2.7 V to 5.5 V, 12-Bit, 8 s
ADC in 8-Lead SOIC/PDIP
AD7896
FEATURES
100 kHz Throughput Rate
Fast 12-Bit Sampling ADC with 8 s Conversion Time
8-Lead PDIP and SOIC
Single 2.7 V to 5.5 V Supply Operation
High Speed, Easy-to-Use Serial Interface
On-Chip Track-and-Hold Amplifier
Analog Input Range Is 0 V to Supply
High Input Impedance
Low Power: 9 mW Typ
FUNCTIONAL BLOCK DIAGRAM
VDD
AD7896
TRACK-AND-HOLD
V IN
12-BIT
ADC
OUTPUT
REGISTER
CONVST
CLOCK
AGND
DGND
BUSY
SCLK
SDATA
GENERAL DESCRIPTION
PRODUCT HIGHLIGHTS
The AD7896 is a fast, 12-bit ADC that operates from a single
2.7 V to 5.5 V supply and is housed in small 8-lead PDIP and
8-lead SOIC packages. The part contains an 8 µs successive
approximation ADC, an on-chip track-and-hold amplifier, an
on-chip clock, and a high speed serial interface.
1. Complete, 12-bit ADC in an 8-Lead Package.
The AD7896 contains an 8 µs ADC, a track-and-hold amplifier, control logic, and a high speed serial interface, all in an
8-lead PDIP. The VDD input is used as the reference for the
part, so no external reference is needed. This offers considerable space saving over alternative solutions.
Output data from the AD7896 is provided via a high speed,
serial interface port. This 2-wire serial interface has a serial
clock input and a serial data output with the external serial
clock accessing the serial data from the part.
In addition to the traditional dc accuracy specifications, such as
linearity, full-scale, and offset errors, the AD7896 is also specified for dynamic performance parameters, including harmonic
distortion and signal-to-noise ratio.
The part accepts an analog input range of 0 V to VDD and operates
from a single 2.7 V to 5.5 V supply, consuming only 9 mW
typical. The VDD input is also used as the reference for the part
so that no external reference is required.
2. Low Power, Single-Supply Operation.
The AD7896 operates from a single 2.7 V to 5.5 V supply
and consumes only 9 mW typical. The automatic powerdown mode, where the part goes into power down once
conversion is complete and “wakes up” before the next conversion cycle, makes the AD7896 ideal for battery-powered
or portable applications.
3. High Speed Serial Interface.
The part provides high speed serial data and serial clock lines
allowing for an easy, 2-wire serial interface arrangement.
The AD7896 features a high sampling rate mode and, for low
power applications, a proprietary automatic power-down mode
where the part automatically goes into power-down once conversion is complete and “wakes up” before the next conversion cycle.
The part is available in a small, 8-lead, 0.3'' wide, plastic or
hermetic dual-in-line package (PDIP) and in an 8-lead, small
outline IC (SOIC).
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 that
may result from its use. No license is granted by implication or otherwise
under any patent or patent rights of Analog Devices. Trademarks and
registered trademarks are the property of their respective companies.
One Technology Way, P.O. Box 9106, Norwood, MA 02062-9106, U.S.A.
Tel: 781/329-4700
www.analog.com
Fax: 781/326-8703
© 2003 Analog Devices, Inc. All rights reserved.
AD7896–SPECIFICATIONS
Parameter
A Version1
DYNAMIC PERFORMANCE 2
Signal-to-(Noise + Distortion) Ratio3
@ 25°C
70
(VDD = 2.7 V to 5.5 V, AGND = DGND = 0 V. All specifications TMIN to TMAX,
unless otherwise noted.)
B Version
J Version S Version Unit
70
70 typ
70
dB min
–80
dB min
dB max
TMIN to TMAX
Total Harmonic Distortion (THD)3
–80
70
–80
–80 typ
Peak Harmonic or Spurious Noise3
–80
–80
–80 typ
–80
–80
–80
–80
–80 typ
–80 typ
–80
–80
dB max
dB max
12
12
12
12
Bits
12
±1
±1
±3
±4
±4
12
± 1/2
±1
± 1.5
±4
±3
12
±1
±1
±3
±5
±5
12
±1
±1
±3
±4
±4
Bits
LSB max
LSB max
LSB max
LSB max
LSB max
0 to +VDD
±2
0 to +VDD
±2
0 to +VDD 0 to +VDD V
±2
±5
µA max
2.0
2.4
0.8
± 10
10
2.0
2.4
0.8
± 10
10
2.0
2.4
0.8
± 10
10
dB max
Intermodulation Distortion (IMD)3
Second Order Terms
Third Order Terms
DC ACCURACY
Resolution
Minimum Resolution for Which No
Missing Codes Are Guaranteed
Relative Accuracy3
Differential Nonlinearity3
Positive Full-Scale Error3
Unipolar Offset Error
ANALOG INPUT
Input Voltage Range
Input Current
LOGIC INPUTS
Input High Voltage, VINH
Input Low Voltage, VINL
Input Current, IIN
Input Capacitance, CIN4
LOGIC OUTPUTS
Output High Voltage, VOH
Output Low Voltage, VOL
Output Coding
CONVERSION RATE
Conversion Time
Mode 1 Operation
Mode 2 Operation5
Track-and-Hold Acquisition Time3
2.4
0.4
8
14
1.5
2.4
2.4
0.4
0.4
Straight (Natural) Binary
8
14
1.5
8
14
1.5
–2–
Test Conditions/
Comments
fIN = 10 kHz Sine Wave,
fSAMPLE = 100 kHz
fIN = 10 kHz Sine Wave,
fSAMPLE = 100 kHz
fIN = 10 kHz Sine Wave,
fSAMPLE = 100 kHz
fa = 9 kHz, fb = 9.5 kHz,
fSAMPLE = 100 kHz
VDD = 5 V ± 10%
VDD = 2.7 V to 3.6 V
2.0
2.4
0.8
± 10
10
V min
VDD = 2.7 V to 3.6 V
VDD = 5 V ± 10%
V max
µA max
pF max
VIN = 0 V to VDD
2.4
0.4
V min
V max
ISOURCE = 400 A
ISINK = 1.6 mA
8.5
14.5
1.5
µs max
µs max
µs max
REV. C
AD7896
Parameter
A Version1
B Version
J Version
S Version Unit
POWER REQUIREMENTS
VDD
IDD
2.7/5.5
4
2.7/5.5
4
2.7/5.5
4
2.7/5.5
4
5
5
5
5
10.8
10.8
10.8
10.8
5
15
50
150
13.5
5
15
50
150
13.5
5 typ
75
50
500
13.5
5
75
50
500
13.5
Power Dissipation
Power-Down Mode
IDD @ 25°C
TMIN to TMAX
IDD @ 25°C
TMIN to TMAX
Power Dissipation @ 25°C
Test Conditions/
Comments
V min/max
mA max
Digital Input @ DGND,
VDD = 2.7 V to 3.6 V
mA max
Digital Inputs @ DGND,
VDD = 5 V ± 10%
mW max VDD = 2.7 V, Typically 9 mW
Digital Inputs @ DGND
µA max
VDD = 2.7 V to 3.6 V
µA max
VDD = 2.7 V to 3.6 V
µA max
VDD = 5 V ± 10%
µA max
VDD = 5 V ± 10%
µW max
VDD = 2.7 V
NOTES
1
Temperature ranges are as follows: A, B Versions: –40°C to +85°C; J Version: 0°C to +70°C; S Version: –55°C to +125°C.
2
Applies to Mode 1 operation. See the section on Operating Modes.
3
See Terminology.
4
Sample tested @ 25°C to ensure compliance.
5
This 14 µs includes the wake-up time from standby. This wake-up time is timed from the rising edge of CONVST, whereas conversion is timed from the falling edge
of CONVST, for narrow CONVST pulsewidth the conversion time is effectively the wake-up time plus conversion time, hence 14 µs. This can be seen from Figure 3.
Note that if the CONVST pulsewidth is greater than 6 µs, the effective conversion time will increase beyond 14 µs.
Specifications subject to change without notice.
TIMING CHARACTERISTICS1 (V
DD
= 2.7 V to 5.5 V, AGND = DGND = 0 V)
Parameter
A, B Versions
J Version
S Version
Unit
Test Conditions/Comments
t1
t2
t3
t4
40
402
402
40
402
402
40
452
452
ns min
ns min
ns min
603
1003
10
504
603
1003
10
504
703
1103
10
504
ns max
ns max
ns min
ns max
CONVST Pulsewidth
SCLK High Pulsewidth
SCLK Low Pulsewidth
Data Access Time after Falling Edge of SCLK
VDD = 5 V ± 10%
VDD = 2.7 V to 3.6 V
Data Hold Time after Falling Edge of SCLK
Bus Relinquish Time after Falling Edge of SCLK
t5
t6
NOTES
1
Sample tested at 25°C to ensure compliance. All input signals are measured with tr = tf = 1 ns (10% to 90% of V DD) and timed from a voltage level of 1.4 V.
2
The SCLK maximum frequency is 10 MHz. Care must be taken when interfacing to account for the data access time, t 4, and the setup time required for the user’s
processor. These two times will determine the maximum SCLK frequency that the user’s system can operate with. See Serial Interface section for more information.
3
Measured with the load circuit of Figure 1 and defined as the time required for an output to cross 0.8 V or 2 V.
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 6, quoted in the timing characteristics is the true bus relinquish time
of the part and as such is independent of external bus loading capacitances.
1.6mA
TO
OUTPUT
PIN
1.6V
50pF
400A
Figure 1. Load Circuit for Access Time and Bus
Relinquish Time
REV. C
–3–
AD7896
PDIP Package, Power Dissipation . . . . . . . . . . . . . . . . 450 mW
JA Thermal Impedance . . . . . . . . . . . . . . . . . . . . . 125°C/W
JC Thermal Impedance . . . . . . . . . . . . . . . . . . . . . . 50°C/W
Lead Temperature (Soldering, 10 sec) . . . . . . . . . . . . 260°C
SOIC Package, Power Dissipation . . . . . . . . . . . . . . . . 450 mW
JA Thermal Impedance . . . . . . . . . . . . . . . . . . . . . 160°C/W
JC Thermal Impedance . . . . . . . . . . . . . . . . . . . . . . 75°C/W
Lead Temperature, Soldering
Vapor Phase (60 sec) . . . . . . . . . . . . . . . . . . . . . . . 215°C
Infrared (15 sec) . . . . . . . . . . . . . . . . . . . . . . . . . . . 220°C
ESD . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .>4000 V
ABSOLUTE MAXIMUM RATINGS*
(TA = 25°C, unless otherwise noted.)
VDD to AGND . . . . . . . . . . . . . . . . . . . . . . . . . . –0.3 V to +7 V
VDD to DGND . . . . . . . . . . . . . . . . . . . . . . . . . . –0.3 V to +7 V
Analog Input Voltage to AGND . . . . . . –0.3 V to VDD + 0.3 V
Digital Input Voltage to DGND . . . . . . –0.3 V to VDD + 0.3 V
Digital Output Voltage to DGND . . . . . –0.3 V to VDD + 0.3 V
Operating Temperature Range
Commercial (J Version) . . . . . . . . . . . . . . . . . 0°C to +70°C
Industrial (A, B Versions) . . . . . . . . . . . . . . –40°C to +85°C
Extended (S Version) . . . . . . . . . . . . . . . . . –55°C to +125°C
Storage Temperature Range . . . . . . . . . . . . . –65°C to +150°C
Junction Temperature . . . . . . . . . . . . . . . . . . . . . . . . . . 150°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
AD7896AN
AD7896BN
AD7896AR
AD7896AR-REEL
AD7896AR-REEL7
AD7896BR
AD7896BR-REEL
AD7896BR-REEL7
AD7896JR
AD7896JR-REEL
AD7896SQ
EVAL-AD7896CB
Temperature
Range
Linearity
SNR
Error (LSB) (dB)
Package
Option*
–40°C to +85°C
–40°C to +85°C
–40°C to +85°C
–40°C to +85°C
–40°C to +85°C
–40°C to +85°C
–40°C to +85°C
–40°C to +85°C
0°C to +70°C
0°C to +70°C
–55°C to +125°C
±1
± 1/2
±1
±1
±1
± 1/2
± 1/2
± 1/2
±1
±1
±1
N-8
N-8
R-8
R-8
R-8
R-8
R-8
R-8
R-8
R-8
Q-8
Evaluation Board
70
70
70
70
70
70
70
70
70
70
70
*N = PDIP; Q = CERDIP; R = SOIC.
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
AD7896 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–
REV. C
AD7896
PIN CONFIGURATION
8 BUSY
V IN 1
AD7896
7 CONVST
TOP VIEW
3
AGND
(Not to Scale) 6 DGND
VDD 2
SCLK 4
5 SDATA
PIN FUNCTION DESCRIPTIONS
Pin No.
Mnemonic
Description
1
VIN
Analog Input. The analog input range is 0 V to VDD.
2
VDD
Positive supply voltage, 2.7 V to 5.5 V.
3
AGND
Analog Ground. Ground reference for track-and-hold, comparator, and DAC.
4
SCLK
Serial Clock Input. An external serial clock is applied to this input to obtain serial data from the AD7896.
A new serial data bit is clocked out on the falling edge of this serial clock. Data is guaranteed valid for
10 ns after this falling edge so data can be accepted on the falling edge when a fast serial clock is used.
The serial clock input should be taken low at the end of the serial data transmission.
5
SDATA
Serial Data Output. Serial data from the AD7896 is provided at this output. The serial data is clocked
out by the falling edge of SCLK, but the data can also be read on the falling edge of the SCLK. This is
possible because data bit N is valid for a specified time after the falling edge of the SCLK (data hold
time) and can be read before data bit N+1 becomes valid a specified time after the falling edge of SCLK
(data access time) (see Figure 4). Sixteen bits of serial data are provided with four leading zeros followed
by the 12 bits of conversion data. On the 16th falling edge of SCLK, the SDATA line is held for the data
hold time and then disabled (three-stated). Output data coding is straight binary.
6
DGND
Digital Ground. Ground reference for digital circuitry.
7
CONVST
Convert Start. Edge-triggered logic input. On the falling edge of this input, the track-and-hold goes into
its hold mode and conversion is initiated. If CONVST is low at the end of conversion, the part goes into
power-down mode. In this case, the rising edge of CONVST “wakes up” the part.
8
BUSY
The BUSY pin is used to indicate when the part is doing a conversion. The BUSY pin goes high on the
falling edge of CONVST and returns low when the conversion is complete.
REV. C
–5–
AD7896
TERMINOLOGY
Relative Accuracy
Total Harmonic Distortion
Total harmonic distortion (THD) is the ratio of the rms sum of
harmonics to the fundamental. For the AD7896, it is defined as:
This is the maximum deviation from a straight line passing
through the endpoints of the ADC transfer function. The endpoints of the transfer function are zero scale (which is VIN =
AGND + 1/2 LSB), a point 1/2 LSB below the first code transition (00 . . . 000 to 00 . . . 001), and full scale (which is VIN =
AGND + VDD – 1/2 LSB), a point 1/2 LSB above the last code
transition (11 . . . 110 to 11 . . . 111).
THD ( dB) = 20 log
Differential Nonlinearity
This is the difference between the measured and the ideal 1 LSB
change between any two adjacent codes in the ADC.
Unipolar Offset Error
This is the deviation of the first code transition (00 . . . 000 to
00 . . . 001) from the ideal VIN voltage (AGND + 1 LSB).
Positive Full-Scale Error
This is the deviation of the last code transition (11 . . . 110 to
11 . . . 111) from the ideal (VIN = AGND + VDD – 1 LSB)
after the offset error has been adjusted out.
V2 2 + V32 + V4 2 + V5 2 + V6 2
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.
Peak Harmonic or Spurious Noise
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.
Intermodulation Distortion
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 distortion 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-and-Hold Acquisition Time
Track-and-hold acquisition time is the time required for the
output of the track-and-hold amplifier to reach its final value,
within ± 1/2 LSB, after the end of conversion (the point at which
the track-and-hold returns into track mode). It also applies to a
situation where there is a step input change on the input voltage
applied to the selected VIN input of the AD7896. It means that
the user must wait for the duration of the track-and-hold acquisition time after the end of conversion or after a step input change
to VIN before starting another conversion, to ensure the part
operates to specification.
Signal-to-(Noise + Distortion) Ratio
This is the measured ratio of signal-to-(noise + distortion) at the
output of the ADC. The signal is the rms amplitude of the fundamental. Noise is the sum of all nonfundamental signals up to
half the sampling frequency (fS/2), excluding dc. The ratio is
dependent on 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:
The AD7896 is tested using the CCIF standard where two
input frequencies near the top end of the input bandwidth are
used. In this case, the second order terms are 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 dB.
Signal-to-(Noise + Distortion) = (6.02N + 1.76) dB
Thus, for a 12-bit converter, this is 74 dB.
–6–
REV. C
AD7896
track-and-hold is greater than the Nyquist rate of the ADC even
when the ADC is operated at its maximum throughput rate of
100 kHz (i.e., the track-and-hold can handle input frequencies
in excess of 50 kHz).
CONVERTER DETAILS
The AD7896 is a fast, 12-bit ADC that operates from a single
2.7 V to 5.5 V supply. It provides the user with a track-andhold, ADC, and serial interface logic functions on a single
chip. The ADC section of the AD7896 consists of a conventional successive approximation converter based on an R-2R
ladder structure. The internal reference for the AD7896 is
derived from VDD, which allows the part to accept an analog
input range of 0 V to VDD. The AD7896 has two operating
modes: the high sampling mode and the auto sleep mode
where the part automatically goes into sleep after the end of
conversion. These modes are discussed in more detail in the
Timing and Control section.
The track-and-hold amplifier acquires an input signal to 12-bit
accuracy in less than 1.5 µs. The operation of the track-andhold is essentially transparent to the user. With the high sampling
operating mode, the track-and-hold amplifier goes from its
tracking mode to its hold mode at the start of conversion (i.e.,
the rising edge of CONVST). The aperture time for the trackand-hold (i.e., the delay time between the external CONVST
signal and the track-and-hold actually going into hold) is typically 15 ns. At the end of conversion (on the falling edge of
BUSY), the part returns to its tracking mode. The acquisition
time of the track-and-hold amplifier begins at this point. For the
auto shutdown mode, the rising edge of CONVST wakes up the
part and the track-and-hold amplifier goes from its tracking
mode to its hold mode 6 µs after the rising edge of CONVST
(provided that the CONVST high time is less than 6 µs). Once
again the part returns to its tracking mode at the end of conversion when the BUSY signal goes low.
A major advantage of the AD7896 is that it provides all of the
preceding functions in an 8-lead package, PDIP or SOIC. This
offers the user considerable space saving advantages over alternative solutions. The AD7896 consumes only 9 mW typical, making
it ideal for battery-powered applications.
Conversion is initiated on the AD7896 by pulsing the CONVST
input. On the falling edge of CONVST, the on-chip track-andhold goes from track to hold mode and the conversion sequence
is started. The conversion clock for the part is generated internally using a laser-trimmed clock oscillator circuit. Conversion
time for the AD7896 is 8 µs in the high sampling mode (14 µs
for the auto sleep mode), and the track-and-hold acquisition
time is 1.5 µs. To obtain optimum performance from the part,
the read operation should not occur during the conversion or
during 400 ns prior to the next conversion. This allows the part
to operate at throughput rates up to 100 kHz and achieves data
sheet specifications (see the Timing and Control section).
Timing and Control
Figure 2 shows the timing and control sequence required to
obtain optimum performance from the AD7896. In the
sequence shown, conversion is initiated on the falling edge of
CONVST and new data from this conversion is available in the
output register of the AD7896 8 µs later. Once the read operation has taken place, another 400 ns should be allowed before
the next falling edge of CONVST to optimize the settling of the
track-and-hold amplifier before the next conversion is initiated.
With the serial clock frequency at its maximum of 10 MHz (5 V
operation), the achievable throughput time for the part is 8 µs
(conversion time) plus 1.6 µs (read time) plus 0.4 µs (acquisition time). This results in a minimum throughput time of 10 µs
(equivalent to a throughput rate of 100 kHz). A serial clock of
less than 10 MHz can be used, but this will in turn mean that
the throughput time will increase.
CIRCUIT DESCRIPTION
Analog Input Section
The analog input range for the AD7896 is 0 V to VDD. The
VIN pin drives the input to the track-and-hold amplifier directly.
This allows for a maximum output impedance of the circuit
driving the analog input of 1 kΩ. This ensures that the part will
be settled to 12-bit accuracy in the 1.5 µs acquisition time. This
input is benign with dynamic charging currents. The designed
code transitions occur on successive integer LSB values (i.e.,
1 LSB, 2 LSB, 3 LSB, . . . , FS – 1 LSB). Output coding is straight
(natural) binary with 1 LSB = FS/4096 = 3.3 V/4096 = 0.81 mV.
The ideal input/output transfer function is shown in Table I.
The read operation consists of 16 serial clock pulses to the output
shift register of the AD7896. After 16 serial clock pulses, the shift
register is reset and the SDATA line is three-stated. If there are
more serial clock pulses after the 16th clock, the shift register will
be moved on past its reset state. However, the shift register will be
reset again on the falling edge of the CONVST signal to ensure
that the part returns to a known state every conversion cycle. As a
result, a read operation from the output register should not
straddle across the falling edge of CONVST as the output shift
register will be reset in the middle of the read operation and the
data read back into the microprocessor will appear invalid.
Table I. Ideal Input/Output Code Table for the AD7896
Analog Input1
2
+FSR – 1 LSB (3.299194)
+FSR – 2 LSB (3.298389)
+FSR/2 – 3 LSB (3.297583)
AGND + 3 LSB (0.002417)
AGND + 2 LSB (0.001611)
AGND + 1 LSB (0.000806)
Code Transition
111 . . . 110 to 111 . . . 111
111 . . . 101 to 111 . . . 110
111 . . . 100 to 111 . . . 101
000 . . . 010 to 000 . . . 011
000 . . . 001 to 000 . . . 010
000 . . . 000 to 000 . . . 001
The throughput rate of the part can be increased by reading
data during conversion. If the data is read during conversion, a
throughput time of 8 µs (conversion time) plus 1.5 µs (acquisition time) is achieved when a 10 MHz, (5 V operation) serial
clock is being used. This minimum throughput time of 9.5 µs is
achieved with a slight reduction in performance from the AD7896.
The advantage of this arrangement is that when the serial clock
is significantly lower than 10 MHz, the throughput time for this
arrangement will be significantly less than the throughput time
where the data is read after conversion. The signal-to-(noise +
distortion) number is likely to degrade by less than 1 dB while
the code flicker from the part will also increase (see the AD7896
Performance section).
NOTES
1
FSR is full-scale range and is 3.3 V with V DD = +3.3 V.
2
1 LSB = FSR/4096 = 0.81 mV with V DD = +3.3 V.
Track-and-Hold Section
The track-and-hold amplifier on the analog input of the AD7896
allows the ADC to accurately convert an input sine wave of fullscale amplitude to 12-bit accuracy. The input bandwidth of the
REV. C
–7–
AD7896
before the next conversion takes place. This is achieved by
keeping CONVST low at the end of conversion, whereas it was
high at the end of conversion for Mode 1 operation. The rising
edge of CONVST “wakes up” the part. This wake-up time is 6
µs, at which point the track-and-hold amplifier goes into its hold
mode. The conversion takes 8 µs after this, provided the
CONVST has gone low, giving a total of 14 µs from the rising
edge of CONVST to the conversion being complete, which is
indicated by the BUSY going low. Note that since the wakeup time from the rising edge of CONVST is 6 µs, when the
CONVST pulsewidth is greater than 6 µs, the conversion will
take more than the 14 µs shown in the diagram from the rising
edge of CONVST. This is because the track-and-hold amplifier
goes into its hold mode on the falling edge of CONVST and
then the conversion will not be complete for a further 8 µs. In
this case, the BUSY will be the best indicator for when the
conversion is complete. Even though the part is in sleep mode,
data can still be read from the part. The read operation consists
of 16 clock cycles as in Mode 1 operation. For the fastest serial
clock of 10 MHz at 5 V operation, the read operation will take
1.6 µs, which must be complete at least 400 ns before the falling
edge of the next CONVST to allow the track-and-hold amplifier
to have enough time to settle. This mode is very useful when the
part is converting at a slow rate as the power consumption will
be significantly reduced from that of Mode 1 operation.
OPERATING MODES
Mode 1 Operation (High Sampling Performance)
The timing diagram in Figure 2 is for optimum performance in
Operating Mode 1 where the falling edge of CONVST starts the
conversion and puts the track-and-hold amplifier into its hold
mode. This falling edge of CONVST also causes the BUSY
signal to go high to indicate that a conversion is taking place.
The BUSY signal goes low when the conversion is complete,
which is 8 µs max after the falling edge of CONVST, and new
data from this conversion is available in the output register of
the AD7896. A read operation accesses this data. This read
operation consists of 16 clock cycles, and the length of this read
operation depends on the serial clock frequency. For the fastest
throughput rate (with a serial clock of 10 MHz at 5 V operation), the read operation will take 1.6 µs. The read operation
must be complete at least 400 ns before the falling edge of
the next CONVST, which gives a total time of 10 µs for the full
throughput time (equivalent to 100 kHz). This mode of operation should be used for high sampling applications.
Mode 2 Operation (Auto Sleep after Conversion)
The timing diagram in Figure 3 is for optimum performance in
Operating Mode 2 where the part automatically goes into sleep
mode once BUSY goes low after conversion and “wakes up”
tCONVERT = 8s
t1 = 40ns MIN
t1
CONVST
BUSY
400ns MIN
SCLK
tCONVERT = 8s
CONVERSION IS
INITIATED AND
TRACK-AND-HOLD GOES
INTO HOLD
CONVERSION ENDS SERIAL READ
8s LATER
OPERATION
READ OPERATION
SHOULD END 400ns
PRIOR TO NEXT
FALLING EDGE OF
CONVST
OUTPUT
SERIAL
SHIFT
REGISTER
IS RESET
Figure 2. Mode 1 Timing Operation Diagram for High Sampling Performance
t1 = 6s
WAKE-UP
TIME
t1
CONVST
BUSY
400ns MIN
SCLK
tCONVERT = 14s
PART
WAKES
UP
CONVERSION
IS INITIATED
TRACK-ANDHOLD GOES
INTO HOLD
CONVERSION
ENDS
14µs LATER
SERIAL READ
OPERATION
READ OPERATION
SHOULD END 400ns
PRIOR TO NEXT
FALLING EDGE OF
CONVST
OUTPUT
SERIAL
SHIFT
REGISTER
IS RESET
Figure 3. Mode 2 Timing Diagram Where Automatic Sleep Function Is Initiated
–8–
REV. C
AD7896
must be low when CONVST goes low in order to reset the
output shift register correctly.
Serial Interface
The serial interface to the AD7896 consists of three wires: a
serial clock input (SCLK), the serial data output (SDATA), and
a conversion status output (BUSY). This allows for an easy-touse interface to most microcontrollers, DSP processors, and
shift registers.
The serial clock input does not need to be continuous during
the serial read operation. The 16 bits of data (four leading zeros
and 12-bit conversion result) can be read from the AD7896 in a
number of bytes. However, the SCLK input must remain low
between the two bytes.
Figure 4 shows the timing diagram for the read operation to the
AD7896. The serial clock input (SCLK) provides the clock
source for the serial interface. Serial data is clocked out from the
SDATA line on the falling edge of this clock and is valid on both
the rising and falling edges of SCLK. The advantage of having
the data valid on both the rising and falling edges of the SCLK
is to give the user greater flexibility in interfacing to the part and
so that a wider range of microprocessor and microcontroller interfaces can be accommodated. This also explains the two timing
figures t4 and t5 that are quoted on the diagram. The time t4 specifies how long after the falling edge of the SCLK that the next data
bit becomes valid, whereas the time t5 specifies how long after the
falling edge of the SCLK that the current data bit is valid for. The
first leading zero is clocked out on the first rising edge of
SCLK; note that the first zero may be valid on the first falling
edge of SCLK even though the data access time is specified
at 60 ns (5 V [A, B, J versions only]) for the other bits (and the
SCLK high time will be 50 ns with a 10 MHz SCLK). The reason
that the first bit will be clocked out faster than the other bits is
due to the internal architecture of the part. Sixteen clock pulses
must be provided to the part to access the full conversion result.
The maximum SCLK frequency is 10 MHz for 5 V operation
(giving a throughput of 100 kHz) and at 2.7 V the maximum
SCLK frequency is less than 10 MHz to allow for the longer
data access time, t4 (60 ns @ 5 V, 100 ns @ 2.7 V (A, B, J
versions), 70 ns @ 5 V, 110 ns @ 2.7 V (S version)). Note that
at 3.0 V operation (A, B, J versions), an SCLK of 10 MHz
(throughput rate of 100 kHz) may be acceptable if the required
processor setup time is 0 ns (this may be possible with an ASIC
or FPGA). The data must be read in the next 10 ns, which is
specified as the data hold time, t5, after the SCLK edge.
The AD7896 counts the serial clock edges to know which bit
from the output register should be placed on the SDATA output. To ensure that the part does not lose synchronization, the
serial clock counter is reset on the falling edge of the CONVST
input provided the SCLK line is low. The user should ensure
that a falling edge on the CONVST input does not occur while
a serial data read operation is in progress.
MICROPROCESSOR/MICROCONTROLLER INTERFACE
The AD7896 provides a 3-wire serial interface that can be
used for connection to the serial ports of DSP processors and
microcontrollers. Figures 5 through 8 show the AD7896
interfaced to a number of different microcontrollers and DSP
processors. The AD7896 accepts an external serial clock and as
a result, in all interfaces shown here, the processor/controller is
configured as the master, providing the serial clock, with the
AD7896 configured as the slave in the system.
The AD7896 provides four leading zeros followed by the 12-bit
conversion result starting with the MSB (DB11). The last data
bit to be clocked out on the penultimate falling clock edge is the
LSB (DB0). On the 16th falling edge of SCLK, the LSB (DB0)
will be valid for a specified time to allow the bit to be read on
the falling edge of SCLK, and then the SDATA line is disabled
(three-stated). After this last bit has been clocked out, the SCLK
input should remain low until the next serial data read operation. If there are extra clock pulses after the 16th clock, the
AD7896 will start over again with outputting data from its output register, and the data bus will no longer be three-stated even
when the clock stops. Provided the serial clock has stopped
before the next falling edge of CONVST, the AD7896 will
continue to operate correctly with the output shift register being
reset on the falling edge of CONVST. However, the SCLK line
AD7896–8051 Interface
Figure 5 shows an interface between the AD7896 and the
8X51/L51 microcontroller. The 8X51/L51 is configured for its
Mode 0 serial interface mode. The diagram shows the simplest
form of the interface where the AD7896 is the only part connected
to the serial port of the 8X51/L51 and, therefore, no decoding
of the serial read operations is required.
t2 = t3 = 40ns MIN, t4 = 60ns MAX, t5 = 10ns MIN, t6 = 50ns MAX @ 5V, A, B, VERSIONS
t2
SCLK (I/P)
1
2
3
t3
THREE-STATE
4
5
6
15
16
t5
t4
t6
4 LEADING ZEROS
DOUT (O/P)
DB11
DB10
Figure 4. Data Read Operation
REV. C
–9–
DB0
THREE-STATE
AD7896
The BUSY line can be connected to the IRQ line of the
68HC11/L11 if an interrupt driven system is preferred. These
two options are shown in the diagram.
To chip select the AD7896 in systems where more than one
device is connected to the 8X51/L51 serial port, a port bit,
configured as an output, from one of the 8X51/L51 parallel
ports can be used to gate on or off the serial clock to the AD7896.
A simple AND function on this port bit and the serial clock from
the 8X51/L51 will provide this function. The port bit should be
high to select the AD7896 and low when it is not selected.
The end of conversion is monitored by using the BUSY signal,
which is shown in the interface diagram of Figure 5, with the
BUSY line from the AD7896 connected to the Port P1.2 of the
8X51/L51 so the BUSY line can be polled by the 8X51/L51.
The BUSY line can be connected to the INT1 line of the
8X51/L51 if an interrupt driven system is preferred. These two
options are shown on the diagram.
The serial clock rate from the 68HC11/L11 is limited to significantly less than the allowable input serial clock frequency with
which the AD7896 can operate. As a result, the time to read
data from the part will actually be longer than the conversion
time of the part. This means that the AD7896 cannot run at its
maximum throughput rate when used with the 68HC11/L11.
PC2 OR
IRQ
Note also that the AD7896 outputs the MSB first during a read
operation while the 8X51/L51 expects the LSB first. Therefore,
the data that is read into the serial buffer needs to be rearranged
before the correct data format from the AD7896 appears in the
accumulator.
P1.2
OR
INT1
8X51/L51
P3.1
SCLK
SDATA
Figure 6. AD7896 to 68HC11/L11 Interface
An interface circuit between the AD7896 and the ADSP2103/ADSP-2105 DSP processor is shown in Figure 7. In the
interface shown, the RFS1 output from the ADSP-2103/
ADSP-2105s SPORT1 serial port is used to gate the serial clock
(SCLK1) of the ADSP-2103/ADSP-2105 before it is applied to
the SCLK input of the AD7896. The RFS1 output is configured
for active high operation. The BUSY line from the AD7896 is
connected to the IRQ2 line of the ADSP-2103/ADSP-2105 so
that at the end of conversion an interrupt is generated telling the
ADSP-2103/ADSP-2105 to initiate a read operation. The interface ensures a noncontinuous clock for the AD7896’s serial
clock input, with only 16 serial clock pulses provided and the
serial clock line of the AD7896 remaining low between data
transfers. The SDATA line from the AD7896 is connected to
the DR1 line of the ADSP-2103/ADSP-2105 serial port.
AD7896
SDATA
MISO
SCLK
AD7896–ADSP-2103/ADSP-2105 Interface
BUSY
P3.0
AD7896
68HC11/L11
SCK
The serial clock rate from the 8X51/L51 is limited to significantly less than the allowable input serial clock frequency with
which the AD7896 can operate. As a result, the time to read
data from the part will actually be longer than the conversion
time of the part. This means that the AD7896 cannot run at its
maximum throughput rate when used with the 8X51/L51.
BUSY
Figure 5. AD7896 to 8X51/L51 Interface
AD7896–68HC11/L11 Interface
An interface circuit between the AD7896 and the 68HC11/L11
microcontroller is shown in Figure 6. For the interface shown,
the 68HC11/L11 SPI port is used and the 68HC11/L11 is configured in its single-chip mode. The 68HC11/L11 is configured
in the master mode with its CPOL bit set to a Logic 0 and its
CPHA bit set to a Logic 1. As with the previous interface, the
diagram shows the simplest form of the interface, where the
AD7896 is the only part connected to the serial port of the
68HC11/L11 and, therefore, no decoding of the serial read
operations is required.
Once again, to chip select the AD7896 in systems where more
than one device is connected to the 68HC11/L11 serial port, a
port bit, configured as an output, from one of the 68HC11/L11
parallel ports can be used to gate on or off the serial clock to the
AD7896. A simple AND function on this port bit and the serial
clock from the 68HC11/L11 will provide this function. The port
bit should be high to select the AD7896 and low when it is
not selected.
The end of conversion is monitored by using the BUSY signal
which is shown in the interface diagram of Figure 6. With the
BUSY line from the AD7896 connected to the Port PC2 of the
68HC11/L11, the BUSY line can be polled by the 68HC11/L11.
The timing relationship between the SCLK1 and RFS1 outputs
of the ADSP-2103/ADSP-2105 are such that the delay between
the rising edge of the SCLK1 and the rising edge of an active
high RFS1 is up to 30 ns. There is also a requirement that data
must be set up 10 ns prior to the falling edge of the SCLK1 to
be read correctly by the ADSP-2103/ADSP-2105. The data
access time for the AD7896 is 60 ns (5 V [A, B versions]) from
the rising edge of its SCLK input. Assuming a 10 ns propagation delay through the external AND gate, the high time of the
SCLK1 output of the ADSP-2105 must be ≥ (30 + 60 +10
+10) ns, i.e., ≥110 ns. This means that the serial clock frequency with which the interface of Figure 7 can work is limited
to 4.5 MHz. However, there is an alternative method that allows
for the ADSP-2105 SCLK1 to run at 5 MHz (which is the max
serial clock frequency of the SCLK1 output). The arrangement
is where the first leading zero of the data stream from the
AD7896 cannot be guaranteed to be clocked into the ADSP2105 due to the combined delay of the RFS signal and the data
access time of the AD7896. In most cases, this is acceptable as
there will still be three leading zeros followed by the 12 data
bits. For the ADSP-2103, the SCLK1 frequency will need to
be limited to
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