ADS805
ADS
805
E
SBAS073B – JANUARY 1997 – REVISED NOVEMBER 2002
12-Bit, 20MHz Sampling
ANALOG-TO-DIGITAL CONVERTER
FEATURES
APPLICATIONS
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HIGH SFDR: 74dB at 9.8MHz fIN
HIGH SNR: 68dB
LOW POWER: 300mW
LOW DLE: 0.25LSB
FLEXIBLE INPUT RANGE
OVER-RANGE INDICATOR
DESCRIPTION
STUDIO CAMERAS
IF AND BASEBAND DIGITIZATION
COPIERS
TEST INSTRUMENTATION
natively, the 5Vp-p input range can be used for the lowest
input-referred noise of 0.09LSBs rms giving superior imaging
performance. There is also the capability to set the input
range between 2Vp-p and 5Vp-p, either single-ended or
differential. The ADS805 also provides an over-range flag
that indicates when the input signal has exceeded the
converter’s full-scale range. This flag can also be used to
reduce the gain of the front end signal conditioning circuitry.
The ADS805 is a 20MHz, high dynamic range, 12-bit, pipelined
Analog-to-Digital Converter ADC. This converter includes a
high-bandwidth track-and-hold that gives excellent spurious
performance up to and beyond the Nyquist rate. This highbandwidth, linear track-and-hold minimizes harmonics and
has low jitter, leading to excellent Signal-to-Noise Ratio
(SNR) performance. The ADS805 is also pin-compatible with
the 10MHz ADS804 and the 5MHz ADS803.
The ADS805 employs digital error techniques to provide
excellent differential linearity for demanding imaging applications. Its low distortion and high SNR give the extra margin
needed for communications, medical imaging, video, and
test instrumentation applications. The ADS805 is available in
an SSOP-28 package.
The ADS805 provides an internal reference or an external
reference can be used. The ADS805 can be programmed for
a 2Vp-p input range which is the easiest to drive with a single
op amp and provides the best spurious performance. Alter-
+VS
CLK
VDRV
ADS805
Timing Circuitry
VIN
IN
12-Bit
Pipelined
ADC Core
T&H
IN
Error
Correction
Logic
3-State
Outputs
D0
•
•
•
D11
CM
OVR
Reference Ladder
and Driver
Reference and
Mode Select
REFT
VREF
SEL
REFB
OE
Please be aware that an important notice concerning availability, standard warranty, and use in critical applications of
Texas Instruments semiconductor products and disclaimers thereto appears at the end of this data sheet.
Copyright © 1997, Texas Instruments Incorporated
PRODUCTION DATA information is current as of publication date.
Products conform to specifications per the terms of Texas Instruments
standard warranty. Production processing does not necessarily include
testing of all parameters.
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ABSOLUTE MAXIMUM RATINGS(1)
ELECTROSTATIC
DISCHARGE SENSITIVITY
+VS ....................................................................................................... +6V
Analog Input ............................................................. –0.3V to (+VS) + 0.3V
Logic Input ............................................................... –0.3V to (+VS) + 0.3V
This integrated circuit can be damaged by ESD. Texas Instruments recommends that all integrated circuits be handled with
appropriate precautions. Failure to observe proper handling
and installation procedures can cause damage.
Case Temperature ......................................................................... +100°C
Junction Temperature .................................................................... +150°C
Storage Temperature ..................................................................... +150°C
ESD damage can range from subtle performance degradation
to complete device failure. Precision integrated circuits may be
more susceptible to damage because very small parametric
changes could cause the device not to meet its published
specifications.
NOTE: (1) Stresses above those listed under “Absolute Maximum Ratings”
may cause permanent damage to the device. Exposure to absolute maximum
conditions for extended periods may affect device reliability.
PACKAGE/ORDERING INFORMATION
PRODUCT
ADS805
"
PACKAGE-LEAD
PACKAGE
DESIGNATOR(1)
SPECIFIED
TEMPERATURE
RANGE
PACKAGE
MARKING
ORDERING
NUMBER(2)
TRANSPORT
MEDIA, QUANTITY
SSOP-28
DB
–40°C to +85°C
ADS805E
"
"
"
"
ADS805E
ADS805E/1K
Rails, 48
Tape and Reel, 1000
NOTE: (1) For the most current specifications and package information, refer to our web site at www.ti.com.
ELECTRICAL CHARACTERISTICS
At TA = full specified temperature range, VS = +5V, specified input range = 1.5V to 3.5V, and single-ended input and sampling rate = 20MHz, unless otherwise specified.
ADS805E
PARAMETER
CONDITIONS
MIN
RESOLUTION
Integral Nonlinearity Error
f = 500kHz
Aperture Delay Time
Aperture Jitter
Over-Voltage Recovery Time
Full-Scale Step Acquisition Time
UNITS
°C
–40 to +85
CONVERSION CHARACTERISTICS
Sample Rate
Data Latency
DYNAMIC CHARACTERISTICS
Differential Linearity Error (Largest Code Error)
f = 500kHz
No Missing Codes
Spurious-Free Dynamic Range(1)
f = 9.8MHz
2-Tone Intermodulation Distortion(3)
f = 7.7MHz and 7.9MHz (–7dB each tone)
Signal-to-Noise Ratio (SNR)
f = 9.8MHz
Signal-to-(Noise + Distortion) (SINAD)
f = 9.8MHz
Effective Number of Bits at 9.8MHz(4)
Input Referred Noise
MAX
12 Bits Tested
SPECIFIED TEMPERATURE RANGE
ANALOG INPUT
Standard Single-Ended Input Range
Optional Single-Ended Input Range
Standard Common-Mode Voltage
Standard Optional Common-Mode Voltage
Input Capacitance
Analog Input Bandwidth
TYP
10k
20M
Samples/s
Clk Cycles
3.5
5
V
V
V
V
pF
MHz
±0.75
LSB
6
1.5
0
2.5
1
20
270
–3dBFS Input
±0.25
Tested
74
dBFS(2)
–70
dBc
63
68
dBFS
62
66
10.7
0.09
0.23
dBFS
Bits
LSBs rms
LSBs rms
65
0V to 5V Input
1.5V to 3.5V Input
1.5x FS Input
±1
3
4
2
20
±2
LSB
ns
ps rms
ns
ns
NOTES: (1) Spurious-Free Dynamic Range refers to the magnitude of the largest harmonic. (2) dBFS means dB relative to full-scale. (3) 2-tone intermodulation
distortion is referred to the largest fundamental tone. This number will be 6dB higher if it is referred to the magnitude of the 2-tone fundamental envelope. (4) Effective
number of bits (ENOB) is defined by (SINAD – 1.76)/6.02. (5) Internal 50kΩ pull-down resistor. (6) Includes internal reference. (7) Excludes internal reference.
2
ADS805
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SBAS073B
ELECTRICAL CHARACTERISTICS (Cont.)
At TA = full specified temperature range, VS = +5V, specified input range = 1.5V to 3.5V, and single-ended input and sampling rate = 20MHz, unless otherwise specified.
ADS805E
PARAMETER
DIGITAL INPUTS
Logic Family
Convert Command
High Level Input Current (VIN = 5V)(5)
Low Level Input Current (VIN = 0V)
High Level Input Voltage
Low Level Input Voltage
Input Capacitance
DIGITAL OUTPUTS
Logic Family
Logic Coding
Low Output Voltage
Low Output Voltage
High Output Voltage
High Output Voltage
3-State Enable Time
3-State Disable Time
Output Capacitance
ACCURACY (5Vp-p Input Range)
Zero-Error (Referred to –FS)
Zero-Error Drift (Referred to –FS)
Gain Error(6)
Gain Error Drift(6)
Gain Error(7)
Gain Error Drift(7)
Power-Supply Rejection of Gain
Reference Input Resistance
Internal Voltage Reference Tolerance (VREF = 2.5V)
Internal Voltage Reference Tolerance (VREF = 1.0V)
POWER-SUPPLY REQUIREMENTS
Supply Voltage: +VS
Supply Current: +IS
Power Dissipation
Thermal Resistance, θJA
SSOP-28
CONDITIONS
MIN
TYP
MAX
UNITS
±100
10
µA
µA
V
V
pF
CMOS Compatible
Rising Edge of Convert Clock
Start Conversion
+3.5
+1.0
5
CMOS/TTL Compatible
Straight Offset Binary
(IOL = 50µA)
(IOL = 1.6mA)
(IOH = 50µA)
(IOH = 0.5mA)
OE = L
OE = H
0.1
0.4
+4.5
+2.4
fS = 2.5MHz
At 25°C
At 25°C
At 25°C
∆VS = ±5%
60
20
2
5
40
10
0.3
±5
0.7
±18
0.2
±10
70
1.6
±1.5
At 25°C
At 25°C
Operating
Operating
Operating
+4.75
50
+5.0
60
300
V
V
V
V
ns
ns
pF
±35
±14
%FS
ppm/°C
%FS
ppm/°C
%FS
ppm/°C
dB
kΩ
mV
mV
+5.25
69
345
V
mA
mW
±2.0
±1.5
°C/W
NOTES: (1) Spurious-Free Dynamic Range refers to the magnitude of the largest harmonic. (2) dBFS means dB relative to full-scale. (3) 2-tone intermodulation
distortion is referred to the largest fundamental tone. This number will be 6dB higher if it is referred to the magnitude of the 2-tone fundamental envelope. (4) Effective
number of bits (ENOB) is defined by (SINAD – 1.76)/6.02. (5) Internal 50kΩ pull-down resistor. (6) Includes internal reference. (7) Excludes internal reference.
ADS805
SBAS073B
www.ti.com
3
PIN CONFIGURATION
PIN DESCRIPTIONS
Top View
SSOP
OVR
1
28
VDRV
B1
2
27
+VS
B2
3
26
GND
B3
4
25
IN
B4
5
24
GND
B5
6
23
IN
B6
7
22
REFT
B7
8
21
CM
B8
9
20
REFB
B9 10
19
VREF
B10 11
18
SEL
B11 12
17
GND
B12 13
16
+VS
CLK 14
15
OE
ADS805
PIN
DESIGNATOR
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
OVR
B1
B2
B3
B4
B5
B6
B7
B8
B9
B10
B11
B12
CLK
OE
16
17
18
19
20
21
22
23
24
25
26
27
28
+VS
GND
SEL
VREF
REFB
CM
REFT
IN
GND
IN
GND
+VS
VDRV
DESCRIPTION
Over-Range Indicator
Data Bit 1 (D11) (MSB)
Data Bit 2 (D10)
Data Bit 3 (D9)
Data Bit 4 (D8)
Data Bit 5 (D7)
Data Bit 6 (D6)
Data Bit 7 (D5)
Data Bit 8 (D4)
Data Bit 9 (D3)
Data Bit 10 (D2)
Data Bit 11 (D1)
Data Bit 12 (D0) (LSB)
Convert Clock Input
Output Enable. H = High Impedance State.
L = LOW or floating, normal operation
(internal pull-down resistor).
+5V Supply
Ground
Input Range Select
Reference Voltage Select
Bottom Reference
Common-Mode Voltage
Top Reference
Complementary Analog Input
Ground
Analog Input (+)
Ground
+5V Supply
Output Driver Voltage
TIMING DIAGRAM
N+2
N+1
Analog In
N+4
N+3
N
tD
N+5
tL
tCONV
N+7
N+6
tH
Clock
6 Clock Cycles
t2
Data Out
N–6
N–5
N–4
N–3
N–2
N–1
N
Data Invalid
SYMBOL
tCONV
tL
tH
tD
t1
t2
4
N+1
t1
DESCRIPTION
MIN
Convert Clock Period
Clock Pulse LOW
Clock Pulse HIGH
Aperture Delay
Data Hold Time, CL = 0pF
New Data Delay Time, CL = 15pF max
50
24
24
TYP
MAX
UNITS
100µs
ns
ns
ns
ns
ns
ns
25
25
3
3.9
12
ADS805
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SBAS073B
TYPICAL CHARACTERISITCS
At TA = full specified temperature range, VS = +5V, specified single-ended input range = 1.5V to 3.5V, and sampling rate = 20MHz, unless otherwise specified.
SPECTRAL PERFORMANCE
SPECTRAL PERFORMANCE
0
0
fIN = 9.8MHz
–20
–40
–40
Amplitude (dB)
Amplitude (dB)
fIN = 500kHz
–20
–60
–80
–100
–60
–80
–100
–120
–120
0
2.0
4.0
6.0
8.0
10.0
0
2.0
4.0
Frequency (MHz)
2-TONE INTERMODULATION DISTORTION
8.0
10.0
DIFFERENTIAL LINEARITY ERROR
0
1.0
fIN = 9.8MHz
Code Width Error (LSB)
f7 = 7.7MHz at –7dBFS
f2 = 7.9MHz at –7dBFS
IMD (3) = –70dBc
–20
Magnitude (dBFS)
6.0
Frequency (MHz)
–40
–60
–80
0.5
0
–0.5
–100
–120
–1.0
0
2.5
5.0
7.5
10.0
0
1024
2048
Frequency (MHz)
3072
INTEGRAL LINEARITY ERROR
SWEPT POWER SFDR
4.0
100
fIN = 9.8MHz
fIN = 500kHz
80
SFDR (dBFS, dBc)
2.0
ILE (LSB)
4096
Output Code
0
–2.0
dBFS
60
dBc
40
20
–4.0
0
0
1024
2048
3072
4096
Output Code
–50
–40
–30
–20
–10
0
Input Amplitude (dBFS)
ADS805
SBAS073B
–60
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5
TYPICAL CHARACTERISITCS (Cont.)
At TA = full specified temperature range, VS = +5V, specified single-ended input range = 1.5V to 3.5V, and sampling rate = 20MHz, unless otherwise specified.
DIFFERENTIAL LINEARITY ERROR
vs TEMPERATURE
DYNAMIC PERFORMANCE vs INPUT FREQUENCY
85
0.6
SFDR
fIN = 9.8MHz
0.4
DLE (LSB)
SFDR, SNR (dBFS)
80
75
70
0.2
fIN = 500kHz
SNR
65
60
0
0.1
1
–50
10
–25
0
25
50
75
100
Temperature (°C)
Frequency (MHz)
SPURIOUS-FREE DYNAMIC RANGE
vs TEMPERATURE
SIGNAL-TO-NOISE RATIO vs TEMPERATURE
72
85
fIN = 500kHz
70
SNR (dBFS)
SFDR (dBFS)
fIN = 500kHz
80
68
fIN = 9.8MHz
75
66
fIN = 9.8MHz
64
70
–50
–25
0
25
50
75
–50
100
–25
0
SIGNAL-TO-(NOISE + DISTORTION)
vs TEMPERATURE
50
75
100
POWER DISSIPATION vs TEMPERATURE
305
72
70
fIN = 500kHz
Power (mW)
SINAD (dBFS)
25
Temperature (°C)
Temperature (°C)
68
300
295
66
fIN = 9.8MHz
290
64
–50
–25
0
25
50
75
–50
100
0
25
50
75
100
Temperature (°C)
Temperature (°C)
6
–25
ADS805
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SBAS073B
TYPICAL CHARACTERISITCS (Cont.)
At TA = full specified temperature range, VS = +5V, specified single-ended input range = 1.5V to 3.5V, and sampling rate = 20MHz, unless otherwise specified.
OUTPUT NOISE HISTOGRAM
(DC Input, VIN = 5Vp-p Range)
800k
800k
600k
600k
Counts
Counts
OUTPUT NOISE HISTOGRAM
(DC Input)
400k
200k
400k
200k
0
0
N–2
N–1
N
N+1
N+2
N–2
N–1
Code
N
N+1
N+2
Code
UNDERSAMPLING
(Differential Input, 2Vp-p)
0
fS = 20MHz
fIN = 41MHz
SNR = 63.2dBFS
SFDR = 76.3dBFS
Magnitude (dB)
–20
–40
–60
–80
–100
–120
0
2.0
4.0
6.0
8.0
10.0
Frequency (MHz)
ADS805
SBAS073B
www.ti.com
7
APPLICATION INFORMATION
configurations. This will decouple the op amp’s output from
the capacitive load and avoid gain peaking, which can result
in increased noise. For best spurious and distortion performance, the resistor value should be kept below 100Ω.
Furthermore, the series resistor, together with the 100pF
capacitor, establish a passive low-pass filter, limiting the
bandwidth for the wideband noise, thus helping improve the
signal-to-noise performance.
DRIVING THE ANALOG INPUT
The ADS805 allows its analog inputs to be driven either
single-ended or differentially. The focus of the following
discussion is on the single-ended configuration. Typically, its
implementation is easier to achieve and the rated specifications for the ADS805 are characterized using the singleended mode of operation.
DC-COUPLED WITHOUT LEVEL SHIFT
AC-COUPLED INPUT CONFIGURATION
In some applications the analog input signal may already be
biased at a level which complies with the selected input
range and reference level of the ADS805. In this case, it is
only necessary to provide an adequately low source impedance to the selected input, IN or IN. Always consider wideband
op amps since their output impedance will stay low over a
wide range of frequencies.
Given in Figure 1 is the circuit example of the most common
interface configuration for the ADS805. With the VREF pin
connected to the SEL pin, the full-scale input range is defined
to be 2Vp-p. This signal is ac-coupled in single-ended form
to the ADS805 using the low distortion voltage-feedback
amplifier OPA642. As is generally necessary for singlesupply components, operating the ADS805 with a full-scale
input signal swing requires a level-shift of the amplifier’s zero
centered analog signal to comply with the ADC’s input range
requirements. Using a DC-blocking capacitor between the
output of the driving amplifier and the converter’s input, a
simple level-shifting scheme can be implemented. In this
configuration, the top and bottom references (REFT, REFB)
provide an output voltage of +3V and +2V, respectively.
Here, two resistor pairs (2 • 2kΩ) are used to create a
common-mode voltage of approximately +2.5V to bias the
inputs of the ADS805 (IN, IN) to the required DC voltage.
DC-COUPLED WITH LEVEL SHIFT
Several applications may require that the bandwidth of the
signal path include DC, in which case the signal has to be DCcoupled to the ADC. In order to accomplish this, the interface
circuit has to provide a DC-level shift. The circuit presented in
Figure 2 utilizes the single-supply, current-feedback op amp
OPA681 (A1), to sum the ground-centered input signal with a
required DC offset. The ADS805 typically operates with a
+2.5V common-mode voltage, which is established with resistors R3 and R4 and connected to the IN input of the converter.
Amplifier A1 operates in inverting configuration. Here, resistors
R1 and R2 set the DC-bias level for A1. Because of the op
amp’s noise gain of +2V/V, assuming RF = RIN, the DC offset
voltage applied to its noninverting input has to be divided down
to +1.25V, resulting in a DC output voltage of +2.5V. DC
voltage differences between the IN and IN inputs of the
ADS805 effectively will produce an offset, which can be corrected for by adjusting the values of resistors R1 and R2. The
bias current of the op amp may also result in an undesired
An advantage of ac-coupling is that the driving amplifier still
operates with a ground-based signal swing. This will keep
the distortion performance at its optimum since the signal
swing stays within the linear region of the op amp and
sufficient headroom to the supply rails can be maintained.
Consider using the inverting gain configuration to eliminate
CMR induced errors of the amplifier. The addition of a small
series resistor (RS) between the output of the op amp and the
input of the ADS805 will be beneficial in almost all interface
+5V –5V
+VIN
2Vp-p
VIN
0.1µF
RS
24.9Ω
2kΩ
IN
OPA642
0V
–VIN
REFT
(+3V)
2kΩ
100pF
RF
402Ω
ADS805
2kΩ
RG
402Ω
+2.5V
IN
0.1µF
2kΩ
(+2V)
REFB
(+1V)
VREF
SEL
FIGURE 1. AC-Coupled Input Configuration for 2Vp-p Input Swing and Common-Mode Voltage at +2.5V Derived from Internal
Top and Bottom Reference.
8
ADS805
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SBAS073B
RF
+VS
RIN
+1V
0
VIN
–1V
R3
2kΩ
RS
50Ω
REFT
IN
OPA691
2Vp-p
22pF
R1
ADS805
R2
+VS
+2.5V
+
0.1µF
IN
0.1µF
10µF
(+1V)
VREF
REFB
R4
2kΩ
SEL
NOTE: RF = RIN, G = –1
FIGURE 2. DC-Coupled, Single-Ended Input Configuration with DC-level Shift.
offset. The selection criteria for an appropriate op amp should
include the input bias current, output voltage swing, distortion,
and noise specification. Note that in this example the overall
signal phase is inverted. To reestablish the original signal
polarity, it is always possible to interchange the IN and IN
connections.
RG
0.1µF
22Ω
1:n
VIN
IN
100pF
RT
ADS805
22Ω
IN
CM
100pF
SINGLE-ENDED-TO-DIFFERENTIAL
CONFIGURATION (TRANSFORMER-COUPLED)
+
In order to select the best suited interface circuit for the
ADS805, the performance requirements must be known. If
an ac-coupled input is needed for a particular application, the
next step is to determine the method of applying the signal;
either single-ended or differentially. The differential input
configuration may provide a noticeable advantage of achieving good SFDR performance based on the fact that, in the
differential mode, the signal swing can be reduced to half of
the swing required for single-ended drive. Secondly, by
driving the ADS805 differentially, the even-order harmonics
will be reduced. Figure 3 shows the schematic for the
suggested transformer-coupled interface circuit. The resistor
across the secondary side (RT) should be set to get an input
impedance match (e.g., RT = n2 • RG).
One application example that will benefit from the differential
input configuration is the digitization of IF signals. The wide
track-and-hold input bandwidth makes the ADS805 well
suited for IF down conversion in both narrow and wideband
applications. The ADS805 maintains excellent dynamic performance in multiple Nyquist regions covering a variety of IF
frequencies (see the Typical Characteristics). Using the
ADS805 for direct IF conversion eliminates the need of an
analog mixer along with subsequent functions like amplifiers
and filters, thus reducing system cost and complexity.
0.1µF
4.7µF
FIGURE 3. Transformer-Coupled Input.
REFERENCE OPERATION
Integrated into the ADS805 is a bandgap reference circuit
including logic that provides either a +1V or +2.5V reference
output, by simply selecting the corresponding pin-strap configuration. Different reference voltages can be generated by
the use of two external resistors, which will set a different
gain for the internal reference buffer. For more design flexibility, the internal reference can be shut off and an external
reference voltage used. Table I provides an overview of the
possible reference options and pin configurations.
MODE
INPUT
FULL-SCALE
RANGE
Internal
Internal
Internal
External
REQUIRED
VREF
CONNECT
2Vp-p
+1V
SEL
VREF
5Vp-p
+2.5V
SEL
GND
TO
2V ≤ FSR < 5V
1V < VREF < 2.5V
R1
VREF and SEL
FSR = 2 • VREF
VREF = 1 + (R1/R2)
R2
SEL and Gnd
1V < FSR < 5V
0.5V < VREF < 2.5V
SEL
+VS
VREF
Ext. VREF
TABLE I. Selected Reference Configuration Examples.
ADS805
SBAS073B
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9
A simple model of the internal reference circuit is shown in
Figure 4. The internal blocks are a 1V-bandgap voltage
reference, buffer, the resistive reference ladder and the
drivers for the top and bottom reference which supply the
necessary current to the internal nodes. As shown, the
output of the buffer appears at the VREF pin. The full-scale
input span of the ADS805 is determined by the voltage at
VREF, according to Equation 1:
Full-Scale Input Span = 2 • VREF
ADS805
VREF
10µF
+
0.1µF
+
0.1µF
0.1µF
10µF
0.1µF
FIGURE 5. Recommended Reference Bypassing Scheme.
The top reference (REFT) and the bottom reference (REFB)
are brought out mainly for external bypassing. For proper
operation with all reference configurations, it is necessary to
provide solid bypassing to the reference pins in order to keep
the clock feedthrough to a minimum. Figure 5 shows the
recommended decoupling network.
In addition, the Common-Mode Voltage (CMV) may be used
as a reference level to provide the appropriate offset for the
driving circuitry. However, care must be taken not to appreciably load this node, which is not buffered and has a high
impedance. An alternate method of generating a commonmode voltage is given in Figure 6. Here, two external precision resistors (tolerance 1% or better) are located between
the top and bottom reference pins. The common-mode level
will appear at the midpoint. The output buffers of the top and
bottom reference are designed to supply approximately 2mA
of output current.
CM
0.1µF
(1)
Note that the current drive capability of this amplifier is limited
to approximately 1mA and should not be used to drive low
loads. The programmable reference circuit is controlled by
the voltage applied to the select pin (SEL). Refer to Table I
for an overview.
REFB
REFT
IN
REFT
0.1µF
R1
CM
ADS805
R2
IN
REFB
0.1µF
FIGURE 6. Alternative Circuit to Generate Common-Mode Voltage.
Disable
Switch
SEL
VREF
to A/D
Converter
1VDC
REFT
Resistor Network
and Switches
800Ω
Bandgap
and Logic
Reference
Driver
CM
800Ω
REFB
ADS805
to A/D
Converter
FIGURE 4. Equivalent Reference Circuit.
10
ADS805
www.ti.com
SBAS073B
SELECTING THE INPUT RANGE AND REFERENCE
EXTERNAL REFERENCE OPERATION
Figures 7 through 9 show a selection of circuits for the most
common input ranges when using the internal reference of
the ADS805. All examples are for single-ended input and
operate with a nominal common-mode voltage of +2.5V.
Depending on the application requirements, it might be
advantageous to operate the ADS805 with an external reference. This may improve the DC accuracy if the external
reference circuitry is superior in its drift and accuracy. To use
the ADS805 with an external reference, the user must
disable the internal reference, as shown in Figure 10. By
connecting the SEL pin to +VS, the internal logic will shut
down the internal reference. At the same time, the output of
the internal reference buffer is disconnected from the VREF
pin, which now must be driven with the external reference.
Note that a similar bypassing scheme should be maintained
as described for the internal reference operation.
5V
VIN
IN
0V
ADS805
IN
VREF
SEL
+2.5V
4.5V
VIN
IN
0.5V
ADS805
FIGURE 7. Internal Reference with 0V to 5V Input Range.
REF1004
+2.5V
+2.5V ext.
IN
+
0.1µF
10µF
VREF
SEL
1.24kΩ
3.5V
VIN
+2VDC
IN
+5V
1.5V
4.99kΩ
ADS805
+2.5V ext.
IN
VREF
SEL
FIGURE 10. External Reference, Input Range 0.5V to 4.5V
(4Vp-p), with +2.5V Common-Mode Voltage.
+1V
FIGURE 8. Internal Reference with 1.5V to 3.5V Input Range.
4V
IN
VIN
1V
ADS805
+2.5V ext.
IN
VREF
SEL
R1
5kΩ
VREF = 1V 1 +
R1
R2
+1.5V
R2
10kΩ
FSR = 2 • VREF
FIGURE 9. Internal Reference with 1V to 4V Input Range.
DIGITAL INPUTS AND OUTPUTS
Over-Range (OVR)
One feature of the ADS805 is its ‘Over-Range’ (OVR) digital
output. This pin can be used to monitor any out-of-range
condition, which occurs every time the applied analog input
voltage exceeds the input range (set by VREF). The OVR
output is LOW when the input voltage is within the defined
input range. It becomes HIGH when the input voltage is
beyond the input range. This is the case when the input
voltage is either below the bottom reference voltage or above
the top reference voltage. OVR will remain active until the
analog input returns to its normal signal range and another
conversion is completed. Using the MSB and its complement
in conjunction with OVR, a simple decode logic can be built
that detects the over-range and under-range conditions, (see
Figure 11). It should be noted that OVR is a digital output
which is updated along with the bit information corresponding
to the particular sampling incidence of the analog signal.
Therefore, the OVR data is subject to the same pipeline
delay (latency) as the digital data.
ADS805
SBAS073B
www.ti.com
11
MSB
Over = H
OVR
Under = H
FIGURE 11. External Logic for Decoding Under-Range and
Over-Range Conditions.
CLOCK INPUT REQUIREMENTS
Clock jitter is critical to the SNR performance of high-speed,
high-resolution ADCs. It leads to aperture jitter (tA) which adds
noise to the signal being converted. The ADS805 samples the
input signal on the rising edge of the CLK input. Therefore, this
edge should have the lowest possible jitter. The jitter noise
contribution to total SNR is given by Equation 2. If this value
is near your system requirements, input clock jitter must be
reduced.
Jitter SNR = 20 log
1
rms signal to rms noise
2π ƒIN t A
(2)
Where: ƒIN is Input Signal Frequency,
tA is rms Clock Jitter
Particularly in undersampling applications, special consideration should be given to clock jitter. The clock input should be
treated as an analog input in order to achieve the highest
level of performance. Any overshoot or undershoot of the
clock signal may cause degradation of the performance.
When digitizing at high sampling rates, the clock should have
a 50% duty cycle (tH = tL), along with fast rise-and-fall times
of 2ns or less.
DIGITAL OUTPUTS
The digital outputs of the ADS805 are designed to be
compatible with both high-speed TTL and CMOS logic families. The driver stage for the digital outputs is supplied
through a separate supply pin, VDRV, which is not connected to the analog supply pins. By adjusting the voltage on
VDRV, the digital output levels will vary respectively. Therefore, it is possible to operate the ADS805 on a +5V analog
supply while interfacing the digital outputs to 3V-logic with
the VDRV pin tied to the +3V digital supply.
If necessary, external buffers or latches may be used which
provide the added benefit of isolating the ADS805 from any
digital noise activities on the bus coupling back high-frequency noise. In addition, resistors in series with each data
line may help maintain the ac performance of the ADS805.
Their use depends on the capacitive loading seen by the
converter. Values in the range of 100Ω to 200Ω will limit the
instantaneous current the output stage has to provide for
recharging the parasitic capacitances, as the output levels
change from LOW to HIGH or HIGH to LOW.
GROUNDING AND DECOUPLING
Proper grounding and bypassing, short lead length, and the
use of ground planes are particularly important for highfrequency designs. Multilayer PC boards are recommended
for best performance since they offer distinct advantages like
minimizing ground impedance, separation of signal layers by
ground layers, etc. It is recommended that the analog and
digital ground pins of the ADS805 be joined together at the
IC and be connected only to the analog ground of the
system.
The ADS805 has analog and digital supply pins, however the
converter should be treated as an analog component and all
supply pins should be powered by the analog supply. This
will ensure the most consistent results, since digital supply
lines often carry high levels of noise that would otherwise be
coupled into the converter and degrade the achievable performance.
Because of the pipeline architecture, the converter also
generates high-frequency current transients and noise that
are fed back into the supply and reference lines. This
requires that the supply and reference pins be sufficiently
bypassed. Figure 12 shows the recommended decoupling
scheme for the analog supplies. In most cases, 0.1µF ceramic chip capacitors are adequate to keep the impedance
low over a wide frequency range. Their effectiveness largely
depends on the proximity to the individual supply pin. Therefore, they should be located as close to the supply pins as
possible. In addition, a larger size bipolar capacitor (1µF to
22µF) should be placed on the PC board in close proximity
to the converter circuit.
ADS805
+VS
27
GND
26
+VS
16
0.1µF
It is recommended to keep the capacitive loading on the data
lines as low as possible (≤ 15pF). Larger capacitive loads
demand higher charging currents as the outputs are changing. Those high-current surges can feed back to the analog
portion of the ADS805 and influence the performance.
GND
17
0.1µF
VDRV
28
0.1µF
2.2µF
+
+5V
+5V/+3V
FIGURE 12. Recommended Bypassing for Analog Supply Pins.
12
ADS805
www.ti.com
SBAS073B
PACKAGE OPTION ADDENDUM
www.ti.com
10-Dec-2020
PACKAGING INFORMATION
Orderable Device
Status
(1)
Package Type Package Pins Package
Drawing
Qty
Eco Plan
(2)
Lead finish/
Ball material
MSL Peak Temp
Op Temp (°C)
Device Marking
(3)
(4/5)
(6)
ADS805E
ACTIVE
SSOP
DB
28
50
RoHS & Green
NIPDAU
Level-2-260C-1 YEAR
-40 to 85
ADS805E
ADS805E/1K
ACTIVE
SSOP
DB
28
1000
RoHS & Green
NIPDAU
Level-2-260C-1 YEAR
-40 to 85
ADS805E
(1)
The marketing status values are defined as follows:
ACTIVE: Product device recommended for new designs.
LIFEBUY: TI has announced that the device will be discontinued, and a lifetime-buy period is in effect.
NRND: Not recommended for new designs. Device is in production to support existing customers, but TI does not recommend using this part in a new design.
PREVIEW: Device has been announced but is not in production. Samples may or may not be available.
OBSOLETE: TI has discontinued the production of the device.
(2)
RoHS: TI defines "RoHS" to mean semiconductor products that are compliant with the current EU RoHS requirements for all 10 RoHS substances, including the requirement that RoHS substance
do not exceed 0.1% by weight in homogeneous materials. Where designed to be soldered at high temperatures, "RoHS" products are suitable for use in specified lead-free processes. TI may
reference these types of products as "Pb-Free".
RoHS Exempt: TI defines "RoHS Exempt" to mean products that contain lead but are compliant with EU RoHS pursuant to a specific EU RoHS exemption.
Green: TI defines "Green" to mean the content of Chlorine (Cl) and Bromine (Br) based flame retardants meet JS709B low halogen requirements of
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