOTE
1
fIN amplitude = –0.2 dB @ 100 kHz and –0.9 dB @ 490 kHz, bipolar mode
unless otherwise indicated. See Definition of Specifications for additional
information.
TE
Figure 18. Data Acquisition System Using the AD684 and the AD671
REV. B
11.3
11.2
–13–
AD671
AD671 TO ADSP-2100A INTERFACE
Figure 19 demonstrates the AD671 to ADSP-2100A interface.
The 2100A with a clock frequency of 12.5 MHz can execute an
instruction in one 80 ns cycle. The AD671 is configured to perform continuous time sampling. The DAV output of the AD671
is asserted at the end of each conversion. DAV can be used to
latch the conversion result into the two 574 octal D-latches. The
falling edge of the sampling clock is used to generate an interrupt (IRQ3) for the processor. Upon interrupt, the ADSP2100A starts a data memory read by providing an address on
the DMA bus. The decoded address generates OE for the
latches and the processor reads their output over the DMA bus.
The conversion result is read within a single processor cycle.
DAV
DMRD
OE
DMA0:13 ADDRESS BUS
574
8
AD671
Q0:7
8
DECODE
D0:7
ADSP-2100A
OE
16
DMA0:15
+5V
4
D0:3
8
DMACK
BIT1:12
574
DATA BUS
Q0:7
D0:7
4
SAMPLING
CLOCK
IRQ3
ENCODE
Figure 19. AD671 to ADSP-2100A Interface
OBS
AD671 TO ADSP-2101/ADSP-2102 INTERFACE
Figure 20 is identical to the 2100A interface except the sampling clock is used to generate an interrupt (IRQ2) for the processor. Upon interrupt the ADSP-2101A starts a data memory
read by providing an address on the Address (A) bus. The decode address generates OE for the D-latches and the processor
reads their output over the Data (D) bus. Reading the conversion result is thus completed within a single processor cycle.
DAV
RD
OE
A0:13 ADDRESS BUS
574
8
OLE
Q0:7
8
DECODE
AD671
D0:7
ADSP-2101
OE
16
D0:15
IRQ2
SAMPLING
CLOCK
BIT1:12
574
DATA BUS
4
D0:3
TE
8
Q0:7
D0:7
4
ENCODE
Figure 20. AD671 to ADSP-2101/ADSP-2102 Interface
Figure 21. PCB Silkscreen and Component Placement
Diagram for Figures 5, 10 and 13
–14–
REV. B
AD671
OBS
OLE
Figure 22. PCB Solder Side Layout for Figures 5, 10 and 13
TE
Figure 23. PCB Component Side Layout for Figures 5, 10 and 13
REV. B
–15–
AD671
OUTLINE DIMENSIONS
C1426a–10–9/91
Dimensions shown in inches and (mm).
24-Pin Plastic DIP (Suffix N)
OBS
OLE
TE
24-Pin Ceramic DIP (Suffix D)
0.295 6 0.01
(7.49 6 0.26)
PIN 1
1
0.300 6 0.010
(7.49 6 0.25)
1.200 6 0.012
(30.48 6 0.31)
SEATING
PLANE
0.085 6 0.009
(2.16 6 0.23)
0.018 6 0.002
(0.46 6 0.05)
TYP
0.100 6 0.005
(2.54 6 0.13)
0.05 (1.27)
TYP
1.100 6 0.005
(27.94 6 0.13)
0.010
+ 0.002
–0.001
+ 0.05
0.025
–0.03
(
)
PRINTED IN U.S.A.
0.175
(4.45)
TOLL NON ACCUM
NOTES
1. LEAD NO. 1 IDENTIFIED BY DOT OR NOTCH.
2. CERAMIC DIP LEADS WILL BE EITHER GOLD OR TIN PLATED
IN ACCORDANCE WITH MIL-M-385 TO REQUIREMENTS.
–16–
REV. B