AD AD678 12-bit 200 ksps complete sampling adc Datasheet

a
FEATURES
AC and DC Characterized and Specified
(K, B and T Grades)
200k Conversions per Second
1 MHz Full Power Bandwidth
500 kHz Full Linear Bandwidth
72 dB S/N+D (K, B, T Grades)
Twos Complement Data Format (Bipolar Mode)
Straight Binary Data Format (Unipolar Mode)
10 M⍀ Input Impedance
8-Bit or 16-Bit Bus Interface
On-Board Reference and Clock
10 V Unipolar or Bipolar Input Range
Commercial, Industrial and Military Temperature
Range Grades
MIL-STD-883 Compliant Versions Available
12-Bit 200 kSPS
Complete Sampling ADC
AD678
FUNCTIONAL BLOCK DIAGRAM
CS
SC OE EOCEN SYNC 12/8 EOC
AD678
REFOUT
VOLTAGE
REF.
CONTROL LOGIC
DB11
12
REFIN
DB2
OUTPUT
REGISTER
12
CONVERSION
LOGIC
12-BIT D/A
CONVERTER
BIPOFF
AIN
DB0
(HBE)
4
SAMPLE/
HOLD
GAIN
STAGE
DB1
(R/L)
4-BIT FLASH
A/D
CONVERTER
AGND
VCC
VEE
VDD
DGND
PRODUCT DESCRIPTION
PRODUCT HIGHLIGHTS
The AD678 is a complete, multipurpose 12-bit monolithic
analog-to-digital converter, consisting of a sample-hold amplifier (SHA), a microprocessor compatible bus interface, a voltage
reference and clock generation circuitry.
1. COMPLETE INTEGRATION: The AD678 minimizes external component requirements by combining a high speed
sample-hold amplifier (SHA), ADC, 5 V reference, clock and
digital interface on a single chip. This provides a fully specified sampling A/D function unattainable with discrete designs.
The AD678 is specified for ac (or “dynamic”) parameters such
as S/N+D ratio, THD and IMD which are important in signal
processing applications. In addition, the AD678K, B and T
grades are fully specified for dc parameters which are important
in measurement applications.
The AD678 offers a choice of digital interface formats; the 12
data bits can be accessed by a 16-bit bus in a single read operation or by an 8-bit bus in two read operations (8+4), with right
or left justification. Data format is straight binary for unipolar
mode and twos complement binary for bipolar mode. The input
has a full-scale range of 10 V with a full power bandwidth of
1 MHz and a full linear bandwidth of 500 kHz. High input impedance (10 MΩ) allows direct connection to unbuffered
sources without signal degradation.
This product is fabricated on Analog Devices’ BiMOS process,
combining low power CMOS logic with high precision, low
noise bipolar circuits; laser-trimmed thin-film resistors provide
high accuracy. The converter utilizes a recursive subranging
algorithm which includes error correction and flash converter
circuitry to achieve high speed and resolution.
The AD678 operates from +5 V and ±12 V supplies and dissipates
560 mW (typ). The AD678 is available in 28-lead plastic DIP,
ceramic DIP, and 44-lead J-leaded ceramic surface mount packages.
2. SPECIFICATIONS: The AD678K, B and T grades provide
fully specified and tested ac and dc parameters. The AD678J,
A and S grades are specified and tested for ac parameters; dc
accuracy specifications are shown as typicals. DC specifications (such as INL, gain and offset) are important in control
and measurement applications. AC specifications (such as
S/N+D ratio, THD and IMD) are of value in signal processing applications.
3. EASE OF USE: The pinout is designed for easy board layout, and the choice of single or two read cycle output provides compatibility with 16- or 8-bit buses. Factory trimming
eliminates the need for calibration modes or external trimming to achieve rated performance.
4. RELIABILITY: The AD678 utilizes Analog Devices’ monolithic BiMOS technology. This ensures long-term reliability
compared to multichip and hybrid designs.
5. UPGRADE PATH: The AD678 provides the same pinout as
the 14-bit, 128 kSPS AD679 ADC.
6. The AD678 is available in versions compliant with MILSTD-883. Refer to the Analog Devices Military Products
Databook or current AD678/883B data sheet for detailed
specifications.
Screening to MIL-STD-883C Class B is also available.
REV. C
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
AD678–SPECIFICATIONS
(TMIN to TMAX, VCC = +12 V ⴞ 5%, VEE = –12 V ⴞ 5%, VDD = +5 V ⴞ 10%, fSAMPLE = 200 kSPS,
1
lN = 10.06 kHz unless otherwise noted)
AC SPECIFICATIONS f
Parameter
Min
AD678J/A/S
Typ
Max
Min
AD678K/B/T
Typ
Max
Units
2
SIGNAL-TO-NOISE AND DISTORTION (S/N+D) RATIO
–0.5 dB Input (Referred to –0 dB Input)
–20 dB Input (Referred to –20 dB Input)
–60 dB Input (Referred to –60 dB Input)
70
71
51
11
71
73
53
13
dB
dB
dB
TOTAL HARMONIC DISTORTION (THD)3
–88
0.004
–80
0.010
–88
0.004
–80
0.010
dB
%
PEAK SPURIOUS OR PEAK HARMONIC COMPONENT
–87
–80
–87
–80
dB
FULL POWER BANDWIDTH
1
FULL LINEAR BANDWIDTH
1
500
INTERMODULATION DISTORTION (IMD)4
2nd Order Products
3rd Order Products
MHz
500
–85
–90
–80
–80
kHz
–85
–90
–80
–80
dB
dB
NOTES
1
fIN amplitude = –0.5 dB (9.44 V p-p) bipolar mode full scale unless otherwise indicated. All measurements referred to a –0 dB (9.997 V p-p) input signal unless
otherwise indicated.
2
See Figures 13 and 14 for higher frequencies and other input amplitudes.
3
See Figure 12.
4
fA = 9.08 kHz, f B = 9.58 kHz, with f SAMPLE = 200 kSPS. See Definition of Specifications section and Figure 16.
Specifications subject to change without notice.
DIGITAL SPECIFICATIONS (All device types T
Parameter
LOGIC INPUTS
High Level Input Voltage
VIH
VIL
Low Level Input Voltage
IIH
High Level Input Current
Low Level Input Current
IIL
CIN
Input Capacitance
LOGIC OUTPUTS
High Level Output Voltage
VOH
VOL
IOZ
COZ
Low Level Output Voltage
High Z Leakage Current
High Z Output Capacitance
MIN
to TMAX, VCC = +12 V ⴞ 5%, VEE = –12 V ⴞ 5%, VDD = +5 V ⴞ 10%)
Test Conditions
Min
Max
Units
VIN = VDD
VIN = 0 V
2.0
0
–10
–10
VDD
0.8
+10
+10
10
V
V
µA
µA
pF
0.4
+10
10
V
V
V
µA
pF
IOH = 0.1 mA
IOH = 0.5 mA
IOL = 1.6 mA
VIN = 0 or VDD
4.0
2.4
–10
Specifications subject to change without notice.
–2–
REV. C
AD678
DC SPECIFICATIONS (T
MIN
to TMAX, VCC = +12 V ⴞ 5%, VEE = –12 V ⴞ 5%, VDD = +5 V ⴞ 10% unless otherwise noted)
Parameter
Min
TEMPERATURE RANGE
J, K Grades
A, B Grades
S, T Grades
0
–40
–55
ACCURACY
Resolution
Integral Nonlinearity (INL)
Differential Nonlinearity (DNL)
Unipolar Zero Error (@ +25°C)1
Bipolar Zero Error (@ +25°C)1
Gain Error (@ +25°C)1, 2
Temperature Drift
Unipolar/Bipolar Zero
J, K Grades
A, B Grades
S, T Grades
Gain3
J, K Grades
A, B Grades
S, T Grades
Gain4
J, K Grades
A, B Grades
S, T Grades
ANALOG INPUT
Input Ranges
Unipolar Range
Bipolar Range
Input Resistance
Input Capacitance
Input Settling Time
Aperture Delay
Aperture Jitter
INTERNAL VOLTAGE REFERENCE
Output Voltage5
External Load
Unipolar Mode
Bipolar Mode
POWER SUPPLIES
Power Supply Rejection
VCC = +12 V ± 5%
VEE = –12 V ± 5%
VDD = +5 V ± 10%
Operating Current
ICC
IEE
IDD
Power Consumption
AD678J/A/S
Typ
Min
+70
+85
+125
0
–40
–55
12
AD678K/B/T
Typ
Max
Units
+70
+85
+125
°C
°C
°C
12
± 0.7
±1
±4
±4
±4
±2
±3
±3
±3
±5
±6
Bits
LSB
Bits
LSB
LSB
LSB
±2
±4
±5
±2
±3
±4
±4
±4
±5
LSB
LSB
LSB
±4
±7
± 10
±4
±5
±8
±6
±9
± 10
LSB
LSB
LSB
±2
±4
±6
±2
±3
±5
±4
±4
±6
LSB
LSB
LSB
+10
+5
V
V
MΩ
pF
µs
ns
ps
±1
12
12
0
–5
+10
+5
0
–5
10
10
10
10
1
1
10
150
4.98
10
150
5.02
4.98
+1.5
+0.5
±2
±2
±2
18
25
8
560
NOTES
1
Adjustable to zero.
2
Includes internal voltage reference error.
3
Includes internal voltage reference drift.
4
Excludes internal voltage reference drift.
5
With maximum external load applied.
Specifications subject to change without notice.
REV. C
Max
–3–
20
34
12
745
18
25
8
560
5.02
V
+1.5
+0.5
mA
mA
±2
±2
±2
LSB
LSB
LSB
20
34
12
745
mA
mA
mA
mW
AD678
(All grades, TMIN to TMAX, VCC = +12 V ⴞ 5%, VEE = –12 V ⴞ 5%, VDD = +5 V ⴞ 10% unless
TIMING SPECIFICATIONS otherwise noted)
Parameter
Symbol
Min
SC Delay
Conversion Time
Conversion Ratel
Convert Pulsewidth
Aperture Delay
Status Delay
Access Time2, 3
tSC
tC
tCR
tCP
tAD
tSD
tBA
50
3.0
Float Delay5
Output Delay
Format Setup
OE Delay
Read Pulsewidth
Conversion Delay
EOCEN Delay
tFD
tOD
tFS
tOE
tRP
tCD
tEO
97
5
0
10
10
10
Max
4.4
5
20
400
100
574
80
0
47
0
97
150
0
Units
ns
µs
µs
ns
ns
ns
ns
ns
ns
ns
ns
ns
ns
ns
ns
NOTES
1
Includes acquisition time.
2
Measured from the falling edge of OE/EOCEN (0.8 V) to the time at which the data lines/EOC cross 2.0 V or 0.8 V. See Figure 3.
3
COUT = 100 pF.
4
COUT = 50 pF.
5
Measured from the rising edge of OE/EOCEN (2.0 V) to the time at which the output voltage changes by 0.5 V. See Figure 3; C OUT = 10 pF.
Specifications subject to change without notice.
Figure 2. EOC Timing
Figure 1. Conversion Timing
Figure 3. Load Circuit for Bus Timing Specifications
–4–
REV. C
AD678
ABSOLUTE MAXIMUM RATINGS*
With
Respect
To
Specification
CS
AD678
Min
Max
Units
REFOUT
VCC
VEE
VCC
VDD
AGND
AIN, REFIN
Digital Inputs
Digital Outputs
Max Junction
Temperature
Operating Temperature
J and K Grades
A and B Grades
S and T Grades
Storage Temperature
Lead Temperature
(10 sec max)
AGND
AGND
VEE
DGND
DGND
AGND
DGND
DGND
SC OE EOCEN SYNC 12/8 EOC
–0.3
–18
–0.3
0
–1
VEE
–0.5
–0.5
0
–40
–55
–65
+18
+0.3
+26.4
+7
+1
VCC
+7
VDD + 0.3
V
V
V
V
V
V
V
V
175
°C
+70
+85
+125
+150
°C
°C
°C
°C
+300
°C
VOLTAGE
REF.
CONTROL LOGIC
DB11
12
REFIN
DB2
OUTPUT
REGISTER
12
12-BIT D/A
CONVERTER
CONVERSION
LOGIC
BIPOFF
AIN
DB0
(HBE)
VCC
4
4-BIT FLASH
A/D
CONVERTER
GAIN
STAGE
SAMPLE/
HOLD
DB1
(R/L)
VEE
VDD
DGND
AGND
Functional Block Diagram
*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 indicated in the operational
sections of this specification is not implied. Exposure to absolute maximum rating
conditions for extended periods may affect device reliability.
ESD SENSITIVITY
The AD678 features input protection circuitry consisting of large “distributed” diodes and polysilicon
series resistors to dissipate both high energy discharges (Human Body Model) and fast, low energy
pulses (Charged Device Model). Per Method 3015.2 of MIL-STD-883C, the AD678 has been
classified as a Category 1 device.
WARNING!
Proper ESD precautions are strongly recommended to avoid functional damage or performance
degradation. Charges as high as 4000 volts readily accumulate on the human body and test equipment
and discharge without detection. Unused devices must be stored in conductive foam or shunts, and
the foam should be discharged to the destination socket before devices are removed. For further
information on ESD precautions, refer to Analog Devices’ ESD Prevention Manual.
ESD SENSITIVE DEVICE
ORDERING GUIDE
Model1
Package
Temperature Range
Tested and Specified
Package Option2
AD678JN
AD678KN
AD678JD
AD678KD
AD678AD
AD678BD
AD678AJ
AD678BJ
AD678SD
AD678TD
28-Lead Plastic DIP
28-Lead Plastic DIP
28-Lead Ceramic DIP
28-Lead Ceramic DIP
28-Lead Ceramic DIP
28-Lead Ceramic DIP
44-Lead Ceramic JLCC
44-Lead Ceramic JLCC
28-Lead Ceramic DIP
28-Lead Ceramic DIP
0°C to +70°C
0°C to +70°C
0°C to +70°C
0°C to +70°C
–40°C to +85°C
–40°C to +85°C
–40°C to +85°C
–40°C to +85°C
–55°C to +125°C
–55°C to +125°C
AC
AC + DC
AC
AC + DC
AC
AC + DC
AC
AC + DC
AC
AC + DC
N-28
N-28
D-28
D-28
D-28
D-28
J-44
J-44
D-28
D-28
NOTES
1
For details on grade and package offerings screened in accordance with MIL-STD-883, refer to Analog Devices Military Products Databook or /883 data sheet.
2
N = Plastic DIP; D = Ceramic DIP; J = J-Leaded Ceramic Chip Carrier.
REV. C
–5–
AD678
PIN DESCRIPTION
Symbol
28-Lead DIP 44-Lead
Pin No.
JLCC Pin No. Type
AGND
AIN
BIPOFF
7
6
10
11
10
15
CS
DGND
DB11–DB4
4
14
26–19
6
DI
23
P
40, 39, 37, 36, DO
35, 34, 33, 31
DB3, DB2
18, 17
30, 27
DO
DB1 (R/L)
DB0 (HBE)
EOC
16
15
27
26
25
42
DO
DO
DO
EOCEN
HBE (DB0)
1
15
1
25
DI
DI
OE
2
3
DI
REFIN
REFOUT
R/L (DB1)
9
8
16
14
12
26
AI
AO
DI
SC
SYNC
3
13
5
21
DI
DI
VCC
VEE
VDD
12/8
11
5
28
12
17
8
43
19
P
P
P
DI
No Connect
P
AI
AI
2, 4, 7, 9, 13,
16, 18, 20, 22,
24, 28, 29, 32,
38, 41, 44
Name and Function
Analog Ground. This is the ground return for AIN only.
Analog Signal Input.
Bipolar Offset. Connect to AGND for +10 V input unipolar mode and straight binary
output coding. Connect to REFOUT through 50 Ω resistor for ± 5 V input bipolar mode
and twos complement binary output coding. See Figures 7 and 8.
Chip Select. Active LOW.
Digital Ground
Data Bits 11 through 4. In 12-bit format (see 12/8 pin), these pins provide the upper 8 bits
of data. In 8-bit format, these pins provide all 12 bits in two bytes (see R/L pin).
Active HIGH.
Data Bits 3 and 2. In 12-bit format, these pins provide Data Bit 3 and Data Bit 2.
Active HIGH. In 8-bit format they are undefined and should be tied to VDD.
In 12-bit format, Data Bit 1. Active HIGH.
In 12-bit format, Data Bit 0. Active HIGH.
End-of-Convert. EOC goes LOW when a conversion starts and goes HIGH when the
conversion is finished. In asynchronous mode, EOC is an open drain output and
requires an external 3 kΩ pull-up resistor. See EOCEN and SYNC pins for information
on EOC gating.
End-Of-Convert Enable. Enables EOC pin. Active LOW.
In 8-bit format, High Byte Enable. If LOW, output contains high byte. If HIGH, output
contains low byte.
Output Enable. The falling edge of OE enables DB11–DB0 in 12-bit format and
DB11–DB4 in 8-bit format. Gated with CS. Active LOW.
Reference Input. +5 V input gives 10 V full-scale range.
+5 V Reference Output. Tied to REFIN through 50 Ω resistor for normal operation.
In 8-bit format, Right/Left justified. Sets alignment of 12-bit result within 16-bit field.
Tied to VDD for right-justified output and tied to DGND for left-justified output.
Start Convert. Active LOW. See SYNC pin for gating.
SYNC Control. If tied to VDD (synchronous mode), SC, EOC and EOCEN are gated
by CS. If tied to DGND (asynchronous mode), SC and EOCEN are independent of CS,
and EOC is an open drain output. EOC requires an external 3 kΩ pull-up resistor in
asynchronous mode.
+12 V Analog Power.
–12 V Analog Power.
+5 V Digital Power.
Twelve/eight-bit format. If tied HIGH, sets output format to 12-bit parallel. If tied
LOW, sets output format to 8-bit multiplexed.
These pins are unused and should be connected to DGND or VDD.
Type: AI = Analog Input; AO = Analog Output; DI = Digital Input (TTL and 5 V CMOS compatible); DO = Digital Output (TTL and 5 V CMOS compatible).
All DO pins are three-state drivers; P = Power.
PIN CONFIGURATIONS
27 EOC
SC 3
26 DB11
CS 4
6 5 4
NC
7
25 DB10
VEE
8
VEE 5
24 DB9
NC
AIN 6
23 DB8
AIN
9
10
22 DB7
AGND
11
REFOUT 8
TOP VIEW
(Not to Scale) 21 DB6
REFOUT
NC
12
13
REFIN 9
20 DB5
REFIN
BIPOFF 10
19 DB4
VCC 11
12/8 12
DB11
EOC
NC
NC
VDD
EOCEN
OE
NC
44 43 42 41 40
PIN 1
39
IDENTIFIER
38
DB10
NC
DB9
36
DB8
AD678
35
DB7
TOP VIEW
34
DB6
33
DB5
14
32
NC
BIPOFF
15
31
DB4
18 DB3
NC
16
30
DB3
17 DB2
VCC
17
29
NC
–6–
NC
DB2
DB1 (R/L)
DB0 (HBE)
NC = NO CONNECT
NC
15 DB0 (HBE)
DGND
DGND 14
18 19 20 21 22 23 24 25 26 27 28
NC
16 DB1 (R/L)
SYNC
SYNC 13
NC
AD678
3 2
37
NC
AGND 7
SC
NC
28 VDD
OE 2
12/8
EOCEN 1
JLCC PACKAGE
CS
DIP PACKAGE
REV. C
Definition of Specifications–AD678
NYQUIST FREQUENCY
APERTURE JITTER
An implication of the Nyquist sampling theorem, the “Nyquist
Frequency” of a converter is that input frequency which is onehalf the sampling frequency of the converter.
Aperture jitter is the variation in aperture delay for successive
samples and is manifested as noise on the input to the A/D.
INPUT SETTLING TIME
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 below the
Nyquist frequency, including harmonics but excluding dc.
Settling time is a function of the SHA’s ability to track fast slewing signals. This is specified as the maximum time required in
track mode after a full-scale step input to guarantee rated conversion accuracy.
TOTAL HARMONIC DISTORTION (THD)
DIFFERENTIAL NONLINEARITY (DNL)
THD is the ratio of the rms sum of the first six harmonic components to the rms value of a full-scale input signal and is expressed as a percentage or in decibels. For input signals or
harmonics that are above the Nyquist frequency, the aliased
component is used.
In an ideal ADC, code transitions are 1 LSB apart. Differential
nonlinearity is the maximum deviation from this ideal value. It
is often specified in terms of resolution for which no missing
codes (NMC) are guaranteed.
UNIPOLAR ZERO ERROR
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.
In unipolar mode, the first transition should occur at a level 1/2
LSB above analog ground. Unipolar zero error is the deviation
of the actual transition from that point. This error can be adjusted as discussed in the Input Connections and Calibration
section.
INTERMODULATION DISTORTION (IMD)
BIPOLAR ZERO ERROR
With inputs consisting of sine waves at two frequencies, fa and
fb, any device with nonlinearities will create distortion products,
of order (m + n), at sum and difference frequencies of mfa ±
nfb, where m, n = 0, 1, 2, 3.... Intermodulation terms are those
for which m or n is not equal to zero. For example, the second
order terms are (fa + fb) and (fa – fb) and the third order terms
are (2 fa + fb), (2 fa – fb), (fa + 2 fb) and (fa – 2 fb). The IMD
products are expressed as the decibel ratio of the rms sum of
the measured input sides to the rms sum of the distortion terms.
The two signals applied to the converter are of equal amplitude and the peak value of their sum is –0.5 dB from full scale
(9.44 V p-p). The IMD products are normalized to a 0 dB
input signal.
In the bipolar mode, the major carry transition (1111 1111
1111 to 0000 0000 0000) should occur at an analog value 1/2
LSB below analog ground. Bipolar zero error is the deviation of
the actual transition from that point. This error can be adjusted
as discussed in the Input Connections and Calibration section.
GAIN ERROR
The last transition should occur at an analog value 1 1/2 LSB
below the nominal full scale (9.9963 volts for a 0–10 V range,
4.9963 volts for a ± 5 V range). The gain error is the deviation of
the actual difference between the first and last code transition
from the ideal difference between the first and last code transition. This error can be adjusted as shown in the Input Connections and Calibration section.
BANDWIDTH
The full-power bandwidth is that input frequency at which the
amplitude of the reconstructed fundamental is reduced by 3 dB
for a full-scale input.
INTEGRAL NONLINEARITY (INL)
The full-linear bandwidth is the input frequency at which the
slew rate limit of the sample-hold-amplifier (SHA) is reached.
At this point, the amplitude of the reconstructed fundamental
has degraded by less than 0.1 dB. Beyond this frequency, distortion of the sampled input signal increases significantly.
The AD678 has been designed to optimize input bandwidth, allowing the AD678 to undersample input signals with frequencies significantly above the converter’s Nyquist frequency.
POWER SUPPLY REJECTION
Variations in power supply will affect the full-scale transition,
but not the converter’s linearity. Power Supply Rejection is the
maximum change in the full-scale transition point due to a
change in power-supply voltage from the nominal value.
APERTURE DELAY
Aperture delay is a measure of the SHA’s performance and is
measured from the falling edge of Start Convert (SC) to when
the input signal is held for conversion. In synchronous mode,
Chip Select (CS) should be LOW before SC to minimize aperture delay.
REV. C
The ideal transfer function for a linear ADC is a straight line
drawn between “zero” and “full scale.” The point used as
“zero” occurs 1/2 LSB before the first code transition. “Full
scale” is defined as a level 1 1/2 LSB beyond the last code transition. Integral nonlinearity is the worst-case deviation of a code
from the straight line. The deviation of each code is measured
from the middle of that code.
TEMPERATURE DRIFT
This is the maximum change in the parameter from the initial
value (@ +25°C) to the value at TMIN or TMAX.
–7–
AD678–Dynamic Performance
Figure 7. Nonaveraged 2048 Point FFT at 200 kSPS,
fIN = 49.902 kHz
Figure 4. Harmonic Distortion vs. Input Frequency
Figure 5. S/N+D vs. Input Amplitude
(fSAMPLE 200 kSPS)
Figure 8. IMD Plot for fIN = 9.08 kHz (fa), 9.58 kHz (fb)
Figure 9. Power Supply Rejection (fIN = 10 kHz,
fSAMPLE = 200 kSPS, VRIPPLE = 0.1 V p-p)
Figure 6. S/N+D vs. Input Frequency and Amplitude
–8–
REV. C
AD678
CONVERSION CONTROL
END-OF-CONVERT
In synchronous mode (SYNC = HIGH), both Chip Select (CS)
and Start Convert (SC) must be brought LOW to start a conversion. CS should be LOW tSC before SC is brought LOW. In
asynchronous mode (SYNC = LOW), a conversion is started by
bringing SC low, regardless of the state of CS.
In asynchronous mode, End-of-Convert (EOC) is an open drain
output (requiring a minimum 3 kΩ pull-up resistor) enabled by
End-of-Convert ENable (EOCEN). In synchronous mode,
EOC is a three-state output which is enabled by EOCEN and
CS. See the Conversion Status Truth Table for details. Access
(tBA) and float (tFD) timing specifications do not apply in asynchronous mode where they are a function of the time constant
formed by the 10 pF output capacitance and the pull-up
resistor.
Before a conversion is started, End-of-Convert (EOC) is HIGH,
and the sample-hold is in track mode. After a conversion is
started, the sample-hold goes into hold mode and EOC goes
LOW, signifying that a conversion is in progress. During the
conversion, the sample-hold will go back into track mode and
start acquiring the next sample. EOC goes HIGH when the conversion is finished.
START CONVERSION TRUTH TABLE
INPUTS
SYNC CS SC
In track mode, the sample-hold will settle to ± 0.01% (12 bits)
in 1 µs maximum. The acquisition time does not affect the
throughput rate as the AD678 goes back into track mode more
than 1 µs before the next conversion. In multichannel systems,
the input channel can be switched as soon as EOC goes LOW if
the maximum throughput rate is needed.
Synchronous
Mode
1
1
1
1
0
X
1
0
0
0
0
0
X
X
X
1
0
12-Bit Mode Coding Format (1 LSB = 2.44 mV)
Unipolar Coding
(Straight Binary)
Bipolar Coding
(Twos Complement)
VIN*
Output Code
VIN*
Output Code
0V
5.000 V
9.9976 V
000 . . . 0
100 . . . 0
111 . . . 1
–5.000 V
–0.002 V
+0.000 V
+2.500 V
+4.9976 V
100 . . . 0
111 . . . 1
000 . . . 0
010 . . . 0
011 . . . 1
Asynchronous
Mode
0
STATUS
No Conversion
Start Conversion
Start Conversion
(Not Recommended)
Continuous Conversion
(Not Recommended)
No Conversion
Start Conversion
Continuous Conversion
(Not Recommended)
NOTES
1 = HIGH voltage level.
0 = LOW voltage level.
X = Don’t care.
X = HIGH to LOW transition. Must stay low for t = t CP.
CONVERSION STATUS TRUTH TABLE
*Code center.
INPUTS
OUTPUT
SYNC CS EOCEN EOC
STATUS
OUTPUT ENABLE TRUTH TABLES
12-BIT MODE (12/8 = HIGH)
INPUTS
(CS U OE)
1
0
OUTPUT
DB11–DB0
High Z
Enable 12-Bit Output
8-BIT MODE (12/8 = LOW)
INPUTS
R/L HBE (CS U OE)
Unipolar
Mode
Bipolar
Mode
X
1
1
0
0
X
0
1
0
1
1
0
0
0
0
0
e
a
i
0
f
b
j
0
g
c
k
1
1
0
0
0
1
0
1
0
0
0
0
a
e
a
i
a
f
b
j
a
g
c
k
NOTES
1 = HIGH voltage level.
0 = LOW voltage level.
X = Don’t care.
U = Logical OR.
REV. C
OUTPUTS
DB11 . . . DB4
High Z
0 a b
h i j
d e f
l 0 0
a
h
d
l
a
i
e
0
b
j
f
0
c
k
g
0
d
l
h
0
c
k
g
0
d
1
h
0
1
1
Synchronous 1
Mode
1
0
0
1
X
0
0
X
1
0
1
High Z
High Z
Converting
Not Converting
Either
Either
0
Asynchronous 0
Mode*
0
X
X
X
0
0
1
0
High Z
High Z
Converting
Not Converting
Either
NOTES
l = HIGH voltage level.
0 = LOW voltage level.
X = Don’t care.
*EOC requires a pull-up resistor in asynchronous mode.
a = MSB.
1 = LSB.
–9–
AD678
OUTPUT ENABLE OPERATION
POWER-UP
The data bits (DB11–DB0) are three-state outputs enabled by
Chip Select (CS) and Output Enable (OE). CS should be LOW
tOE before OE is brought LOW. Bits DB1 (R/L) and DB0
(HBE) are bidirectional. In 12-bit mode they are data output
bits. In 8-bit mode they are inputs which define the format of
the output register.
The AD678 typically requires 10 µs after power-up to reset
internal logic.
In unipolar mode (BIPOFF tied to AGND), the output coding
is straight binary. In bipolar mode (BIPOFF tied to REFOUT),
output coding is twos complement binary.
When EOC goes HIGH, the conversion is completed and the
output data may be read. Bringing OE LOW tOE after CS is
brought LOW makes the output register contents available on
the data bits. A period of time tCD is required after OE is
brought HIGH before the next SC instruction may be issued.
Figure 10 illustrates the 8-bit read mode (12/8 = LOW), where
only DB11–DB4 are used as output lines onto an 8-bit bus. The
output is read in two steps, with the high byte read first, followed
by the low byte. High Byte Enable (HBE) controls the output
sequence. The 12-bit result can be right or left justified depending on the state of R/L.
In 12-bit read mode (12/8 = HIGH), a single READ operation
accesses all 12 output bits on DB11-DB0 for interface to a
16-bit bus. Figure 11 provides the output timing relationships.
Note that tCR must be observed, in that SC pulses should not be
issued at intervals closer than 5 µs. If SC is asserted sooner than
5 µs, conversion accuracy may deteriorate. For this reason, SC
should not be held LOW in an attempt to operate in a continuously converting mode.
APPLICATION INFORMATION
INPUT CONNECTIONS AND CALIBRATION
The high (10 MΩ) input impedance of the AD678 eases the
task of interfacing to high source impedances or multiplexer
channel-to-channel mismatches of up to 1000 Ω. The 10 V p-p
full-scale input range accepts the majority of signal voltages
without the need for voltage divider networks which could deteriorate the accuracy of the ADC.
The AD678 is factory trimmed to minimize linearity, offset and
gain errors. In unipolar mode, the only external component that
is required is a 50 Ω ± 1% resistor. Two resistors are required in
bipolar mode. If offset and gain are not critical (as in some ac
applications), even these components can be eliminated.
In some applications, offset and gain errors need to be trimmed
out completely. The following sections describe the correct procedure for these various situations.
UNIPOLAR RANGE INPUTS
Offset and gain errors can be trimmed out by using the configuration shown in Figure 12. This circuit allows approximately
± 25 mV of offset trim range (± 10 LSB) and ± 0.5% of gain trim
(± 20 LSB).
The first transition (from 0000 0000 0000 to 0000 0000 0001)
should nominally occur for an input level of +1/2 LSB (1.22 mV
above ground for a 10 V range). To trim unipolar zero to this
nominal value, apply a 1.22 mV signal to AIN and adjust R1
until the first transition is located.
The gain trim is done by adjusting R2. If the nominal value is
required, apply a signal 1 1/2 LSB below full scale (9.9963 V for
a 10 V range) and adjust R2 until the last transition is located
(1111 1111 1110 to 1111 1111 1111).
If offset adjustment is not required, BIPOFF should be connected directly to AGND. If gain adjustment is not required, R2
should be replaced with a fixed 50 Ω ± 1% metal film resistor. If
REFOUT is connected directly to REFIN, the additional gain
error will be approximately 1%.
BIPOLAR RANGE INPUTS
Figure 10. Output Timing, 8-Bit Read Mode
The connections for the bipolar mode are shown in Figure 13.
In this mode, data output coding will be in twos complement
binary. This circuit will allow approximately ± 25 mV of offset
trim range (± 10 LSB) and ± 0.5% of gain trim range (20 LSB).
Either or both of the trim pots can be replaced with 50 Ω ± 1%
fixed resistors if the AD678 accuracy limits are sufficient for the
application. If the pins are shorted together, the additional offset
and gain errors will be approximately 1%.
NOTE
1IN
ASYNCHRONOUS MODE, SC IS INDEPENDENT OF CS
Figure 11. Output Timing, 12-Bit Read Mode
To trim bipolar zero to its nominal value, apply a signal 1/2 LSB
below midrange (–1.22 mV for a ± 5 V range) and adjust R1
until the major carry transition is located (1111 1111 1111 to
0000 0000 0000). To trim the gain, apply a signal 1 1/2 LSB
below full scale (+4.9963 V for a ± 5 V range) and adjust R2 to
give the last positive transition (0111 1111 1110 to 0111 1111
1111). These trims are interactive so several iterations may be
necessary for convergence.
–10–
REV. C
AD678
The AD678 incorporates several features to help the user’s
layout. Analog pins (VEE) AIN, AGND, REFOUT, REFIN,
BIPOFF, VCC) are adjacent to help isolate analog from digital
signals. In addition, the 10 MΩ input impedance of AIN minimizes input trace impedance errors. Finally, ground currents
have been minimized by careful circuit design. Current through
AGND is 200 µA, with no code-dependent variation. The current through DGND is dominated by the return current for
DB11–DB0 and EOC.
A single-pass calibration can be done by substituting a bipolar
offset trim (error at minus full scale) for the bipolar zero trim
(error at midscale), using the same circuit. First, apply a signal
1/2 LSB above minus full scale (–4.9988 V for a ± 5 V range)
and adjust R1 until the minus full-scale transition is located
(1000 0000 0000 to 1000 0000 0001). Then perform the gain
error trim as outlined above.
SUPPLY DECOUPLING
The AD678 power supplies should be well filtered, well regulated,
and free from high-frequency noise. Switching power supplies
are not recommended. These supplies generate spikes which can
induce noise in the analog system.
Decoupling capacitors should be located as close as possible to
all power supply pins. A 10 µF tantalum capacitor in parallel
with a 0.1 µF ceramic provides adequate decoupling. The power
supply pins should be decoupled directly to AGND.
Figure 12. Unipolar Input Connections with Gain and
Offset Trims
An effort should be made to minimize the trace length between
the capacitor leads and the respective converter power supply
and common pins. The circuit layout should attempt to locate
the AD678, associated analog input circuitry and interconnections as far as possible from logic circuitry. A solid analog ground
plane around the AD678 will isolate large switching ground
currents. For these reasons, the use of wire wrap circuit construction is not recommended; careful printed circuit construction
is preferred.
GROUNDING
Figure 13. Bipolar Input Connections with Gain and Offset
Trims
BOARD LAYOUT
Designing with high-resolution data converters requires careful
attention to layout. Trace impedance is a significant issue. At the
12-bit level, a 5 mA current through a 0.5 Ω trace will develop a
voltage drop of 2.5 mV, which is 1 LSB for a 10 V full-scale span.
In addition to ground drops, inductive and capacitive coupling
need to be considered, especially when high- accuracy analog
signals share the same board with digital signals. Finally, power
supplies need to be decoupled in order to filter out ac noise.
Analog and digital signals should not share a common path.
Each signal should have an appropriate analog or digital return
routed close to it. Using this approach, signal loops enclose a
small area, minimizing the inductive coupling of noise. Wide PC
tracks, large gauge wire, and ground planes are highly recommended to provide low impedance signal paths. Separate analog
and digital ground planes are also desirable, with a single interconnection point to minimize ground loops. Analog signals should
be routed as far as possible from digital signals and should cross
them at right angles.
REV. C
If a single AD678 is used with separate analog and digital
ground planes, connect the analog ground plane to AGND and
the digital ground plane to DGND keeping lead lengths as short
as possible. Then connect AGND and DGND together at the
AD678. If multiple AD678s are used or the AD678 shares analog supplies with other components, connect the analog and
digital returns together once at the power supplies rather than at
each chip. This prevents large ground loops which inductively
couple noise and allow digital currents to flow through the analog system.
INTERFACING THE AD678 TO MICROPROCESSORS
The I/O capabilities of the AD678 allow direct interfacing to
general purpose and DSP microprocessor buses. The asynchronous conversion control feature allows complete flexibility and
control with minimal external hardware.
The following examples illustrate typical AD678 interface
configurations.
–11–
AD678
AD678 TO TMS320C25
In Figure 14 the AD678 is mapped into the TMS320C25 I/O
space. AD678 conversions are initiated by issuing an OUT
instruction to Port 8. EOC status and the conversion result are
read in with an IN instruction to Port 8. A single wait state is
inserted by generating the processor READY input from IS,
Port 8 and MSC. This configuration supports processor clock
speeds of 20 MHz and is capable of supporting processor clock
speeds of 40 MHz if a NOP instruction follows each AD678
read instruction.
byte of data as soon as “high byte read” is complete. The low
byte read operation executes in a similar manner to the first and
is completed during the next 160 ns.
AD678 TO 80186
Figure 15 shows the AD678 interfaced to the 80186 microprocessor. This interface allows the 80186’s built-in DMA controller to transfer the AD678 output into a RAM based FIFO
buffer of any length, with no microprocessor intervention.
Figure 14. AD678 to TMS320C25 Interface
In this application the AD678 is configured in the asynchronous
mode, which allows conversions to be initiated by an external
trigger source independent of the microprocessor clock. After
each conversion, the AD678 EOC signal generates a DMA
request to Channel 1 (DRQ1). The subsequent DMA READ
operation resets the interrupt latch. The system designer must
assign a sufficient priority to the DMA channel to ensure that
the DMA request will be serviced before the completion of the
next conversion. This configuration can be used with 6 MHz
and 8 MHz 80186 processors.
AD678 TO ANALOG DEVICES ADSP-2101
Figure 16 demonstrates the AD678 interfaced to an ADSP-2101.
With a clock frequency of 12.5 MHz, and instruction execution in
one 80 ns cycle, the digital signal processor supports the AD678
interface with one wait state.
The converter is configured to run asynchronously using a sampling clock. The EOC output of the AD678 gets asserted at the
end of each conversion and causes an interrupt. Upon interrupt,
the ADSP-2101 immediately asserts its FO pin LOW. In the
following cycle, the processor starts a data memory read by providing an address on the DMA bus. The decoded address generates OE for the converter, and the high byte of the conversion
result is read over the data bus. The read operation is extended
with one wait state and thus started and completed within two
processor cycles (160 ns). Next, the ADSP-2101 asserts its FO
pin HIGH. This allows the processor to start reading the lower
byte of data. This read operation executes in a similar manner to
the first and is completed during the next 160 ns.
AD678 TO ANALOG DEVICES ADSP-2100A
Figure 15. AD678 to 80186 DMA Interface
Figure 16. AD678 to ADSP-2101 Interface
Figure 17 demonstrates the AD678 interfaced to an ADSP-2100A.
With a clock frequency of 12.5 MHz, and instruction execution
in one 80 ns cycle, the digital signal processor will support the
AD678 data memory interface with three hardware wait states.
The converter is configured to run asynchronously using a sampling clock. The EOC output of the AD678 gets asserted at the
end of each conversion and causes an interrupt. Upon interrupt,
the ADSP-2100A immediately executes a data memory write
instruction which asserts HBE. In the following cycle, the processor starts a data memory read (high byte read) by providing
an address on the DMA bus. The decoded address generates
OE for the converter. OE, together with logic and latch, is used
to force the ADSP-2100A into a one cycle wait state by generating DMACK. The read operation is thus started and completed
within two processor cycles (160 ns). HBE is released during
“high byte read.” This allows the processor to read the lower
Figure 17. AD678 to ADSP-2100A Interface
–12–
REV. C
AD678
OUTLINE DIMENSIONS
Dimensions shown in inches and (mm).
28-Lead Ceramic DIP Package (D-28)
28-Lead Plastic DIP Package (N-28A)
REV. C
–13–
AD678
OUTLINE DIMENSIONS
Dimensions shown in inches and (mm).
PRINTED IN U.S.A.
C1381b–0–3/00 (rev. C)
44-Terminal Lead Ceramic (J-44)
–14–
REV. C