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PDF AD872A Data sheet ( Hoja de datos )
| Número de pieza | AD872A | |
| Descripción | Complete 12-Bit 10 MSPS Monolithic A/D Converter | |
| Fabricantes | Analog Devices | |
| Logotipo |  |
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FEATURES
Monolithic 12-Bit 10 MSPS A/D Converter
Low Noise: 0.26 LSB RMS Referred-to-Input
No Missing Codes Guaranteed
Differential Nonlinearity Error: 0.5 LSB
Signal-to-Noise and Distortion Ratio: 68 dB
Spurious-Free Dynamic Range: 75 dB
Power Dissipation: 1.03 W
Complete: On-Chip Track-and-Hold Amplifier and
Voltage Reference
Twos Complement Binary Output Data
Out-of-Range Indicator
28-Lead Ceramic DIP or 44-Terminal Leadless Chip
Carrier Package
Complete 12-Bit 10 MSPS
Monolithic A/D Converter
AD872A
VINA
VINB
CLOCK
REF IN
REF OUT
REF GND
FUNCTIONAL BLOCK DIAGRAM
AVDD AGND AVSS
DVDD DGND *DRVDD *DRGND
AD872A
T/H + T/H
+ T/H
+ A/D
A/D DAC
4
+2.5V
REFERENCE
A/D DAC
A/D DAC
4
CORRECTION LOGIC
3
OUTPUT BUFFERS
4
*OEN OTR MSB *MSB BIT2–BIT12
*ONLY AVAILABLE ON 44-TERMINAL SURFACE MOUNT PACKAGE
PRODUCT DESCRIPTION
The AD872A is a monolithic 12-bit, 10 MSPS analog-to-digital
converter with an on-chip, high performance track-and-hold
amplifier and voltage reference. The AD872A uses a multistage
differential pipelined architecture with error correction logic to
provide 12-bit accuracy at 10 MSPS data rates and guarantees
no missing codes over the full operating temperature range. The
AD872A is a redesigned version of the AD872 which has been
optimized for lower noise. The AD872A is pin compatible with
the AD872, allowing the parts to be used interchangeably as sys-
tem requirements change.
The low noise input track-and-hold (T/H) of the AD872A is
ideally suited for high-end imaging applications. In addition, the
T/H’s high input impedance and fast settling characteristics al-
low the AD872A to easily interface with multiplexed systems
that switch multiple signals through a single A/D converter. The
dynamic performance of the T/H also renders the AD872A suit-
able for sampling single channel inputs at frequencies up to and
beyond the Nyquist rate. The AD872A provides both reference
output and reference input pins, allowing the onboard reference
to serve as a system reference. An external reference can also be
chosen to suit the dc accuracy and temperature drift require-
ments of the application. A single clock input is used to control
all internal conversion cycles. The digital output data is pre-
sented in twos complement binary output format. An out-of-
range signal indicates an overflow condition, and can be used
with the most significant bit to determine low or high overflow.
The AD872A is fabricated on Analog Devices’ ABCMOS-l
process that utilizes high speed bipolar and CMOS transistors
on a single chip.
The AD872A is packaged in a 28-lead ceramic DIP and a 44-
terminal leadless ceramic surface mount package (LCC). Opera-
tion is specified from 0°C to +70°C and –55°C to +125°C.
PRODUCT HIGHLIGHTS
The AD872A offers a complete single-chip sampling, 12-bit
10 MSPS analog-to-digital conversion function in a 28-lead DIP
or 44-terminal LCC.
Low Noise—The AD872A features 0.26 LSB rms referred to-
input noise.
Low Power—The AD872A at 1.03 W consumes a fraction of the
power of presently available hybrids.
On-Chip Track-and-Hold (T/H)—The low noise, high imped-
ance T/H input eliminates the need for external buffers and can
be configured for single-ended or differential inputs.
Ease of Use—The AD872A is complete with T/H and voltage
reference and is pin-compatible with the AD872.
Out of Range (OTR)—The OTR output bit indicates when the
input signal is beyond the AD872A’s input range.
REV. A
Information furnished by Analog Devices is believed to be accurate and
reliable. However, no responsibility is assumed by Analog Devices for its
use, nor for any infringements of patents or other rights of third parties
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., 1997
1 page  
Symbol
VINA
VINB
AVSS
AVDD
AGND
DGND
DVDD
BIT 12 (LSB)
BIT 2–BIT 11
MSB
OTR
CLK
REF OUT
REF GND
REF IN
DRVDD
NC
DRGND
OEN
BIT 1
AD872A
PIN DESCRIPTION
DIP LCC
Pin No. Pin No.
1
2
3, 25
4
5, 24
6, 23
7, 22
8
1
2
5, 40
6, 38
9, 36
10
33
16
18–9 26–17
19 29
Type
AI
AI
P
P
P
P
P
DO
DO
DO
20 30
DO
21 31
DI
26 41
AO
27 42
AI
28 43
AI
N/A 12, 32
P
N/A 3, 4, 7, 8, 14, 15,
28, 35, 37, 39, 44
N/A 11, 34
P
N/A 13
N/A 27
DI
DO
Name and Function
(+) Analog Input Signal on the differential input amplifier.
(–) Analog Input Signal on the differential input amplifier.
–5 V Analog Supply.
+5 V Analog Supply.
Analog Ground.
Digital Ground.
+5 V Digital Supply.
Least Significant Bit.
Data Bits 2 through 11.
Inverted Most Significant Bit. Provides twos complement output
data format.
Out of Range is Active HIGH on the leading edge of Code 0 or the
trailing edge of Code 4096. See Output Data Format Table III.
Clock Input. The AD872A will initiate a conversion on the rising
edge of the clock input. See the Timing Diagram for details.
+2.5 V Reference Output. Tie to REF IN for normal operation.
Reference Ground.
Reference Input. +2.5 V input gives ± 1 V full-scale range.
+5 V Digital Supply for the output drivers.
No Connect.
Digital Ground for the output drivers. (See section on Power Supply
Decoupling for details on DRVDD and DRGND.)
Output Enable. See the Three State Output Timing Diagram for details.
Most Significant Bit.
TYPE: AI = Analog Input; AO = Analog Output; DI = Digital Input; DO = Digital Output; P = Power; N/A = Not Available on 28-lead DIP. Only available on
44-terminal surface mount package.
28-Lead Ceramic DIP
PIN CONFIGURATIONS
44-Terminal LCC
VINA 1
VINB 2
28 REF IN
27 REF GND
AVSS 3
26 REF OUT
AVDD 4
25 AVSS
AGND 5 AD872A 24 AGND
DGND 6 TOP VIEW 23 DGND
DVDD 7 (Not to Scale) 22 DVDD
BIT 12 (LSB) 8
21 CLK
BIT 11 9
BIT 10 10
20 OTR
19 MSB
BIT 9 11
18 BIT 2
BIT 8 12
17 BIT 3
BIT 7 13
16 BIT 4
BIT 6 14
15 BIT 5
REV. A
6 5 4 3 2 1 44 43 42 41 40
NC 7
NC 8
AGND 9
DGND 10
DRGND 11
DRVDD 12
OEN 13
NC 14
NC 15
BIT 12 (LSB) 16
BIT 11 17
AD872A
TOP VIEW
(NOT TO SCALE)
39 NC
38 AVDD
37 NC
36 AGND
35 NC
34 DRGND
33 DVDD
32 DRVDD
31 CLK
30 OTR
29 MSB
18 19 20 21 22 23 24 25 26 27 28
–5–
NC = NO CONNECT
5 Page 
AD872A
The wide input bandwidth and superior dynamic performance
of the input THA make the AD872A suitable for undersam-
pling applications where the input frequency exceeds half the
sample frequency. The input THA is designed to recover rap-
idly from input overdrive conditions, returning from a 50%
overdrive in less than 40 ns.
Because of the THA’s exceptionally wide input bandwidth,
some users may find the AD872A is sensitive to noise at fre-
quencies from 10 MHz to 50 MHz that other converters are
incapable of responding to. This sensitivity can be mitigated by
careful use of the differential inputs (see previous paragraphs).
Additionally, Figure 18 shows how a small capacitor (10 pF-
20 pF for 50 Ω terminated inputs) may be placed between VINA
and VINB to help reduce high frequency noise in applications
where limiting the input bandwidth is acceptable.
؎1V
10 OR 20pF
1 VINA
AD872A
2 VINB
The AD872A’s reference input impedance is equal to 5 kΩ
(± 20%), and its effective noise bandwidth is 10 MHz, with a
referred-to-input noise gain of 0.8. For example, the internal
reference, with an rms noise of 28 µV (using an external 1 µF
capacitor), contributes 24 µV (0.05 LSB) of noise to the transfer
function of the AD872A.
The full-scale peak-to-peak input voltage is a function of the
reference voltage, according to the equation:
(VINA = VINB ) Full Scale = 0.8 × (VREF – REF GND )
Note that the AD872A’s performance was optimized for a 2.5 V
reference input: performance may degrade somewhat for other
reference voltages. Figure 20 illustrates the S/(N+D) perfor-
mance vs. reference voltage for a 1 MHz, –0.5 dB input signal.
Note also that if the reference is changed during a conversion,
all three conversions in the pipeline will be invalidated.
75
70
65
Figure 18. Optional High Frequency Noise Reduction
The AD872A will contribute its own wideband thermal noise.
As a result of the integrated wideband noise (0.26 LSB rms,
referred-to-input), applying a dc analog input may produce
more than one code at the output. A histogram of the ADC
output codes, for a dc input voltage, will be between one and
three codes wide, depending on how well the input is centered
on a given code and how many samples are taken. Figure 8
shows a typical AD872A code histogram, and Figure 9 illus-
trates the AD872A’s transition noise.
REFERENCE INPUT
The nominal reference input should be 2.5 V, taken with re-
spect to REFERENCE GROUND (REF GND). Figure 19 il-
lustrates the equivalent model for the reference input: there is
no clock or signal-dependent activity associated with the refer-
ence input circuitry, therefore, no “kickback” into the reference.
REF IN 1
AD872A
5k⍀
(؎20%)
REF GND 2
AVSS
Figure 19. Equivalent Reference Input Circuit
However, in order to realize the lowest noise performance of the
AD872A, care should be taken to minimize noise at the refer-
ence input.
60
55
50
1.5 1.7 1.9 2.1 2.3 2.5 2.7 2.9 3.1 3.3 3.5
REFERENCE INPUT VOLTAGE – V
Figure 20. S/(N+D) vs. Reference Input Voltage,
fIN = 1 MHz, FS = 10 MHz
Table II summarizes various 2.5 V references suitable for use
with the AD872A, including the onboard bandgap reference
(see REFERENCE OUTPUT section).
REF43B
AD680JN
Internal
Table II. Suitable 2.5 V References
Drift (ppm/؇C)
Initial Accuracy %
6 (max)
10 (max)
30 (typ)
0.2
0.4
0.4
If an external reference is connected to REF IN, REF OUT
must be connected to +5 V. This should lower the current in
REF GND to less than 350 µA and eliminate the need for a
1 µF capacitor, although decoupling the reference for noise
reduction purposes is recommended.
Alternatively, Figure 21 shows how the AD872A may be driven
from other references by use of an external resistor. The exter-
nal resistor forms a resistor divider with the on-chip 5 kΩ resis-
tor to realize 2.5 V at the reference input pin (REF IN). A trim
potentiometer is needed to accommodate the tolerance of the
AD872A’s 5 kΩ resistor.
REV. A
–11–
11 Page | ||
| Páginas | Total 20 Páginas | |
| PDF Descargar | [ Datasheet AD872A.PDF ] | |
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