ADC08100
8-Bit, 20 MSPS to 100 MSPS, 1.3 mW/MSPS A/D
Converter
General Description
The ADC08100 is a low-power, 8-bit, monolithic analog-todigital converter with an on-chip track-and-hold circuit. Optimized for low cost, low power, small size and ease of use,
this product operates at conversion rates of 20 MSPS to
100 MSPS with outstanding dynamic performance over its
full operating range while consuming just 1.3 mW per MHz of
clock frequency. That’s just 130 mW of power at 100 MSPS.
Raising the PD pin puts the ADC08100 into a Power Down
mode where it consumes just 1 mW.
The unique architecture achieves 7.4 Effective Bits with
41 MHz input frequency. The excellent DC and AC characteristics of this device, together with its low power consumption and single +3V supply operation, make it ideally suited
for many imaging and communications applications, including use in portable equipment. Furthermore, the ADC08100
is resistant to latch-up and the outputs are short-circuit proof.
The top and bottom of the ADC08100’s reference ladder are
available for connections, enabling a wide range of input
possibilities. The digital outputs are TTL/CMOS compatible
with a separate output power supply pin to support interfacing with 3V or 2.5V logic. The digital inputs (CLK and PD) are
TTL/CMOS compatible.
The ADC08100 is offered in a 24-lead plastic package
(TSSOP) and is specified over the industrial temperature
range of −40˚C to +85˚C.
Features
n Single-ended input
n Internal sample-and-hold function
n Low voltage (single +3V) operation
n Small package
n Power-down feature
Key Specifications
n Resolution 8 bits
n Maximum sampling frequency 100 MSPS (min)
n DNL 0.4 LSB (typ)
n ENOB 7.4 bits (typ) at f
IN
=41MHz
n THD −60 dB (typ)
n Guaranteed no missing codes
n Power consumption
— Operating: 1.3 mW/MSPS (typ)
— Power down: 1 mW (typ)
Applications
n Flat panel displays
n Projection systems
n Set-top boxes
n Battery-powered instruments
n Communications
n Medical scan converters
n X-ray imaging
n High speed viterbi decoders
n Astronomy
Pin Configuration
10137101
December 2002
ADC08100 8-Bit, 100 MSPS, 1.3 mW/MSPS A/D Converter
© 2002 National Semiconductor Corporation DS101371 www.national.com
Ordering Information
ADC08100CIMT TSSOP
ADC08100CIMTX TSSOP (tape and reel)
Block Diagram
10137102
Pin Descriptions and Equivalent Circuits
Pin No. Symbol Equivalent Circuit Description
6V
IN
Analog signal input. Conversion range is VRBto VRT.
3V
RT
Analog Input that is the high (top) side of the reference
ladder of the ADC. Nominal range is 1.0V to V
A
. Voltage
on V
RT
and VRBinputs define the VINconversion range.
Bypass well. See Section 2.0 for more information.
9V
RM
Mid-point of the reference ladder. This pin should be
bypassed to a clean, quiet point in the analog ground
plane with a 0.1 µF capacitor.
10 V
RB
Analog Input that is the low side (bottom) of the
reference ladder of the ADC. Nominal range is 0.0V to
(V
RT
– 1.0V). Voltage on VRTand VRBinputs define the
V
IN
conversion range. Bypass well. See Section 2.0 for
more information.
ADC08100
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Pin Descriptions and Equivalent Circuits (Continued)
Pin No. Symbol Equivalent Circuit Description
23 PD
Power Down input. When this pin is high, the converter is
in the Power Down mode and the data output pins hold
the last conversion result.
24 CLK
CMOS/TTL compatible digital clock Input. V
IN
is sampled
on the falling edge of CLK input.
13 thru 16
and
19 thru 22
D0–D7
Conversion data digital Output pins. D0 is the LSB, D7 is
the MSB. Valid data is output just after the rising edge of
the CLK input.
7V
IN
GND Reference ground for the single-ended analog input, VIN.
1, 4, 12 V
A
Positive analog supply pin. Connect to a clean, quiet
voltage source of +3V. V
A
should be bypassed with a
0.1 µF ceramic chip capacitor for each pin, plus one
10 µF capacitor. See Section 3.0 for more information.
18 DR V
D
Power supply for the output drivers. If connected to VA,
decouple well from V
A
.
17 DR GND The ground return for the output driver supply.
2, 5, 8, 11 AGND The ground return for the analog supply.
ADC08100
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Absolute Maximum Ratings (Notes 1,
2)
If Military/Aerospace specified devices are required,
please contact the National Semiconductor Sales Office/
Distributors for availability and specifications.
Supply Voltage (V
A
) 3.8V
Driver Supply Voltage (DR V
D
)V
A
+ 0.3V
Voltage on Any Input or Output Pin −0.3V to V
A
Reference Voltage (VRT,VRB)V
A
to AGND
CLK, OE Voltage Range
−0.3V to
(V
A
+ 0.3V)
Digital Output Voltage (V
OH,VOL
) DR GND to DR V
D
Input Current at Any Pin (Note 3)
±
25 mA
Package Input Current (Note 3)
±
50 mA
Power Dissipation at T
A
= 25˚C See (Note 4)
ESD Susceptibility (Note 5)
Human Body Model
Machine Model
2500V
250V
Soldering Temperature, Infrared,
10 seconds (Note 6) 235˚C
Storage Temperature −65˚C to +150˚C
Operating Ratings (Notes 1, 2)
Operating Temperature Range −40˚C ≤ T
A
≤ +85˚C
Supply Voltage (V
A
) +2.7V to +3.6V
Driver Supply Voltage (DR V
D
) +2.4V to V
A
Ground Difference |GND - DR GND| 0V to 300 mV
Upper Reference Voltage (V
RT
) 1.0V to (VA+ 0.1V)
Lower Reference Voltage (V
RB
) 0Vto(VRT− 1.0V)
V
IN
Voltage Range VRBto V
RT
Converter Electrical Characteristics
The following specifications apply for VA=DRVD= +3.0VDC,VRT= +1.9V, VRB= 0.3V, CL= 10 pF, f
CLK
= 100 MHz at 50%
duty cycle. Boldface limits apply for T
A=TMIN
to T
MAX
: all other limits TA= 25˚C (Notes 7, 8)
Symbol Parameter Conditions
Typical
(Note 9)
Limits
(Note 9)
Units
(Limits)
DC ACCURACY
INL Integral Non-Linearity
±
0.5
±
1.3 LSB (max)
DNL Differential Non-Linearity
±
0.4
+1.0
−0.95
LSB (max)
LSB (min)
Missing Codes 0 (max)
FSE Full Scale Error 18
±
28 mV (max)
V
OFF
Zero Scale Offset Error 26
±
35 mV (max)
ANALOG INPUT AND REFERENCE CHARACTERISTICS
V
IN
Input Voltage 1.6
V
RB
V (min)
V
RT
V (max)
C
IN
VINInput Capacitance
V
IN
= 0.75V +0.5
Vrms
(CLK LOW) 3 pF
(CLK HIGH) 4 pF
R
IN
RINInput Resistance
>
1MΩ
BW Full Power Bandwidth 200 MHz
V
RT
Top Reference Voltage 1.9
V
A
V (max)
1.0 V (min)
V
RB
Bottom Reference Voltage 0.3
V
RT
− 1.0 V (max)
0 V (min)
V
RT
-
V
RB
Reference Delta 1.6
1.0 V (min)
2.3 V(max)
R
REF
Reference Ladder Resistance VRTto V
RB
220
150 Ω (min)
300 Ω (max)
I
REF
Reference Ladder Current 7.3
5.3 mA (min)
10.6 mA (max)
CLK, PD DIGITAL INPUT CHARACTERISTICS
V
IH
Logical High Input Voltage DR VD=VA= 3.3V 2.0 V (min)
V
IL
Logical Low Input Voltage DR VD=VA= 2.7V 0.8 V (max)
I
IH
Logical High Input Current VIH=DRVD=VA= 3.3V 10 nA
ADC08100
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Converter Electrical Characteristics (Continued)
The following specifications apply for VA=DRVD= +3.0VDC,VRT= +1.9V, VRB= 0.3V, CL= 10 pF, f
CLK
= 100 MHz at 50%
duty cycle. Boldface limits apply for T
A=TMIN
to T
MAX
: all other limits TA= 25˚C (Notes 7, 8)
Symbol Parameter Conditions
Typical
(Note 9)
Limits
(Note 9)
Units
(Limits)
CLK, PD DIGITAL INPUT CHARACTERISTICS
I
IL
Logical Low Input Current VIL= 0V, DR VD=VA= 2.7V −50 nA
C
IN
Logic Input Capacitance 3 pF
DIGITAL OUTPUT CHARACTERISTICS
V
OH
High Level Output Voltage VA=DRVD= 2.7V, IOH= −400 µA 2.6 2.4 V (min)
V
OL
Low Level Output Voltage VA=DRVD= 2.7V, IOL= 1.0 mA 0.4 0.5 V (max)
DYNAMIC PERFORMANCE
ENOB Effective Number of Bits
f
IN
= 4 MHz, VIN= FS − 0.25 dB 7.5 Bits
f
IN
= 10 MHz, VIN= FS − 0.25 dB 7.5 7.0 Bits (min)
f
IN
= 41 MHz, VIN= FS − 0.25 dB,
T
A
= 25˚C
7.3 6.9 Bits (min)
f
IN
= 41 MHz, VIN= FS − 0.25 dB, T
A
=T
MIN
to T
MAX
7.3 6.8 Bits (min)
f
IN
= 49.8 MHz, VIN= FS − 0.25 dB 7.2 Bits
SINAD Signal-to-Noise & Distortion
f
IN
= 4 MHz, VIN= FS − 0.25 dB 47 dB
f
IN
= 10 MHz, VIN= FS − 0.25 dB 47 43.9 dB (min)
f
IN
= 41 MHz, VIN= FS − 0.25 dB,
T
A
= 25˚C
46 43.3 dB (min)
f
IN
= 41 MHz, VIN= FS − 0.25 dB, T
A
=T
MIN
to T
MAX
46 42.7 dB (min)
f
IN
= 49.8 MHz, VIN= FS − 0.25 dB 45 dB
SNR Signal-to-Noise Ratio
f
IN
= 4 MHz, VIN= FS − 0.25 dB 47 dB
f
IN
= 10 MHz, VIN= FS − 0.25 dB 47 44 dB (min)
f
IN
= 41 MHz, VIN= FS − 0.25 dB 46.5 42.8 dB (min)
f
IN
= 49.8 MHz, VIN= FS − 0.25 dB 45.8 dB
SFDR Spurious Free Dynamic Range
f
IN
= 4 MHz, VIN= FS − 0.25 dB 61 dBc
f
IN
= 10 MHz, VIN= FS − 0.25 dB 60 dBc
f
IN
= 41 MHz, VIN= FS − 0.25 dB 63 dBc
f
IN
= 49.8 MHz, VIN= FS − 0.25 dB 54 dBc
THD Total Harmonic Distortion
f
IN
= 4 MHz, VIN= FS − 0.25 dB −61 dBc
f
IN
= 10 MHz, VIN= FS − 0.25 dB −60 dBc
f
IN
= 41 MHz, VIN= FS − 0.25 dB -60 dBc
f
IN
= 49.8 MHz, VIN= FS − 0.25 dB −54 dBc
HD2 2nd Harmonic Distortion
f
IN
= 4 MHz, VIN= FS − 0.25 dB -62 dBc
f
IN
= 10 MHz, VIN= FS − 0.25 dB −60 dBc
f
IN
= 41 MHz, VIN= FS − 0.25 dB -63 dBc
f
IN
= 49.8 MHz, VIN= FS − 0.25 dB −54 dBc
HD3 3rd Harmonic Distortion
f
IN
= 4 MHz, VIN= FS − 0.25 dB −68 dBc
f
IN
= 10 MHz, VIN= FS − 0.25 dB −65 dBc
f
IN
= 41 MHz, VIN= FS − 0.25 dB -64 dBc
f
IN
= 49.8 MHz, VIN= FS − 0.25 dB −68 dBc
IMD Intermodulation Distortion
f
1
= 9 MHz, VIN= FS − 6.25 dB
f
2
= 10 MHz, VIN= FS − 6.25 dB
-48 dBc
POWER SUPPLY CHARACTERISTICS
I
A
Analog Supply Current
DC Input 41 50 mA (max)
f
IN
= 10 MHz, VIN=FS−3dB 41 mA(max)
ADC08100
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Converter Electrical Characteristics (Continued)
The following specifications apply for VA=DRVD= +3.0VDC,VRT= +1.9V, VRB= 0.3V, CL= 10 pF, f
CLK
= 100 MHz at 50%
duty cycle. Boldface limits apply for T
A=TMIN
to T
MAX
: all other limits TA= 25˚C (Notes 7, 8)
Symbol Parameter Conditions
Typical
(Note 9)
Limits
(Note 9)
Units
(Limits)
POWER SUPPLY CHARACTERISTICS
DR I
D
Output Driver Supply Current
DC Input 1 2 mA (max)
f
IN
= 10 MHz, VIN=FS−3dB 8 mA(max)
I
A
+
DRI
D
Total Operating Current
DC Input 42 52
mA (max)
f
IN
= 10 MHz, VIN=FS−3dB,
PD = Low
49
CLK Low, PD = Hi 0.2
PC Power Consumption
DC Input 126 156 mW (max)
f
IN
= 10 MHz, VIN=FS−3dB,
PD = Low
147 mW
CLK Low, PD = Hi 0.6 mW
PSRR
1
Power Supply Rejection Ratio
FSE change with 2.7V to 3.3V change
in V
A
54 dB
PSRR
2
Power Supply Rejection Ratio
SNR change with 200 mV at 1 MHz
on supply
TBD dB
AC ELECTRICAL CHARACTERISTICS
f
C1
Maximum Conversion Rate 125 100 MHz (min)
f
C2
Minimum Conversion Rate 20 MHz
t
CL
Minimum Clock Low Time 4.5 ns (min)
t
CH
Minimum Clock High Time 4.5 ns (min)
t
OH
Output Hold Time CLK Rise to Data Invalid 4.4 ns
t
OD
Output Delay CLK Rise to Data Valid 5.9 8.5 ns (max)
Pipeline Delay (Latency) 2.5 Clock Cycles
t
AD
Sampling (Aperture) Delay CLK Fall to Acquisition of Data 1.5 ns
t
AJ
Aperture Jitter 2 ps rms
Note 1: Absolute Maximum Ratings indicate limits beyond which damage to the device may occur. Operating Ratings indicate conditions for which the device is
functional, but do not guarantee specific performance limits. For guaranteed specifications and test conditions, see the Electrical Characteristics. The guaranteed
specifications apply only for the test conditions listed. Some performance characteristics may degrade when the device is not operated under the listed test
conditions.
Note 2: All voltages are measured with respect to GND = AGND = DR GND = 0V, unless otherwise specified.
Note 3: When the input voltage at any pin exceeds the power supplies (that is, less than AGND or DR GND, or greater than V
A
or DR VD), the current at that pin
should be limited to 25 mA. The 50 mA maximum package input current rating limits the number of pins that can safely exceed the power supplies with an input
current of 25 mA to two.
Note 4: The absolute maximum junction temperature (T
J
max) for this device is 150˚C. The maximum allowable power dissipation is dictated by TJmax, the
junction-to-ambient thermal resistance (θ
JA
), and the ambient temperature (TA), and can be calculated using the formula PDMAX=(TJmax − TA)/θJA. In the 24-pin
TSSOP, θ
JA
is 92˚C/W, so PDMAX = 1,358 mW at 25˚C and 435 mW at the maximum operating ambient temperature of 85˚C. Note that the power consumption of
this device under normal operation will typically be about 162 mW (126 mW quiescent power + 12 mW reference ladder power + 24 mW to drive the output bus
capacitance). The values for maximum power dissipation listed above will be reached only when the ADC08100 is operated in a severe fault condition (e.g., when
input or output pins are driven beyond the power supply voltages, or the power supply polarity is reversed). Obviously, such conditions should always be avoided.
Note 5: Human body model is 100 pF capacitor discharged through a 1.5 kΩ resistor. Machine model is 220 pF discharged through ZERO Ohms.
Note 6: See AN-450, “Surface Mounting Methods and Their Effect on Product Reliability”, or the section entitled “Surface Mount” found in any post 1986 National
Semiconductor Linear Data Book, for other methods of soldering surface mount devices.
Note 7: The analog inputs are protected as shown below. Input voltage magnitudes up to V
A
+ 300 mV or to 300 mV below GND will not damage this device.
However, errors in the A/D conversion can occur if the input goes above DR V
D
or below GND by more than 100 mV. For example, if VAis 2.7VDCthe full-scale input
voltage must be ≤2.6V
DC
to ensure accurate conversions.
ADC08100
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Converter Electrical Characteristics (Continued)
10137107
Note 8: To guarantee accuracy, it is required that VAand DR VDbe well bypassed. Each supply pin must be decoupled with separate bypass capacitors.
Note 9: Typical figures are at T
J
= 25˚C, and represent most likely parametric norms. Test limits are guaranteed to National’s AOQL (Average Outgoing Quality
Level).
Typical Performance Characteristics V
A
=DRVD= 3V, f
CLK
= 100 MHz, fIN= 41 MHz, unless oth-
erwise stated
INL INL vs Temperature
10137108
10137114
INL vs Supply Voltage INL vs Sample Rate
10137115
10137110
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Typical Performance Characteristics V
A
=DRVD= 3V, f
CLK
= 100 MHz, fIN= 41 MHz, unless
otherwise stated (Continued)
DNL DNL vs Temperature
10137109
10137117
DNL vs Supply Voltage DNL vs Sample Rate
10137118
10137111
SNR vs Temperature SNR vs Supply Voltage
10137120
10137121
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Typical Performance Characteristics V
A
=DRVD= 3V, f
CLK
= 100 MHz, fIN= 41 MHz, unless
otherwise stated (Continued)
SNR vs Sample Rate SNR vs Input Frequency
10137112
10137123
SNR vs Clock Duty Cycle Distortion vs Temperature
10137124 10137125
Distortion vs Supply Voltage Distortion vs Sample Rate
10137126
10137113
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Typical Performance Characteristics V
A
=DRVD= 3V, f
CLK
= 100 MHz, fIN= 41 MHz, unless
otherwise stated (Continued)
Distortion vs Input Frequency Distortion vs Clock Duty Cycle
10137128
10137129
SINAD/ENOB vs Temperature SINAD/ENOB vs Supply Voltage
10137130
10137138
SINAD/ENOB vs Sample Rate SINAD/ENOB vs Input Frequency
10137116
10137139
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Typical Performance Characteristics V
A
=DRVD= 3V, f
CLK
= 100 MHz, fIN= 41 MHz, unless
otherwise stated (Continued)
SINAD/ENOB vs Clock Duty Cycle Power Consumption vs Sample Rate
10137140
10137119
Spectral Response@fIN= 10 MHz Spectral Response@fIN=41MHz
10137144 10137145
Spectral Response@fIN= 76 MHz Intermodulation Distortion (IMD)
10137142 10137143
ADC08100
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Specification Definitions
APERTURE (SAMPLING) DELAY is that time required after
the fall of the clock input for the sampling switch to open. The
Sample/Hold circuit effectively stops capturing the input signal and goes into the “hold” mode t
AD
after the clock goes
low.
APERTURE JITTER is the variation in aperture delay from
sample to sample. Aperture jitter shows up as input noise.
BOTTOM OFFSET is the difference between the input volt-
age that just causes the output code to transition to the first
code and the negative reference voltage. Bottom Offset is
defined as E
OB=VZT–VRB
, where VZTis the first code
transition input voltage. V
RB
is the lower reference voltage.
Note that this is different from the normal Zero Scale Error.
CLOCK DUTY CYCLE is the ratio of the time that the clock
waveform is at a logic high to the total time of one clock
period.
DIFFERENTIAL NON-LINEARITY (DNL) is the measure of
the maximum deviation from the ideal step size of 1 LSB.
Measured at 100 MSPS with a ramp input.
EFFECTIVE NUMBER OF BITS (ENOB, or EFFECTIVE
BITS) is another method of specifying Signal-to-Noise and
Distortion Ratio, or SINAD. ENOB is defined as (SINAD –
1.76) / 6.02 and says that the converter is equivalent to a
perfect ADC of this (ENOB) number of bits.
FULL POWER BANDWIDTH is a measure of the frequency
at which the reconstructed output fundamental drops 3 dB
below its low frequency value for a full scale input. The test
is performed with f
IN
equal to 100 kHz plus integer multiples
of f
CLK
. The input frequency at which the output is −3 dB
relative to the low frequency input signal is the full power
bandwidth.
FULL-SCALE ERROR is a measure of how far the last code
transition is from the ideal 1
1
⁄2LSB below VRTand is defined
as:
V
max
+ 1.5 LSB – V
RT
where V
max
is the voltage at which the transition to the
maximum (full scale) code occurs.
INTEGRAL NON-LINEARITY (INL) is a measure of the
deviation of each individual code from a line drawn from zero
scale (
1
⁄2LSB below the first code transition) through positive
full scale (
1
⁄2LSB above the last code transition). The deviation of any given code from this straight line is measured
from the center of that code value. The end point test method
is used. Measured at 100 MSPS with a ramp input.
INTERMODULATION DISTORTION (IMD) is the creation of
additional spectral components as a result of two sinusoidal
frequencies being applied to the ADC input at the same time.
It is defined as the ratio of the power in the second and third
order intermodulation products to the power in one of the
original frequencies. IMD is usually expressed in dBFS.
MISSING CODE are those output codes that are skipped
and will never appear at the ADC outputs. These codes
cannot be reached with any input value.
OUTPUT DELAY is the time delay after the rising edge of
the input clock before the data update is present at the
output pins.
OUTPUT HOLD TIME is the length of time that the output
data is valid after the rise of the input clock.
PIPELINE DELAY (LATENCY) is the number of clock cycles
between initiation of conversion and when that data is presented to the output driver stage. New data is available at
every clock cycle, but the data lags the conversion by the
Pipeline Delay plus the Output Delay.
SIGNAL TO NOISE RATIO (SNR) is the ratio, expressed in
dB, of the rms value of the input signal at the output to the
rms value of the sum of all other spectral components below
one-half the sampling frequency, not including harmonics or
dc.
SIGNAL TO NOISE PLUS DISTORTION (S/(N+D) or
SINAD) is the ratio, expressed in dB, of the rms value of the
input signal at the output to the rms value of all of the other
spectral components below half the clock frequency, including harmonics but excluding dc.
SPURIOUS FREE DYNAMIC RANGE (SFDR) is the difference, expressed in dB, between the rms values of the input
signal at the output and the peak spurious signal, where a
spurious signal is any signal present in the output spectrum
that is not present at the input.
TOTAL HARMONIC DISTORTION (THD) is the ratio, expressed in dB, of the total of the first nine harmonic levels at
the output to the level of the fundamental at the output. THD
is calculated as
where F1is the RMS power of the fundamental (input)
frequency and f
2
through f10is the power in the first 9
harmonics in the output spectrum.
ZERO SCALE OFFSET ERROR is the error in the input
voltage required to cause the first code transition. It is defined as
V
OFF=VZT−VRB
where VZTis the first code transition input voltage.
ADC08100
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Timing Diagram
Functional Description
The ADC08100 uses a new, unique architecture that
achieves over 7 effective bits at input frequencies up to and
beyond 50 MHz.
The analog input signal that is within the voltage range set by
V
RT
and VRBis digitized to eight bits. Input voltages below
V
RB
will cause the output word to consist of all zeroes. Input
voltages above V
RB
will cause the output word to consist of
all ones.
Incorporating a switched capacitor bandgap, the ADC08100
exhibits a power consumption that is proportional to frequency, limiting power consumption to what is needed at the
clock rate that is used. This and its excellent performance
over a wide range of clock frequencies makes it an ideal
choice as a single ADC for many 8-bit needs.
Data is acquired at the falling edge of the clock and the
digital equivalent of that data is available at the digital outputs 2.5 clock cycles plus t
OD
later. The ADC08100 will
convert as long as the clock signal is present. The device is
in the active state when the Power Down pin (PD) is low.
When the PD pin is high, the device is in the power down
mode, where the output pins hold the last conversion before
the PD pin went high and the device consumes just 1 mW.
Applications Information
1.0 REFERENCE INPUTS
The reference inputs V
RT
and VRBare the top and bottom of
the reference ladder, respectively. Input signals between
these two voltages will be digitized to 8 bits. External voltages applied to the reference input pins should be within the
range specified in the Operating Ratings table (1.0V to (V
A
+
0.1V) for V
RT
and −100 mV to (VRT− 1.0V) for VRB). Any
device used to drive the reference pins should be able to
source sufficient current into the V
RT
pin and sink sufficient
current from the V
RB
pin.
10137131
FIGURE 1. ADC08100 Timing Diagram
ADC08100
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Applications Information (Continued)
The reference bias circuit of Figure 2 is very simple and the
performance is adequate for many applications. However,
circuit tolerances will lead to a wide referance voltage range.
Superior performance can generally be achieved by driving
the reference pins with a low impedance source.
The circuit of Figure 3 will allow a more accurate setting of
the reference voltages. The lower amplifier must have bipolar supplies as its output voltage must go negative to force
V
RB
to any voltage below the VBEof the PNP transistor. Of
course, the divider resistors at the amplifier input could be
changed to suit your reference voltage needs, or the divider
can be replaced with potentiometers for precise settings.
The bottom of the ladder (V
RB
) may simply be returned to
ground if the minimum input signal excursion is 0V. Be sure
that the driving source can source sufficient current into the
V
RT
pin and sink enough current from the VRBpin to keep
these pins stable.
The LMC662 amplifier shown was chosen for its low offset
voltage and low cost.
10137132
FIGURE 2. Simple, low component count reference biasing. Because of the ladder and external resistor tolerances,
the reference voltage can vary too much for some applications.
ADC08100
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Applications Information (Continued)
V
RT
should always be at least 1.0V more positive than V
RB
to minimize noise.
The V
RM
pin is the center of the reference ladder and should
be bypassed to a clean, quiet point in the analog ground
plane with a 0.1 µF capacitor. DO NOT allow this pin to float.
2.0 THE ANALOG INPUT
The analog input of the ADC08100 is a switch followed by an
integrator. The input capacitance changes with the clock
level, appearing as 3 pF when the clock is low, and 4 pF
when the clock is high. Since a dynamic capacitance is more
difficult to drive than is a fixed capacitance, choose an
amplifier that can drive this type of load. The LMH6702 and
the LMH6628 have been found to be good amplifiers to drive
the ADC08100.
Figure 4 shows an example of an input circuit using the
LMH6702. Any input amplifier should incorporate some gain
as operational amplifiers exhibit better phase margin and
transient response with gains above 2 or 3 than with unity
gain. If an overall gain of less than 3 is required, attenuate
the input and operate the amplifier at a higher gain, as
shown in Figure 4.
10137133
FIGURE 3. Driving the reference to force desired values requires driving with a low impedance source.
ADC08100
www.national.com15
Applications Information (Continued)
The RC at the amplifier output filters the clock rate energy
that comes out of the analog input due to the input sampling
circuit. The optimum time constant for this circuit depends
not only upon the amplifier and ADC, but also on the circuit
layout and board material. A resistor value should be chosen
between 18Ω and 47Ω and the capacitor value chose according to the formula
This will provide optimum SNR performance. Best THD performance is realized when the capacitor and resistor values
are both zero. To optimize SINAD, reduce the capacitor
value until SINAD performance is optimized. That is, until
SNR = −THD. This value will usually be in the range of 40%
to 65% of the value calculated with the above formula. An
accurate calculation is not possible because of the board
material and layout dependance.
The above is intended for oversampling or Nyquist applications. There should be no resistor or capacitor between the
ADC input and any amplifier for undersampling applications.
The circuit of Figure 4 has both gain and offset adjustments.
If you eliminate these adjustments normal circuit tolerances
may cause signal clipping unless care is exercised in the
worst case analysis of component tolerance and the input
signal excursion is appropriately limited to account for the
worst case conditions. Of course, this means that the designer will not be able to count on getting a full scale output
with maximum signal input.
3.0 POWER SUPPLY CONSIDERATIONS
A/D converters draw sufficient transient current to corrupt
their own power supplies if not adequately bypassed. A
10 µF tantalum or aluminum electrolytic capacitor should be
placed within an inch (2.5 cm) of the A/D power pins, with a
0.1 µF ceramic chip capacitor placed within one centimeter
of the converter’s power supply pins. Leadless chip capacitors are preferred because they have low lead inductance.
While a single voltage source is recommended for the V
A
and DR VDsupplies of the ADC08100, these supply pins
should be well isolated from each other to prevent any digital
noise from being coupled into the analog portions of the
ADC. A choke or 27Ω resistor is recommended between
these supply lines with adequate bypass capacitors close to
the supply pins.
As is the case with all high speed converters, the ADC08100
should be assumed to have little power supply rejection.
None of the supplies for the converter should be the supply
that is used for other digital circuitry in any system with a lot
of digital power being consumed. The ADC supplies should
be the same supply used for other analog circuitry.
No pin should ever have a voltage on it that is in excess of
the supply voltage or below ground by more than 300 mV,
not even on a transient basis. This can be a problem upon
application of power and power shut-down. Be sure that the
supplies to circuits driving any of the input pins, analog or
digital, do not come up any faster than does the voltage at
the ADC08100 power pins.
10137134
FIGURE 4. The input amplifier should incorporate some gain for best performance (see text).
ADC08100
www.national.com 16
Applications Information (Continued)
4.0 THE DIGITAL INPUT PINS
The ADC08100 has two digital input pins: The PD pin and
the Clock pin.
4.1 The PD Pin
The Power Down (PD) pin, when high, puts the ADC08100
into a low power mode where power consumption is reduced
to 1 mW. Output data is valid and accurate about 1 millisecond after the PD pin is brought low.
The digital output pins retain the last conversion output code
when either the clock is stopped or the PD pin is high.
4.2 The ADC08100 Clock
Although the ADC08100 is tested and its performance is
guaranteed with a 100 MHz clock, it typically will function
well with clock frequencies from 20 MHz to 125 MHz.
Halting the clock will provide nearly as much power saving
as raising the PD pin high. Typical power consumption with a
stopped clock is 3 mW, compared to 1 mW when PD is high.
The digital outputs will remain in the same state as they were
before the clock was halted.
Once the clock is restored (or the PD pin is brought low),
there is a time of 2.5 clock cycles plus t
OD
before the output
data is valid. However, because of the linear relationship
between total power consumption and clock frequency, the
part requires several microseconds after the clock is restarted or substantially changed in frequency before the part
returns to its specified accuracy.
The low and high times of the clock signal can affect the
performance of any A/D Converter. Because achieving a
precise duty cycle is difficult, the ADC08100 is designed to
maintain performance over a range of duty cycles. While it is
specified and performance is guaranteed with a 50% clock
duty cycle and 100 Msps, ADC08100 performance is typically maintained with clock high and low times of 4.5 ns,
corresponding to a clock duty cycle range of 45% to 55%
with a 100 MHz clock.
The CLOCK line should be series terminated at the clock
source in the characteristic impedence of that line. If the
clock line is longer than
where tris the clock rise time and tPDis the propagation rate
of the signal along the trace, the CLOCK pin should be a.c.
terminated with a series RC to ground such that the resistor
value is equal to the characteristic impedance of the clock
line and the capacitor value is
where tPDis the signal propagation rate down the clock line,
"L" is the line length and Z
o
is the characteristic impedance
of the clock line. This termination should be located as close
as possible to, but within one centimeter of, the ADC08100
clock pin. Typical t
PD
is about 150 ps/inch on FR-4 board
material. For FR-4 board material, the value of C becomes
where L is the length of the clock line in inches.
5.0 LAYOUT AND GROUNDING
Proper grounding and proper routing of all signals are essential to ensure accurate conversion. A combined analog
and digital ground plane should be used.
Since digital switching transients are composed largely of
high frequency components, total ground plane copper
weight will have little effect upon the logic-generated noise
because of the skin effect. Total surface area is more important than is total ground plane volume. Capacitive coupling
between the typically noisy digital circuitry and the sensitive
analog circuitry can lead to poor performance that may seem
impossible to isolate and remedy. The solution is to keep the
analog circuitry well separated from the digital circuitry.
High power digital components should not be located on or
near a straight line between the ADC or any linear component and the power supply area as the resulting common
return current path could cause fluctuation in the analog
input “ground” return of the ADC.
Generally, analog and digital lines should cross each other at
90˚ to avoid getting digital noise into the analog path. In high
frequency systems, however, avoid crossing analog and
digital lines altogether. Clock lines should be isolated from
ALL other lines, analog AND digital. Even the generally
accepted 90˚ crossing should be avoided as even a little
coupling can cause problems at high frequencies. Best performance at high frequencies is obtained with a straight
signal path.
The analog input should be isolated from noisy signal traces
to avoid coupling of spurious signals into the input. Any
external component (e.g., a filter capacitor) connected between the converter’s input and ground should be connected
to a very clean point in the analog ground plane.
10137136
FIGURE 5. Layout Example
ADC08100
www.national.com17
Applications Information (Continued)
Figure 5 gives an example of a suitable layout. All analog
circuitry (input amplifiers, filters, reference components, etc.)
should be placed together away from any digital components.
6.0 DYNAMIC PERFORMANCE
The ADC08100 is ac tested and its dynamic performance is
guaranteed. To meet the published specifications, the clock
source driving the CLK input must exhibit less than 3 ps
(rms) of jitter. For best ac performance, isolating the ADC
clock from any digital circuitry should be done with adequate
buffers, as with a clock tree. See Figure 6.
It is good practice to keep the ADC clock line as short as
possible and to keep it well away from any other signals.
Other signals can introduce jitter into the clock signal. The
clock signal can also introduce noise into the analog path.
7.0 COMMON APPLICATION PITFALLS
Driving the inputs (analog or digital) beyond the power
supply rails. For proper operation, all inputs should not go
more than 300 mV below the ground pins or 300 mV above
the supply pins. Exceeding these limits on even a transient
basis may cause faulty or erratic operation. It is not uncommon for high speed digital circuits (e.g., 74F and 74AC
devices) to exhibit undershoot that goes more than a volt
below ground. A 51Ω resistor in series with the offending
digital input will usually eliminate the problem.
Care should be taken not to overdrive the inputs of the
ADC08100. Such practice may lead to conversion inaccuracies and even to device damage.
Attempting to drive a high capacitance digital data bus.
The more capacitance the output drivers must charge for
each conversion, the more instantaneous digital current is
required from DR V
D
and DR GND. These large charging
current spikes can couple into the analog section, degrading
dynamic performance. Buffering the digital data outputs (with
a 74F541, for example) may be necessary if the data bus
capacitance exceeds 10 pF. Dynamic performance can also
be improved by adding 47Ω to 56Ω series resistors at each
digital output, reducing the energy coupled back into the
converter input pins.
Using an inadequate amplifier to drive the analog input.
As explained in Section 2.0, the capacitance seen at the
input alternates between 3 pF and 4 pF with the clock. This
dynamic capacitance is more difficult to drive than is a fixed
capacitance, and should be considered when choosing a
driving device. The LMH6702 and the LMH6628 have been
found to be good devices for driving the ADC08100.
Driving the V
RT
pin or the VRBpin with devices that can
not source or sink the current required by the ladder. As
mentioned in Section 1.0, care should be taken to see that
any driving devices can source sufficient current into the V
RT
pin and sink sufficient current from the VRBpin. If these pins
are not driven with devices than can handle the required
current, these reference pins will not be stable, resulting in a
reduction of dynamic performance.
Using a clock source with excessive jitter, using an
excessively long clock signal trace, or having other
signals coupled to the clock signal trace. This will cause
the sampling interval to vary, causing excessive output noise
and a reduction in SNR performance. The use of simple
gates with RC timing is generally inadequate as a clock
source.
10137137
FIGURE 6. Isolating the ADC Clock from Digital
Circuitry
ADC08100
www.national.com 18
Physical Dimensions inches (millimeters) unless otherwise noted
NOTES: UNLESS OTHERWISE SPECIFIED
REFERENCE JECED REGISTRATION mo-153, VARIATION AD, DATED 7/93.
24-Lead Package TC
Order Number ADC08100CIMT
NS Package Number MTC24
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www.national.com
ADC08100 8-Bit, 100 MSPS, 1.3 mW/MSPS A/D Converter
National does not assume any responsibility for use of any circuitry described, no circuit patent licenses are implied and National reserves the right at any time without notice to change said circuitry and specifications.
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