AD AD1385KD 16-bit 500 khz wide temperature range sampling adc Datasheet

a
16-Bit 500 kHz
Wide Temperature Range Sampling ADC
AD1385
FEATURES
16-Bit Resolution
500 kHz Sampling Rate
Differential Linearity Autocalibration
Specified over –558C to +1258C Range
SNR 90 dB @ 100 kHz (min)
THD –88 dB @ 100 kHz (min)
0.0006% FSR DNL (typ)
0.0015% FSR INL (typ)
No Missing Codes
65, 610 V Bipolar Input Ranges
Zero Offset Autocalibration
FUNCTIONAL BLOCK DIAGRAM
APPLICATIONS
Medical Imaging
CAT
Magnetic Resonance
Radar
Vibration Analysis
Parametric Measurement Unit (ATE)
Digital Storage Oscilloscopes
Waveform Recorders
Analytical Instruments
PRODUCT DESCRIPTION
The AD1385 is a complete 500 kHz, 16-bit, sampling analogto-digital converter contained in a single package. Its differential
linearity autocalibration feature allows this high resolution, high
speed converter to offer outstanding noise and distortion performance, as well as excellent INL and DNL specifications, over
the full military temperature range. Autocalibration effectively
eliminates DNL drift over temperature.
The AD1385 architecture includes a low noise, low distortion
track/hold, a three pass digitally corrected subranging ADC, and
linearity calibration circuitry. A complete linearity calibration
requires only 15 ms. Precision thin-film resistors and a proprietary DAC contribute to the part’s outstanding dynamic and
static performance.
The AD1385 uses four power supplies, ± 5 V and ± 15 V, and an
external 10 MHz clock. Power dissipation is nominally 2.76 W.
Two user selectable bipolar input ranges, ± 5 V and ± 10 V, are
provided. Careful attention to grounding and a single package
make it easy to design PCBs to achieve specified performance.
The AD1385’s pinout is nearly identical to that of the AD1382,
a factory calibrated 16-bit, 500 kHz SADC. Just two additional
connections, to enable and monitor autocalibration, are required.
This commonality provides an easy upgrade path to extend system performance and operating temperature range.
REV. 0
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
(TA = +258C, VS = 615 V, VDD = +5 V, VSS = –5 V, 10 MHz External Clock,
AD1385–SPECIFICATIONS unless otherwise noted)
Parameter
Min
RESOLUTION
16
ANALOG INPUT
Input Ranges
Input Impedance
2.45
TRANSFER CHARACTERISTICS
(Combined ADC/Track/Hold)
Integral Nonlinearity1, 2, TMIN to TMAX
Differential Nonlinearity1
Drift, TMIN to TMAX
Missing Codes, TMIN to TMAX
Gain Error4
Drift, TMIN to TMAX
Bipolar Zero4
Drift, TMIN to TMAX
PSRR
Noise
DYNAMIC CHARACTERISTICS2
± 5 V FSR, VIN = –0.4 dB, TMIN to TMAX
Sample Rate
Signal-to-Noise Ratio5
f = 5 kHz
f = 100 kHz
f = 200 kHz
Peak Distortion
f = 5 kHz
f = 100 kHz
f = 200 kHz
Total Harmonic Distortion6
f = 5 kHz
f = 100 kHz
f = 200 kHz
DYNAMIC CHARACTERISTICS2
± 10 V FSR, VIN = –0.4 dB, TMIN to TMAX
Sample Rate
Signal-to-Noise Ratio5
f = 5 kHz
f = 100 kHz
f = 200 kHz
Peak Distortion
f = 5 kHz
f = 100 kHz
f = 200 kHz
Total Harmonic Distortion6
f = 5 kHz
f = 100 kHz
f = 200 kHz
DIGITAL INPUTS
Input Voltage
VIL
VIH
Input Current
Input Capacitance
Clock
Frequency
Duty Cycle
Aperture Delay7
DIGITAL OUTPUTS
Output Voltage
VOL @ IOL = 3.2 mA
VOH @ IOH = –3.2 mA
Output Capacitance
Leakage, Outputs Disabled
AD1385KD
Typ
Max
Min
AD1385TD
Typ
Max
16
± 5, ± 10
2.5
± 0.0015
± 0.0006
0.3
± 0.05
8
± 0.05
5
± 0.006
70
2.55
2.45
Bits
± 5, ± 10
2.5
± 0.0015
± 0.0006
0.3
± 0.0015
None
± 0.15
15
± 0.10
15
± 0.10
Units
± 0.05
8
± 0.05
5
± 0.006
70
500
2.55
± 0.0015
None
± 0.15
15
± 0.10
15
± 0.10
500
V
kΩ
% FSR3
% FSR
ppm/°C
% FSR
ppm/°C
% FSR
ppm/°C
% FSR/V
µV rms
kHz
90
90
88
93
92
91
90
90
88
93
92
91
dB
dB
dB
–90
–88
–82
–107
–95
–88
–90
–88
–82
–107
–95
–88
dB
dB
dB
–90
–88
–82
–105
–95
–88
–90
–88
–82
–105
–95
–88
dB
dB
dB
500
500
kHz
90
90
88
95
94
93
90
90
88
95
94
93
dB
dB
dB
–90
–80
–74
–108
–87
–82
–90
–80
–74
–108
–87
–82
dB
dB
dB
–90
–80
–74
–105
–87
–82
–90
–80
–74
–105
–87
–82
dB
dB
dB
0.8
2.25
2.4
± 200
0.8
2.25
± 200
2
2
2.5–10
40-60
7
2.5–10
40-60
7
0.2
4.5
4
–2–
0.4
2.4
± 200
0.2
4.5
4
V
V
µA
pF
MHz
%
ns
0.4
± 200
V
V
pF
µA
REV. 0
AD1385
Parameter
Min
OUTPUT CODING
AD1385KD
Typ
Max
Min
AD1385TD
Typ
Max
Units
Complementary Offset Binary or Complementary Twos Complement
INTERNAL REFERENCE
Voltage
Current
Drift
9.990
2
TEMPERATURE RANGE, CASE
Specified
Storage
POWER REQUIREMENTS
Specified Operating Range
± VS
+VDD
–VSS
Current Drains
+VS
–VS
+VDD
–VSS
Power Dissipation
10.010
5
5
9.990
2
10.010
5
5
15
15
V
mA
ppm/°C
0
–65
+70
+150
–55
–65
+125
+150
°C
°C
14.25
4.75
–5.25
15.75
5.25
–4.75
14.25
4.75
–5.25
15.75
5.25
–4.75
V
V
V
80
75
160
200
4.125
mA
mA
mA
mA
Watts
52
48
104
148
2.76
80
75
160
200
4.125
52
48
104
148
2.76
NOTES
1
Integral linearity is inferred from FFTs. Differential linearity is derived from histograms.
2
Performance over temperature is specified at the temperature at which the last calibration was performed.
3
FSR = Full-Scale Range.
4
Adjustable to zero.
5
SNR excludes harmonics 2-9 of the fundamental.
6
THD includes harmonics 2-9 of the fundamental.
7
Aperture delay is the time from the rising edge on the Hold Command Input to the opening of the switch in the Track/Hold.
Specifications subject to change without notice.
TIMING SPECIFICATIONS1, 2 (T = –558C to +1258C, V
A
Parameter
Design Minimum
START COMMAND
tSCS
tSCH
AUTOZERO
tAZS
tAZH
S
= 615 V, VDD = +5 V, VSS = –5 V)
Typ
Unit
Description
10
10
ns
ns
Setup Time
Hold Time
10
20
ns
ns
Setup Time
Hold Time
DATA VALID
tDVS
tDVH
1.5
0.5
CP3
CP3
Setup Time
Hold Time
HOLD COMMAND
tH
tD
13
7
CP3
ns
Hold Time
Delay Time
DATA STROBE
tDS
tDSD
2
16.5
CP3
CP3
Pulse Width
Delay
CALIBRATE PULSE WIDTH
20
ns
CALIBRATION STATUS
15
ms
Duration
NOTES
1
Refer to Figures 17, 18 and 24.
2
Design minimums are derived from worst case design analysis and/or simulation results. Typical values are based on characterization data. These specifications are
not guaranteed or tested.
3
The time duration for this parameter varies in direct proportion to the width of the Clock Pulse (CP).
REV. 0
–3–
AD1385
ABSOLUTE MAXIMUM RATINGS*
AD1385 PIN CONNECTIONS
+VS to AGND . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 18 V
–VS to AGND . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . –18 V
VDD to PGND . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7 V
VSS to PGND . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . –7 V
AGND to PGND . . . . . . . . . . . . . . . . . . . . . . . . . . . . ± 0.3 V
Analog Inputs . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . ± VS
Reference Input . . . . . . . . . . . . . . . . . . . . . . . . . .0 V to +11 V
Digital Inputs . . . . . . . . . . . . . . . . . . . . –0.3 V to VDD + 0.3 V
Output Short Circuit Duration
Reference Output . . . . . . . . . . . . . . . . . . . . . . . . . Indefinite
Track/Hold Output . . . . . . . . . . . . . . . . . . . . . . . . . . . 1 sec
Digital Outputs . . . . . . . . . . . . . . 1 sec for Any One Output
Case Temperature (Operating) . . . . . . . . . . –55°C to +125°C
Storage Temperature . . . . . . . . . . . . . . . . . . –65°C to +150°C
The AD1385 is housed in a 48-pin bottom-brazed ceramic
bathtub package. The pinout is as follows:
*Stresses greater than 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 section of this specification is not implied. Exposure to absolute
maximum rating conditions for extended periods may affect device reliability.
ORDERING GUIDE
Model
Temperature
Range (Case)
Package Option*
AD1385KD
AD1385TD
AD1385TD/883B
0°C to +70°C
–55°C to +125°C
–55°C to +125°C
DH-48A
DH-48A
DH-48A
*DH-48A = Bottom Brazed Ceramic DIP.
Pin Function
Pin Function
11
12
13
14
15
16
17
18
19
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
48
47
46
45
44
43
42
41
40
39
38
37
36
35
34
33
32
31
30
29
28
27
26
25
CLOCK IN
POWER GROUND
B1/B9 (MSB)
B2/B10
B3/B11
B4/B12
B5/B13
B6/B14
B7/B15
B8/B16 (LSB)
VDD1 (+5 V SIGNAL)
POWER GROUND
VSS1 (–5 V SIGNAL)
SIGNAL GROUND
DATA STROBE
HI/LO BYTE SELECT
OE DATA ENABLE
START CONVERT
HOLD COMMAND OUT
SIGNAL GROUND
+VS2 (+15 V)
HOLD COMMAND IN
–VS2 (–15 V)
POWER GROUND
VDD2 (+5 V POWER)
POWER GROUND
VSS2 (–5 V POWER)
AUTOZERO
B1 SELECT
POWER GROUND
POWER GROUND
CAL
GAIN ADJUST
+10 V REFERENCE OUT
–VS1 (–15 V)
SIGNAL GROUND
+VS1 (+15 V)
SIGNAL GROUND
DNC
DNC
+10 V REFERENCE IN
VIN B
VIN A
OFFSET ADJUST
CAL STATUS
TRACK/HOLD OUTPUT
SIGNAL GROUND
TRACK/HOLD INPUT
DNC = DO NOT CONNECT.
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 AD1385 features proprietary ESD protection circuitry, permanent damage may
occur on devices subjected to high energy electrostatic discharges. Therefore, proper ESD
precautions are recommended to avoid performance degradation or loss of functionality.
–4–
WARNING!
ESD SENSITIVE DEVICE
REV. 0
AD1385
Figure 1. Spurious-Free Range vs. Input Amplitude,
± 5 V Range, 2048-Point FFT, 500 kHz Sample Rate
Figure 4. Full-Scale Sine Wave Power Spectral Density,
± 5 V Range, 16384-Point FFT, 500 kHz Sample Rate
Figure 2. Spurious-Free Range vs. Input Amplitude,
± 10 V Range, 2048-Point FFT, 500 kHz Sample Rate
Figure 5. Full-Scale Sine Wave Power Spectral Density,
± 5 V Range, 16384-Point FFT, 500 kHz Sample Rate
Figure 6. 100 kHz Intermodulation Performance, ± 5 V
Range, 16384-Point FFT, 500 kHz Sample Rate
Figure 3. Full-Scale Sine Wave Power Spectral Density,
± 5 V Range, 16384-Point FFT, 500 kHz Sample Rate
REV. 0
–5–
AD1385
Figure 7. 200 kHz Intermodulation Performance, ± 5 V
Range, 16384-Point FFT, 500 kHz Sample Rate
Figure 10. Full-Scale Sine Wave Power Spectral Density,
± 10 V Range, 16384-Point FFT, 500 kHz Sample Rate
Figure 8. Full-Scale Sine Wave Power Spectral Density,
± 10 V Range, 16384-Point FFT, 500 kHz Sample Rate
Figure 11. 100 kHz Intermodulation Performance, ± 10 V
Range, 16384-Point FFT, 500 kHz Sample Rate
Figure 12. 200 kHz Intermodulation Performance, ± 10 V
Range, 16384-Point FFT, 500 kHz Sample Rate
Figure 9. Full-Scale Sine Wave Power Spectral Density,
± 10 V Range, 16384-Point FFT, 500 kHz Sample Rate
–6–
REV. 0
AD1385
Figure 13. AD1385 Functional Block Diagram
S4 routes this error signal to the flash ADC, which is strobed a
second time after Error Amp 1 has settled. The new 8-bit result
is used to correct the previous result, increasing the accuracy of
this intermediate answer to 13-bit precision. Following this the
Reference DAC is updated.
THEORY OF OPERATION
The AD1385 performs conversions using a three-pass subranging technique. This proven circuit concept, implemented with
state of the art components, allows the ADC, track-hold, and a
low noise reference to fit into a single hermetic package, simplifying the task of board design. The T/H and ADC portions of
the AD1385 are distinct circuits with inputs and outputs available on separate pins. This functional division allows greatest
application flexibility. The AD1385’s major functional blocks
are shown in Figure 13.
The T/H uses a low noise high performance hybrid amplifier and
high speed analog switches to achieve precision performance. It
operates as an inverting amplifier during Track mode. Summing
junction switch S1 disconnects the analog input to place the circuit into Hold mode; the amplifier’s output stays constant because
the dc path to its inverting input is broken. S1 also grounds the
junction of R1 and R2 to minimize signal feedthrough. Pedestal
is independent of the analog input level because all switching
is done near ground. This ensures very low nonlinearity and
distortion.
A precision Reference DAC and an 8-bit flash ADC form the
heart of the AD1385’s subranging design. High speed amplifiers
combine the analog input and DAC output to produce the voltages encoded by the flash ADC during each pass. A logic array
provides all necessary timing, control, and computation.
The output data are placed on the data bus in two 8-bit bytes
to be read by the host system. The Data Strobe output synchronizes the data transfer by providing a rising edge for the first
byte and a falling edge for the second byte. The Hi/Lo Byte
Select input allows the user to choose which data byte is presented first. B1 Select sets the polarity of the MSB to provide
either complementary twos complement or complementary offset binary data.
The AD1385’s internal linearity calibration capability may be
used to compensate for shifts in Reference DAC linearity with
time and temperature. The calibration sequence uses the
AD1385’s error amplifiers and flash converter to directly measure Reference DAC linearity errors. The routine calculates the
Corrections required to each of the Reference DAC’s 8 MSBs
and stores these in an internal memory; the memory address is
determined by the Reference DAC’s codes. The RAM data control a Correction DAC whose output is summed with the Reference DAC’s output. Together the two DACs provide the 18-bit
linearity required for accurate A/D conversions. Calibration
corrects only linearity errors, and has a negligible effect on
gain and offset errors. A calibration cycle requires 15 ms and
may be initiated at any time (see Autozero).
The first rising clock edge after Start Convert goes high begins
the conversion (provided the previous conversion is complete).
The Hold Command goes high and switches the T/H into hold.
The held signal from the T/H goes through S2, S3, and Error
Amp 2 to the flash ADC. During this pass Error Amp 2 actually
attenuates the ADC input to keep the voltage within the flash
ADC’s input range. The flash ADC is strobed after a 100 ns
settling period. The 8-bit result is saved in the logic array and is
routed to the MSBs of the Reference DAC.
Error Amp 1 amplifies the difference between the Reference
DAC output and the held input signal during the second pass.
REV. 0
Both error amplifiers are active during the third pass. S2 is
closed, allowing Error Amp 2 to amplify Error Amp 1’s output.
S3 now brings Error Amp 2’s output to the flash ADC. The
flash ADC is strobed a final time after the DAC and both error
amplifiers have settled. The logic array combines the data from
the third flash conversion with the earlier 13-bit word to produce the final 16-bit result. The T/H is returned to track mode,
and Error Amp 2 is reconnected as an attenuator 50 ns after the
completion of the third flash conversion to prepare for the next
conversion.
–7–
AD1385
CONNECTION AND OPERATION OF THE AD1385
Analog Input
The analog input should be connected to the Track/Hold Input
(Pin 25). Two pin programmable operating ranges are available:
± 5 V and ± 10 V. Connect the Track/Hold Output to VIN A and/
or VIN B as follows:
Desired Scale
Connect VIN A to
Connect VIN B to
±5 V
± 10 V
Track/Hold Output
Track/Hold Output
Track/Hold Output
Analog Signal GND
Harmonic distortion is lower when using the ± 5 V range, while
noise is lower when using the ± 10 V range.
The AD1385’s noise and distortion performance exceed the
capability of most signal sources. Maintaining this performance
at the system level requires attention to every detail of grounding, bypassing, and signal sources. A low impedance high bandwidth signal source is essential to achieve low distortion. Few
monolithic amplifiers exist which can maintain signal fidelity at
levels comparable with the AD1385’s performance, even at low
frequencies. High bandwidth means increased noise and decreased SNR. See Testing the AD1385 for techniques of achieving the lowest possible noise and distortion.
Grounding
Proper treatment of the AD1385’s power and ground connections is vital to achieve the best possible system performance.
The ideal grounding arrangement is to have a single, solid, low
impedance ground plane beneath the device to which all ground
and supply bypassing connections are made. This results in the
lowest possible ground noise and minimizes undesired interactions between the sensitive circuits inside the AD1385. Aperture
uncertainty, for example, can be degraded by noise in Power
Ground because the Hold Command signals are referenced to
this ground. The digital interface between the AD1385 and the
rest of the user’s system is also critical. The following discussion
will help in obtaining optimal performance. These guidelines are
general and apply equally well to other high performance analog
and digital circuits.
The AD1385 must connect to three other parts of the system:
the input signal(s), the power supplies, and the digital interface.
The system designer must determine the magnitude and type of
ground currents and whether they are constant or dynamic. A
system block diagram is a valuable aid to understanding how
grounds should be connected for good performance. Figure 14
shows recommended ground connections for the AD1385 in a
typical system.
The AD1385 has a net ground current of about 40 mA. Most of
this flows in the power grounds. There are also substantial dynamic currents in the power grounds. The signal grounds have
primarily low level static (dc) currents. Signal and power
grounds are separated inside the hybrid because the resistance
and inductance inherent in thick-film construction would cause
interactions between ground currents, leading to poor performance. (Remember that an LSB can be as small as 156 µV.)
Care must be taken to prevent the AD1385’s ground currents
from flowing in the signal ground between the signal source and
the AD1385 if this ground has significant resistance. This is not
usually a problem if the signal source is located on the same
board as the AD1385 because the resistance can be made very
low through the use of a ground plane.
The signal source’s ground and supply currents must be considered when the source and ADC share common power supplies.
A ground loop formed by the AD1385, the signal source, and
the power supplies can cause significant errors.
The connection between the AD1385’s ground plane and the
system’s digital ground is best made away from the AD1385.
This will prevent noisy system ground currents from passing
through critical parts of the ADC. In a very noisy environment it
may be wise to isolate the entire analog circuit. Figure 14 shows
the required isolation provided by a digital buffer. The buffer
can then drive resistive and/or capacitive loads without compromising ground at the ADC. Using separate isolated supplies for
the ADC and signal source will result in a single-point connection between system digital ground and the ADC’s ground plane
at the digital buffer.
Power Supplies and Bypassing
The AD1385 has four sets of power supply pins. These are:
± 5 V Analog
± 15 V
± 15 V
± 5 V Power
(VDD1/VSS1)
(+VS1/–VS1)
(+VS2/–VS2)
(VDD2/VSS2)
A single source may be used to supply like voltages (e.g., VDD1,
VDD2 from the same +5 V supply). Each of the four ± 5 V supply
pins should have a distinct low impedance connection to a
well-bypassed central source node. This is required because
each pin draws large transient currents. These dynamic currents, if passed through a common supply path, would introduce crosstalk and increase the AD1385’s apparent noise. The
two sets of ± 15 V supplies need not be split in this fashion.
Every AD1385 supply pin should be bypassed to the ground
plane with a high quality ceramic capacitor of 0.01 µF to 0.1 µF.
This capacitor should be located as close as possible to the
AD1385 to minimize lead lengths. Each VDD and VSS pin must
also be bypassed to the ground plane with a 10 µF solid tantalum bypass capacitor located close to the AD1385. Ten microfarad bypass capacitors for ± VS2 (Pins 21 and 23) are also
necessary. These power distribution concepts are shown in
Figure 15.
All power supplies should be of the linear type. Switching power
supplies are not recommended as they can introduce considerable high frequency noise into sensitive analog signal paths, degrading the AD1385’s apparent performance.
Figure 14. AD1385 Grounding
Supply pins of equivalent voltage should not be allowed to differ
by more than 0.3 V.
–8–
REV. 0
AD1385
DIGITAL INTERFACES
10 MHz Clock
The AD1385 requires a stable external clock. A 10 MHz clock
provides a sample rate of 500 kilosamples per second. Since
the ADC operates synchronously with this clock, clock phase
noise will appear as jitter in the aperture time. Lower clock
frequencies may be used, and the sample rate will be reduced
proportionately.
Standard TTL and CMOS crystal oscillator modules may be
used successfully to generate the required 10 MHz clock signal.
These oscillators often create considerable power supply transient noise. The oscillator should be bypassed with both ceramic and solid tantalum capacitors using minimum lead
lengths. A 10 Ω resistor in series with the +5 V supply provides
additional isolation and low-pass filtering of transients produced by the oscillator. See Figure 16.
Figure 15. Recommended AD1385 Power Distribution. All
10 µ F and 0.01 µ F capacitors must have minimum lead
length and be located as close as possible to the bypassed pins. Make all ground connections directly to the
groundplane.
Figure 16. Isolating Clock Noise. Bypass Capacitors
Should Be Located Close to the Oscillator
Transmission line effects cannot be ignored when supplying the
AD1385’s 10 MHz clock. The large impedance mismatch between typical PCB traces and the AD1385’s CMOS clock input
can give rise to reflections and high frequency transients when
the 10 MHz clock source is located more than a few inches
from the AD1385. This noise can corrupt local ground and
cause degradation in the AD1385’s apparent SNR performance. A series termination resistor of 50 Ω to 100 Ω, located
at the clock source, will usually eliminate this problem.
If separate ground planes are used for Signal and Power
Ground, the supplies should be bypassed as follows:
Supply
Bypass to
± 5 V Analog
± 15 V (+VS1/–VS1)
± 15 V (+VS2/–VS2)
± 5 V Power
Signal Ground
Signal Ground
Power Ground
Power Ground
Care is also required when using a +5 V powered crystal oscillator to provide the AD1385’s clock signal. These devices produce
considerable supply noise and proper bypassing is essential.
The oscillator should be bypassed with both ceramic and solid
tantalum capacitors using minimum lead lengths. A 10 Ω resistor in series with the +5 V supply provides additional isolation
and low pass filtering of transients produced by the oscillator.
START CONVERT (PIN 18)
Synchronous Operation
The Start Convert signal acts like the data input of a flip-flop. A
conversion begins on the first rising clock edge after Start Convert goes high (provided setup time requirements are met). This
edge drives Hold Command Out high, switching the T/H into
Hold mode. Hold Command Out (Pin 19) should be connected
to Hold Command In (Pin 22) for synchronous operation.
Continuous conversions at a 500 kHz rate may be obtained by
holding Start Convert high. The 10 MHz clock may be divided
down and used to drive the Start Convert input when a lower
conversion rate is desired. This will provide clock-synchronized
conversions at the lower rate. Synchronous conversion timing is
shown in Figures 17 and 18.
Reference
The AD1385 has an excellent internal reference with a typical
temperature coefficient of 5 ppm/°C. The Reference Out (Pin
39) is normally connected to Reference In (Pin 32). An external reference may be connected to the reference input if desired.
The reference input pin requires negligible current. The reference input voltage should not exceed +11 V and must remain
more positive than 0 V. The reference output requires no bypassing and should not be capacitively loaded. If an external
reference is used, it must have low noise to avoid degrading the
signal to noise ratio of the AD1385.
Start Convert may also be used as a gate to capture data in a
time window. The rising and falling edges of Start Convert
define the beginning and end of the window during which
conversions are desired.
The reference output can source up to 2 mA of static (dc) current without affecting the performance of the AD1385. By using
the AD1385’s internal reference as the system reference, gain
error over temperature can be minimized.
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Some restrictions apply when using a pulse to drive the Start
Convert input. Start Convert is ignored during a conversion for
seven clock periods after Hold Command Out goes low to signal the end of a conversion. The state of Start Convert is sampled
on each rising clock edge, beginning with the seventh edge after
Hold Command Out goes low, until a logical high is detected.
–9–
AD1385
Figure 17. Start-Convert Controlled Conversion Timing
Figure 18. Free Running Conversion Timing
At this point a new conversion will be initiated. The minimum
setup and hold times for Start Convert relative to the rising
clock edge are 10 ns. Start Convert transitions should not be
placed in the window which begins 100 ns (one clock period)
after the rising edge of Hold Command Out and which ends
1300 ns (thirteen clock periods) after this rising edge (see Figure 17). This minimizes internal coupling between Start Convert and sensitive internal circuit nodes.
rising edge of Start Convert places the T/H into Hold mode; the
A/D conversion cycle begins with the first rising clock edge after
the Start Convert transition, and Start Convert must remain
high during at least one rising clock edge in order to begin the
conversion. The width of Start Convert should be either less
than 150 ns or greater than 1400 ns to minimize coupling between the falling edge of Start Convert and sensitive internal
nodes. In asynchronous operation the T/H will remain in Hold
Transmission line effects at the Start Convert input should be
considered when designing circuit boards for the AD1385. A series termination resistor of 50 Ω to 100 Ω is recommended when
the source of Start Convert is more than a few inches away from
the AD1385. This will control reflections and transients which
could otherwise degrade the part’s performance.
Asynchronous Operation
In synchronous operation the T/H is placed into Hold mode by
the first rising clock edge after Start Convert goes high. This
mode of operation provides maximum rejection of system clock
noise. Some applications may require the AD1385 to operate
asynchronously, that is, with the Start Convert input directly
controlling the track-to-hold transition. This may be achieved
using a 2-input OR gate connected as shown in Figure 19. The
Figure 19. Connecting the AD1385 to Sample the Input
Signal Asynchronously from the Clock
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AD1385
mode as long either Hold Command Out or Start Convert is
high. Care is needed in defining system timing to ensure that
the T/H has a minimum of 700 ns for signal acquisition before
another conversion begins. The minimum width of Start Convert is 20 ns, the sum of tSCS and tSCH, the minimum setup and
hold times.
Transmission line effects at the Start Convert and Hold Command In inputs should be considered when designing circuit
boards for the AD1385. A series termination resistor of 50 Ω to
100 Ω is recommended when the source of either of these signals is more than a few inches away from the AD1385. This will
control reflections and transients which could otherwise degrade
the part’s performance.
Output Data
The output data are multiplexed in two bytes onto an 8-bit data
bus. Data are guaranteed to be stable at the time of the edges of
Data Strobe (Pin 15). Hi/Lo Byte Select (Pin 16) controls
which byte is presented first. If Hi/Lo Byte Select is high, then
BYTE0 is B9–B16 and BYTE1 is B1–B8. The order of the data
bytes is interchanged when Hi/Lo Byte Select is low. BYTE 0
and BYTE 1 are defined in the timing diagram Figure 17. B1 is
the most significant bit of the reconstructed 16-bit data.
Figure 20. Full-Scale Power Spectral Density after Powerup at TCASE = +25°C Without Calibration, ± 5 V Range,
16384-Point FFT, 500 kHz Sample Rate. Compare with
Figure 4.
B1 SELECT (Pin 44) determines whether data is presented in
complementary twos complement or complementary offset binary form. Complementary twos complement data is provided
when B1 Select is LOW. OE may be used to place the data bus
into a high impedance state.
The arithmetic unit in the AD1385 saturates at all 0s or all 1s if
the input range is exceeded.
Table I.
B1 Select
0
1
Data Format
–Full-Scale Data
0 V Data
+Full-Scale Data
Complementary Twos
Complement
7FFFH
FFFFH
8000H
Complementary Offset
Binary
FFFFH
8000H
0000H
Figure 21. Full-Scale Power Spectral Density at TCASE =
+125°C, Calibration Performed at TCASE = +25°C, ± 5 V
Range, 16384-Point FFT, 500 kHz Sample Rate
CALIBRATION (Pins 28 and 41)
Calibration corrects for linearity errors in the Reference DAC
arising from internal component mismatches or temperature
changes. It has a negligible effect on gain and offset errors, and
these should be corrected by other means. The AD1385 must
be calibrated after power-up, and recalibration is recommended
whenever the part’s temperature has changed by more than
15°C. Performance degrades gracefully with temperature
changes, resulting in small but gradual decreases in SNR and
increases in distortion which may be eliminated by recalibration. Calibration codes are stored in internal RAM and are lost
when power is removed. Figures 20–22 show the effects of
uncalibrated versus calibrated operation.
Figure 22. Same as Figure 21 Following Recalibration at
TCASE = +125°C
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–11–
AD1385
No external switches or relays are required for calibration and
all connections to the AD1385 may remain in place. The
Track/Hold is internally isolated from the analog input by analog switches and used as a buffer during the calibration process.
Its signal output (Pin 27) must remain connected to the A/D
input(s) (Pin 30 and/or Pin 31, as appropriate) for successful
calibration. Hold Command Out (Pin 19) must also remain
connected to Hold Command In (Pin 22), either via direct connection (synchronous sampling) or with an external OR-gate
(asynchronous sampling, Figure 19).
A calibration sequence may be initiated at any time by bringing
the CAL input (Pin 41) low. The calibration request remains
pending if a conversion is in process, and calibration begins on
the first rising clock edge after the end of that conversion. Calibration begins on the first rising clock edge after CAL is asserted if the AD1385 is idle when calibration is requested. The
minimum pulse width for the CAL input is 20 ns. The CAL input has priority over the Start Convert signal in all cases.
The CAL Status output (Pin 28) goes high as soon as calibration begins and remains high until the calibration cycle is completed. Pulsing CAL low while CAL Status is high has no
effect. A full calibration requires about 15 ms with a 10 MHz
clock and proportionately longer with slower clocks.
Calibration has no effect on the contents of the Autozero register. The apparent zero point may shift a few LSBs as a result of
the calibration. Autozero after recalibration will provide the
greatest possible accuracy (see Autozero).
The AD1385 controller allocates 17 clock periods after the conclusion of a calibration cycle for Track/Hold recovery and signal
acquisition. Activity at the Start Convert input during this
interval is ignored. Figure 23 shows the timing associated with
the resumption of synchronous conversions following a calibration cycle.
Start-Convert should remain low during the calibration period
when using asynchronous sampling (Figure 19).
The CAL input may be held low indefinitely, causing repeated
calibration cycles. The AD1385 will complete the calibration in
progress when CAL goes high and will then begin normal conversions after the 17-clock-period delay. This simplifies the
system-level implementation of the power-up reset function.
The AD1385 requires a 5 minute warmup to reach thermal
equilibrium after power is applied, and calibration may drift
slightly during this time. Occasional recalibration will provide a
slight improvement in distortion and noise performance during
warmup.
AUTOZERO (Pin 45)
The Autozero function may be used to digitally correct internal
offsets in the Track/Hold and ADC as well as external offsets.
To use Autozero the Track/Hold input must be connected to a
zero reference prior to the zeroing conversion. This connection
is external to the AD1385 and must be provided by the user;
the resistance of this connection is not critical but should be less
than 1000 Ω. An Autozero cycle forces the AD1385’s digital
output to indicate exactly zero when its input is at the zero
point, nominally 0 V. (This assumes that the complementary
twos complement data format is used. Autozero forces the digital output to midscale when the selected data format is complementary offset binary.) Autozero operates by storing the digital
result of a zeroing conversion and subtracting it from all subsequent conversion results. This reduces the maximum nonsaturating input of the AD1385 a small amount at one end of its
range depending on the magnitude and polarity of the offset.
The Autozero feature is enabled by driving the Autozero input
(Pin 45) low before a falling edge at the Data Strobe output.
Offset data will be stored on the first rising edge of Data Strobe
after Autozero is brought high; the offset data are also available
on the AD1385’s data bus during this Data Strobe pulse. Autozero operation is illustrated in Figure 24. All subsequent A/D
conversions will be digitally corrected by the offset term as long
as Autozero remains high. The offset register is cleared when
Autozero goes low and the contents of the data output registers
will revert to their uncorrected value. Figure 24 shows Autozero
timing requirements. Autozero cannot be activated until the
first conversion after power-up has been completed.
The Autozero feature may be disabled by keeping Autozero low.
Figure 23. Resumption of Synchronous Conversions
Following Completion of Calibration
Figure 24. Autozero Cycle Operation
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AD1385
GAIN ADJUST (Pin 40)
The internal reference of the AD1385 may be adjusted by varying the voltage applied to the Gain Adjust pin. The input impedance of this pin is nominally 20 kΩ, with a tolerance of
± 20%. A change of 1 V on Pin 40 will change the reference voltage by about 10 mV. The reference may be adjusted by ±150 mV
without degrading the AD1385’s performance. The simplest
method of implementing the gain adjust is to connect a potentiometer between the ± 15 V supplies, with the wiper connected to
the Gain Adjust pin. Care should be taken to ensure that noise
does not enter the ADC through the Gain Adjust pin.
Figure 25a. AD1385 Gain Adjust Circuit
OFFSET ADJUST (Pin 29)
The ADC’s offset voltage may be adjusted by means of a voltage
applied to the Offset Adjust pin. The nominal adjustment sensitivity is 0.005% FSR/V. The input impedance is 20 kΩ with a
± 20% tolerance. The simplest way to implement the offset adjust is to connect a potentiometer between the ± 15 V supplies,
with the wiper connected to the Offset Adjust pin. Care should
be taken to ensure that noise does not enter the ADC through
the Offset Adjust pin.
Figure 25b. AD1385 Offset Adjust Circuit
APPLICATIONS
Mounting and Thermal Considerations
The AD1385’s operation is specified over a case temperature
range of –55°C to +125°C. Case temperature in still air is normally about 20°C above ambient, and a heat sink and/or air flow
is required to guarantee specified performance when high ambient temperatures are expected. A thin heat transfer plate,
mounted beneath the package to conduct heat into the ground
plane, may be sufficient. This plate may be made of metal provided care is taken to prevent shorting the package pins. An
excellent alternative is to use an elastomeric heat conducting
material. These materials will conform to the board and to the
AD1385 package to improve heat transfer while reducing mechanical stress. Elastomeric materials normally will not require
thermally conductive grease.
Testing the AD1385
It is difficult to test the AD1385 with ordinary test methods because of the part’s very low distortion and noise. The number of
output codes and the nature of the analog to digital conversion
make static tests of performance especially cumbersome. Subranging converters with error correction circuitry can have flaws
at any place in their transfer function and all codes must be exercised for a complete test.
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Histograms provide a convenient way to measure all codes in a
modest amount of time. Even histograms can be slow, when 20
million conversions (40 seconds) may be required to achieve
statistically valid results.
Dynamic tests based on FFTs are the most powerful. They
quantify noise and distortion as a function of input frequency.
From them one can infer qualitative integral and differential
nonlinearity performance while determining the ADC’s specific
dynamic performance. FFTs are especially useful for systems
which require excellent dynamic response, such as magnetic
resonance imaging. They also uncover performance problems
that don’t show up in static tests of linearity.
The difficulty in doing FFT tests stems from the requirement
for ultra pure sine wave inputs at various frequencies over the
operating bandwidth of the ADC. Even the best available generators are not capable of supplying signals with sufficiently low
noise and low distortion for testing the AD1385. Few generators
permit phase-locking to the ADC clock. (Phase-locking makes it
possible to obtain an integral number of cycles of the input sine
wave within the FFT data window, which in turn eliminates the
need for windowing functions and the spectral spreading they
cause.)
The best generator currently available for this purpose is the
Brüel and Kjaer Model 1051 (or 1049). This generator provides
a programmable output frequency up to 250 kHz with better
than 0.001 Hz resolution. The generator’s distortion performance at frequencies below 20 kHz is better than the AD1385
but degrades at 100 kHz and higher. Noise is a problem at all
frequencies, being about –85 dB over the AD1385’s bandwidth.
Both noise and distortion can be reduced to acceptable levels
with filters. Passive narrow bandwidth filters will reduce harmonic distortion to less than –100 dB. Inductors wound on
large pot cores with air gaps can be made quite linear, and with
careful winding will provide low loss and low capacitance. Such
filters will reduce noise to negligible levels outside their pass
band to provide a much better view of actual ADC performance.
The effect of aperture jitter, for example, cannot be observed
without a filter.
The FFTs shown in Figures 3-12 were produced using these
methods. These tests are done as a normal part of production
testing to guarantee the dynamic performance of the AD1385.
Multiplexing and High Impedance Inputs
Multiplexing the AD1385’s input presents several challenges in
component selection. The ON-resistance of most available multiplexers and switches is a function of the applied voltage. This,
coupled with the AD1385’s 2.5 kΩ input resistance, can introduce significant harmonic distortion unless the multiplexer output is buffered. All monolithic switches and multiplexers exhibit
this behavior to some extent, with CMOS-based designs generally worse than those using JFET technology.
An acceptable alternative is the DG180 family produced by
Siliconix. These hybrid switches use discrete JFET pass devices
to provide an extremely low ON-resistance virtually independent of signal level. Care should be taken to match the switch’s
common-mode signal capability with operating range desired for
the AD1385. The finite on-resistance of any unbuffered switch
driving the AD1385 will introduce a gain error, and that error
may change appreciably over temperature.
–13–
AD1385
Buffering the multiplexer’s output will eliminate the problems
caused by its ON-resistance. The choice of buffer depends on
the nature of the system’s input signals. There are two cases to
consider: static inputs and dynamic inputs.
filter tuned to the fundamental frequency greatly improves measurement resolution. It is also possible to use the AD1385 as
the measuring device by performing FFTs on the output data.
Refer to the discussion of signal sources in Testing the AD1385.
“Static” Applications
Amplifier noise, CMRR linearity, and settling time are of primary importance when the inputs are low frequency or DC.
This is the case in a CAT-scan imager, for example, when signals are produced by integrating photocurrents. Noise limits
ultimate system resolution. The AD1385 has a typical inputreferred noise of 70 µV rms. Buffer noise must be added to this
in a root-sum-squares fashion to determine total system noise. A
buffer amplifier which adds noise of 18 µV rms, for example,
will result in a system noise level of (182 +702)1/2 = 72 µV rms, a
negligible increase. Detailed system noise calculations require
knowledge of the buffer’s noise spectral density and equivalent noise bandwidth. The AD1385’s equivalent noise bandwidth is 2.2 MHz. Low Noise Electronic Design (C.D.
Motchenbacher and F.C. Fitchen, John Wiley and Sons, New
York, 1973) provides excellent discussions of noise analysis and
calculations.
Unipolar Operation
The AD1385 does not provide a direct unipolar input capability. Unipolar inputs can be achieved using the circuits of Figures 26 and 27. The circuit in Figure 26 is suitable when a low
input impedance is acceptable. The AD845 is an excellent amplifier choice for this application. Multiplexed applications
should use the circuit of Figure 27. The discussions under High
Impedance Inputs also apply to amplifier selection for high
impedance unipolar operation.
Buffer amplifier CMRR produces only gain error as long as the
value of CMRR is independent of signal level. The size of this
“gain error” is directly related to the actual value of CMRR; an
amplifier with 60 dB CMRR will create an apparent gain error
of 0.1%. The precise value of CMRR is not critical as long as it
remains independent of signal level. Any variation in CMRR
with input level will introduce nonlinearity. The smaller the
value of CMRR (in dB), the more critical variations in this value
become. An amplifier with CMRR ranging from 100 dB to
110 dB over the range of –10 V to +10 V will produce negligible
nonlinearity, while an amplifier whose CMRR varies from 60 dB
to 70 dB over the same range would be completely unacceptable.
Buffer settling time will affect the system’s throughput. The system sample rate can be maintained at 500 kHz provided the
buffer’s settling time is less than about 1.7 microseconds. The
input channel should be switched just after the AD1385’s SHA
enters Hold mode as indicated by a rising edge at Hold Command In (Pin 22).
“Dynamic” Applications
Dynamic applications complicate the choice of buffer amplifier.
The amplifier’s harmonic distortion performance now becomes
as important as its noise, CMRR linearity, and settling behavior.
Few manufacturers specify amplifier THD in the noninverting
configuration. These specifications, when available, seldom address signals greater than 10 V p-p or frequencies above 1 kHz.
It may be necessary to characterize candidate amplifiers from
several vendors to find the best fit to the amplitude and frequency requirements of a particular application. Such evaluations are easily performed using a spectrum analyzer. A notch
Figure 26. Unipolar-to-Bipolar Conversion (Low Input
Impedance)
Figure 27. High Input Impedance Unipolar-to-Bipolar
Conversion Circuit
Data Bus Interface
The AD1385’s data outputs are 4 mA CMOS drivers and are
not intended to be connected directly to a system data bus.
Charging and discharging a capacitive data bus creates large
supply transients and ground spikes which can interfere with
the AD1385’s operation and result in erroneous data. Registers
and/or buffers should be used to isolate the AD1385 from the
bus. Buffering devices should be located close to the AD1385 to
minimize the capacitive load presented to the converter’s data
outputs. Control will be simplified by permanently grounding
the AD1385’s OE input when using buffers. A schematic of a
typical 16-bit bus interface is shown in Figure 28.
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AD1385
Figure 28. Basic AD1385 Digital Interface (16-Bit Complementary 2s Complement Data,
Autozero Not Used, Other Digital and Analog Inputs Not Shown)
Sample Board Layout
Figures 29-34 show the layout of an evaluation board for the
AD1385. This layout incorporates the grounding, power distribution, and interface concepts described in previous sections.
This 4-layer layout makes extensive use of ground and power
planes and provides optimal AD1385 performance.
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The layout accommodates buffer amplifiers with standard op
amp pinouts in both 14- and 8-pin DIP packages. The pin
numbers shown for U12 in Figure 29 refer to the 14-pin format.
An 8-pin op amp such as the AD845 should be positioned with
package Pin 4 inserted in layout Pin 6. The AD845 provides
slightly better distortion performance than the AD842, an amplifier in a 14-pin package, with no significant increase in noise.
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AD1385
Figure 29. AD1385 Evaluation Board Schematic
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AD1385
Figure 30. AD1385 Evaluation Board Layout, Layer 1 (Component Side)
Figure 31. AD1385 Evaluation Board Layout, Layer 2 (± 15 V Planes)
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–17–
AD1385
Figure 32. AD1385 Evaluation Board Layout, Layer 3 (± 5 V Planes)
Figure 33. AD1385 Evaluation Board Layout, Layer 4 (Solder Side)
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AD1385
Figure 34. AD1385 Evaluation Board Silkscreen
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–19–
AD1385
OUTLINE DIMENSIONS
AD1385 EVALUATION BOARD PARTS LIST
1
C1
Ceramic Capacitor, 10 pF, 50 V
(Mallory CEC100J)
13
C2, C3, C6, C7, C11, Tantalum Capacitor, 10 µF, 35 V
C12, C21, C22, C26, (Mallory TDL106K035S1D)
C27, C29–C31
16
C4, C5, C8–C10,
C13–C20, C23–C25
Ceramic Cap, 0.1 µF, 100 V(Murata
Erie RPE122Z5U104M100V)
2
C28, C32
Tantalum Capacitor, 39 µF, 10 V
(Kemet T110B396K010AS)
4
CR1–CR4
1N4001 Diode
3
J7, J13, J14
BNC Female, PC Mount
(Pomona 4578)
8
JMP2–JMP6, JMP8,
JMP9, JMP13
Jumper, 2 Position (3M 929950-00)
3
R1, R5, R6
RN55C Resistor, 2.00k
2
R2, R3
50k 20-Turn Trimpot*
(Bourns 3299W-1-503)
2
R4, R8, R9
RN55C Resistor, 10.0k
1
R7
Carbon Composition Resistor,
100 Ω, 1/2 W
1
R10
RN55C Resistor, 10 Ω
1
SW1
Momentary SPST, C & K
KS l l-R2-C-Q
1
U7
74ALS74
1
U9
AD1385KD (Analog Devices)
2
U10, U11
74ALS574
1
U 12
AD845KN (Analog Devices)
1
U13
10 MHz DIP Crystal Oscillator
1
U14
74ALS04
1
U15
74ALS32
2
—
Socket Strip (SPC MPS1P-32-GG)
1
—
Pin Strip (3M 929647-01-36)
1
—
Socket, 14-Pin Oscillator
(Augat 504-AG10D)
4
—
Socket, 14-Pin (Augat 514-AG11D)
2
—
Socket, 20-Pin (Augat 520-AG11D)
2
—
Ejector Latch (3M 3505-3)
1
—
50-Pin Connector (3M 3433-5002)
2
—
Screw, 2-56 × 1/2
2
—
Hex Nut, 2-56
48-Pin Bottom Brazed Ceramic DIP (DH-48A)
C1579–10–4/92
Description (Manufacturer/PN)
PRINTED IN U.S.A.
Qty. Ref. Des.
Dimensions shown in inches and (mm).
*Trimpot is a trademark of Bourns.
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