LTC1411
Single Supply
14-Bit 2.5Msps ADC
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FEATURES
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DESCRIPTIO
The LTC ®1411 is a 2.5Msps sampling 14-bit A/D converter in a 36-pin SSOP package, which typically dissipates only 195mW from a single 5V supply. This device
comes complete with a high bandwidth sample-andhold, a precision reference, programmable input ranges
and an internally trimmed clock. The ADC can be powered
down with either the Nap or Sleep mode for low power
applications.
Sample Rate: 2.5Msps
80dB S/(N + D) and 90dB THD at 100kHz fIN
Single 5V Operation
No Pipeline Delay
Programmable Input Ranges
Low Power Dissipation: 195mW (Typ)
True Differential Inputs Reject Common Mode Noise
Out-of-Range Indicator
Internal or External Reference
Sleep (1µA) and Nap (2mA) Shutdown Modes
36-Pin SSOP Package
The LTC1411 converts either differential or single-ended
inputs and presents data in 2’s complement format.
Maximum DC specs include ±2LSB INL and 14-bit no
missing code over temperature. Outstanding dynamic
performance includes 80dB S/(N + D) and 90dB THD at
100kHz input frequency.
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APPLICATIO S
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Telecommunications
High Speed Data Acquisition
Digital Signal Processing
Multiplexed Data Acquisition Systems
Spectrum Analysis
Imaging Systems
The LTC1411 has four programmable input ranges selected by two digital input pins, PGA0 and PGA1. This
provides input spans of ±1.8V, ±1.27V, ±0.9V and ±0.64V.
An out-of-the-range signal together with the D13 (MSB)
will indicate whether a signal is over or under the ADC’s
input range. A simple conversion start input and a data
ready signal ease connections to FIFOs, DSPs and microprocessors.
, LTC and LT are registered trademarks of Linear Technology Corporation.
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BLOCK DIAGRA
10
2
3
30
OVDD
AIN–
REFOUT
D13
2.5V
BANDGAP
REFERENCE
+
14-BIT
ADC
–
REFIN
5k
14
OUTPUT
DRIVERS
INTERNAL
CLOCK
BUSY
OTR
6
12
•
•
•
D0
2k
REFCOM1
REFCOM2
S/(N + D) and Effective Bits
vs Input Frequency
29
OGND 28
5k
5
DVP
25
86
14
80
13
74
12
68
11
62
10
56
50
44
38
27
32
26
26
EFFECTIVE BITS
4
AVP
S/(N + D) (dB)
1
AIN+
20
X1.62/
X1.15
14
CONTROL LOGIC
10
100
1000
INPUT FREQUENCY (kHz)
10000
1411 TA02
7, 8, 9
AGND
11
AVM
36
SLP
35
NAP
34
PGA0
33
PGA1
32
CONVST
31
DGND
1411 BD
1411f
1
LTC1411
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ABSOLUTE
RATI GS
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PACKAGE/ORDER I FOR ATIO
AVP = DVP = OVDD = VDD (Notes 1, 2)
Supply Voltage (VDD) ................................................. 6V
Analog Input Voltage (Note 3) ... – 0.3V to (VDD + 0.3V)
Digital Input Voltage (Note 4) .................. – 0.3V to 10V
Digital Output Voltage ............... – 0.3V to (VDD + 0.3V)
Power Dissipation .............................................. 500mW
Operating Temperature Range
LTC1411C ............................................... 0°C to 70°C
LTC1411I ............................................ – 40°C to 85°C
Storage Temperature Range ................. – 65°C to 150°C
Lead Temperature (Soldering, 10 sec).................. 300°C
ORDER PART
NUMBER
TOP VIEW
AIN+
AIN–
1
36 SLP
2
35 NAP
REFOUT
3
34 PGA0
REFIN
4
33 PGA1
REFCOM1
5
32 CONVST
REFCOM2
6
31 DGND
AGND1
7
30 DVP
AGND2
8
AGND3
9
29 OVDD
28 OGND
AVP 10
27 BUSY
AVM 11
26 OTR
D13 (MSB) 12
25 D0
D12 13
24 D1
D11 14
23 D2
D10 15
22 D3
D9 16
21 D4
D8 17
20 D5
D7 18
19 D6
LTC1411CG
LTC1411IG
G PACKAGE
36-LEAD PLASTIC SSOP
TJMAX = 125°C, θJA = 95°C/ W
Consult LTC Marketing for parts specified with wider operating temperature ranges.
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CO VERTER CHARACTERISTICS
The ● denotes specifications which apply over the full operating
temperature range, otherwise specifications are TA = 25°C. (Notes 5, 6)
PARAMETER
CONDITIONS
Resolution (No Missing Codes)
●
Integral Linearity Error
(Note 7)
Offset Error
(Note 8)
Full-Scale Error
External Reference = 2.5V
Full-Scale Tempco
IOUT(REF) = 0
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DY A IC ACCURACY
MIN
TYP
MAX
14
UNITS
Bits
●
±2
LSB
●
±16
±24
LSB
LSB
± 60
LSB
±15
ppm/°C
TA = 25°C (Note 5)
SYMBOL PARAMETER
CONDITIONS
S/(N + D) Signal-to-Noise Plus Distortion Ratio
100kHz Input Signal
500kHz Input Signal
80.0
77.5
dB
dB
THD
Total Harmonic Distortion
100kHz Input Signal, Up to 5th Harmonic
500kHz Input Signal, Up to 5th Harmonic
– 90
– 82
dB
dB
Peak Harmonic or Spurious Noise
100kHz Input Signal
500kHz Input Signal
90
82
dB
dB
Full Linear Bandwidth
S/(N + D) ≥ 74dB
Transition Noise
MIN
TYP
MAX
UNITS
1.0
MHz
0.66
LSBRMS
1411f
2
LTC1411
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A ALOG I PUT
TA = 25°C (Note 5)
SYMBOL PARAMETER
CONDITIONS
VIN
Analog Input Range (Note 9)
(AIN+) – (AIN–), PGA0 = PGA1 = 5V
(AIN+) – (AIN–), PGA0 = 5V, PGA1 = 0V
(AIN+) – (AIN–), PGA0 = 0V, PGA1 = 5V
(AIN+) – (AIN–), PGA0 = PGA1 = 0V
MIN
Common Mode Input Range
AIN+ or AIN–
CIN
Analog Input Capacitance
Between Conversions (Sample Mode)
During Conversions (Hold Mode)
tACQ
Sample-and-Hold Acquisition Time
tAP
Sample-and-Hold Aperture Delay Time
7
ns
tjitter
Sample-and-Hold Aperture Delay Time Jitter
1
psRMS
CMRR
Analog Input Common Mode Rejection Ratio
0
0V < (AIN– = AIN+) < VDD
Input Leakage Current (Pins 1, 2)
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I TER AL REFERE CE CHARACTERISTICS
PARAMETER
TYP
MAX
UNITS
±1.8
±1.27
±0.9
±0.64
V
V
V
V
VDD
V
10
4
pF
pF
100
ns
62
dB
0.1
µA
TA = 25°C (Note 5)
CONDITIONS
MIN
TYP
MAX
UNITS
VREF Output Voltage
IOUT = 0
2.480
2.500
2.520
VREF Output Tempco
IOUT = 0
±15
ppm/°C
VREF Line Regulation
4.75V ≤ VDD ≤ 5.25V
0.01
LSB/ V
V
VREF Load Regulation
0 ≤ IOUT ≤ 1mA
REFCOM2 Output Voltage
IOUT = 0, PGA0 = PGA1 = 5V
4.05
V
REFIN Input Current
REFIN = External Reference 2.5V
250
µA
2
LSB/mA
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DIGITAL I PUTS A D DIGITAL OUTPUTS
The ● denotes specifications which apply over the full operating temperature range, otherwise specifications are TA = 25°C. (Note 5)
SYMBOL PARAMETER
CONDITIONS
VIH
High Level Input Voltage
VDD = 5.25V
●
VIL
Low Level Input Voltage
VDD = 4.75V
●
IIN
Digital Input Current
VIN = 0V to VDD, Except SLP, NAP (Note 11)
●
CIN
Digital Input Capacitance
VOH
High Level Output Voltage
VOL
Low Level Output Voltage
MIN
VDD = 4.75V, IO = – 10µA
VDD = 4.75V, IO = – 200µA
●
VDD = 4.75V, IO = 160µA
VDD = 4.75V, IO = 1.6mA
●
TYP
MAX
UNITS
2.4
V
0.8
V
±10
µA
2
pF
4.75
V
V
4.0
0.05
0.10
V
V
0.4
ISOURCE
Output Source Current
VOUT = 0V
– 10
mA
ISINK
Output Sink Current
VOUT = VDD
10
mA
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POWER REQUIRE E TS
The ● denotes specifications which apply over the full operating temperature range, otherwise specifications are TA = 25°C. (Note 5)
SYMBOL PARAMETER
CONDITIONS
MIN
VDD
Supply Voltage
(Note 9)
4.75
IDD
Supply Current
Nap Mode
Sleep Mode
NAP = 0V (Note 11)
SLP = 0V
Power Dissipation
Nap Mode
Sleep Mode
NAP = 0V
SLP = 0V
PD
TYP
MAX
UNITS
5.25
V
●
39
2
1
65
mA
mA
µA
●
195
10
5
325
mW
mW
µW
1411f
3
LTC1411
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TI I G CHARACTERISTICS
The ● denotes specifications which apply over the full operating temperature
range, otherwise specifications are TA = 25°C. (Notes 5) (See Figures 11a, 11b)
SYMBOL
PARAMETER
CONDITIONS
fSAMPLE(MAX)
Maximum Sampling Frequency
(Note 9)
tCONV
Conversion Time
tACQ
Acquisition Time
t0
SLP↑ to CONVST↓ Wake-Up Time
t1
NAP↑ to CONVST↓ Wake-Up Time
t2
CONVST Low Time
(Note 10)
t3
CONVST to BUSY Delay
CL = 25pF
t4
Data Ready After BUSY↑
t5
CONVST High Time
t6
Aperture Delay of Sample-and-Hold
MIN
MAX
250
350
2.5
●
UNITS
MHz
●
10µF Bypass Capacitor at REFCOM2 Pin
ns
100
ns
210
ms
250
ns
20
●
(Note 10)
TYP
ns
12
ns
7
ns
20
●
ns
7
Note 1: Absolute Maximum Ratings are those values beyond which the life
of a device may be impaired.
Note 2: All voltage values are with respect to ground with DGND, OGND,
AVM and AGND wired together unless otherwise noted.
Note 3: When these pin voltages are taken below AGND or above VDD,
they will be clamped by internal diodes. This product can handle input
currents greater than 100mA without latchup.
Note 4: When these pin voltages are taken below AGND, they will be
clamped by internal diodes. This product can handle input currents greater
than 100mA below AGND without latchup. These pins are not clamped to
VDD.
Note 5: VDD = 5V, PGA1 = PGA0 = 5V, fSAMPLE = 2.5MHz at 25°C and
t r = t f = 5ns unless otherwise specified.
Note 6: Linearity, offset and full-scale specifications apply for a singleended AIN+ input with AIN– tied to an external 2.5V reference voltage.
ns
Note 7: Integral nonlinearity is defined as the deviation of a code from a
straight line passing through the actual endpoints of the transfer curve.
The deviation is measured from the center of the quantization band.
Note 8: Bipolar offset is the offset voltage measured from – 0.5LSB
when the output code flickers between 0000 0000 0000 00 and
1111 1111 1111 11.
Note 9: Recommended operating conditions.
Note 10: The falling CONVST edge starts a conversion. If CONVST returns
high at a critical point during the conversion it can create small errors. For
best performance ensure that CONVST returns high within 20ns after
conversion start of after BUSY rises.
Note 11: SLP and NAP have an internal pull-down so the pins will draw
approximately 7µA when tied high and less than 1µA when tied low.
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TYPICAL PERFOR A CE CHARACTERISTICS
Signal-to-Noise Ratio
vs Input Frequency
Distortion vs Input Frequency
86
86
80
80
–10
74
74
–20
68
62
56
50
44
0
–30
DISTORTION (dB)
68
62
SNR (dB)
S/(N + D) (dB)
S/(N + D) vs Input Frequency
56
50
44
–40
–50
–60
–70
38
38
32
32
–80
26
26
–90
20
20
–100
14
14
10
100
1000
INPUT FREQUENCY (kHz)
10000
1411 G01
2ND
THD
3RD
–110
10
100
1000
INPUT FREQUENCY (kHz)
10000
1411 G02
10
100
1000
INPUT FREQUENCY (kHz)
10000
1411 G03
1411f
4
LTC1411
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TYPICAL PERFOR A CE CHARACTERISTICS
Spurious Free Dynamic Range
vs Input Frequency
S/(N + D) vs Input Frequency
and Amplitude
0
86
–10
80
–20
74
1.0
0dB
0.8
0.6
68
–20dB
62
SINAD (dB)
–40
–50
–60
–70
0.4
56
INL (LSB)
–30
DISTORTION (dB)
Integral Nonlinearity
vs Output Code
50
44
–40dB
38
–80
32
0.2
0
–0.2
–0.4
–0.6
–90
26
–100
20
–0.8
–110
14
–1.0
10
100
1000
INPUT FREQUENCY (kHz)
100
1000
INPUT FREQUENCY (kHz)
10
10000
10000
Differential Nonlinearity
vs Output Code
44
0.6
43
–0.4
40
39
38
37
36
8192
12288
OUTPUT CODE
16384
41.5
39.0
36.5
34.0
35
–50
31.5
–25
25
50
0
TEMPERATURE (°C)
75
1411 G08
4.5
100
4.75
5.0
VDD (V)
Histogram for 4096 Conversions
0
3000
–20
2500
–40
1500
5.5
4096 Points FFT Plot (100kHz)
3500
2000
5.25
1411 G12
1411 G11
AMPLITUDE (dB)
4096
TA = 25°C
44.0
41
–0.8
–1.0
VDD = 5V
42
–0.6
COUNTS
DNL (LSB)
0
–0.2
16384
Supply Current vs Supply Voltage
46.5
SUPPLY CURRENT (mA)
0.8
SUPPLY CURRENT (mA)
45
0.4
8192
12288
OUTPUT CODE
1411 G07
Supply Current vs Temperature
1.0
0.2
4096
1411 G05
1411 G04
0
0
SINAD = 78.8dB
SFDR = 95dB
fSAMPLE = 2.5MHz
fIN = 100kHz
–60
–80
1000
–100
500
–120
–140
0
–1
0
CODE
0
1
1411 G13
250
500
750
1000
INPUT FREQUENCY (kHz)
1250
1411 G14
1411f
5
LTC1411
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TYPICAL PERFOR A CE CHARACTERISTICS
Acquisition Time
vs Source Resistance
4096 Points FFT Plot (1MHz)
0
ACQUISITION TIME (µs)
–20
AMPLITUDE (dB)
100
SINAD = 75dB
SFDR = 81dB
fSAMPLE = 2.5MHz
fIN = 1MHz
–40
–60
–80
–100
10
1
0.1
–120
–140
250
0
500
750
1000
1250
FREQUENCY (kHz)
1411 G15
0.01
1
10
1000 10000 100000
100
SOURCE RESISTANCE (Ω)
1411 G16
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PI FU CTIO S
AIN+ (Pin 1): Positive Analog Input. The ADC converts the
difference voltage between AIN+ and AIN– with programmable input ranges of ±1.8V, ±1.27V, ±0.9V and ±0.64V
depending on PGA selection. AIN+ has common mode
range between 0V and VDD.
AIN– (Pin 2): Negative Analog Input. This pin can be tied
to the REFOUT pin of the ADC or tied to an external DC
voltage. This voltage is also the bipolar zero for the ADC.
AIN– has common mode range between 0V and VDD.
REFOUT (Pin 3): 2.5V Reference Output. Bypass to AGND1
with a 22µF tantalum capacitor if REFOUT is tied to AIN–.
No capacitor is needed if the external reference is used to
drive AIN–.
REFIN (Pin 4): Reference Buffer Input. This pin can be
tied to REFOUT or to an external reference if more
precision is required.
REFCOM1 (Pin 5): Noise Reduction Pin. Put a 10µF
bypass capacitor at this pin to reduce the noise going into
the reference buffer.
REFCOM2 (Pin 6): 4.05V Reference Compensation Pin.
Bypass to AGND1 with a 10µF tantalum capacitor in
parallel with a 0.1µF ceramic.
AGND (Pins 7 to 9): Analog Ground. AGND1 is the ground
for the reference. AGND2 is the ground for the comparator
and AGND3 is the ground for the remaining analog
circuitry.
AVP (Pin 10): 5V Analog Power Supply. Bypass to AGND
with a 10µF tantalum capacitor.
AVM (Pin 11): Analog and Digital Substrate Pin. Tie this
pin to AGND.
D13 to D0 (Pins 12 to 25): Digital Data Outputs. D13 is the
MSB (Most Significant Bit).
OTR (Pin 26): Out-of-the-Range Pin. This pin can be used
in conjunction with D13 to determine if a signal is less than
or greater than the analog input range. If D13 is low and
OTR is high, the analog input to the ADC exceeds the
maximum voltage of the input range.
BUSY (Pin 27): Busy Output. Converter status pin. It is
low during conversion.
OGND (Pin 28): Digital Ground for Output Drivers (Data
Bits, OTR and BUSY).
OVDD (Pin 29): 3V or 5V Digital Power Supply for Output
Drivers (Data Bits, OTR and BUSY). Bypass to OGND with
a 10µF tantalum capacitor.
1411f
6
LTC1411
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PI FU CTIO S
Table 1. Input Spans for LTC1411
DVP (Pin 30): 5V Digital Power Supply Pin. Bypass to
OGND with a 10µF tantalum capacitor.
DGND (Pin 31): Digital Ground.
CONVST (Pin 32): Conversion Start Signal. This active
low signal starts a conversion on its falling edge.
PGA1, PGA0 (Pins 33, 34): Logic Inputs for Programmable Input Range. This ADC has four input ranges (or
four REFCOM2 voltages) controlled by these two pins.
For the logic inputs applied to PGA0 and PGA1, the
following summarizes the gain levels and the analog
input range with AIN– tied to 2.5V.
PGA1
LEVEL
INPUT
SPAN
REFCOM2
VOLTAGE
5V
5V
0dB
±1.8V
4V
5V
0V
– 3dB
±1.28V
2.9V
0V
5V
– 6dB
±0.9V
2V
0V
0V
– 9dB
±0.64V
1.45V
NAP (Pin 35): Nap Input. Driving this pin low will put the
ADC in the Nap mode and will reduce the supply current to
2mA and the internal reference will remain active.
SLP (Pin 36): Sleep Input. Driving this pin low will put the
ADC in the Sleep mode and the ADC draws less than 1µA
of supply current.
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TYPICAL CO
PGA0
ECTIO DIAGRA
5V
+
10
1
2
+
3
30
REFOUT
2.5V
BANDGAP
REFERENCE
+
14-BIT
ADC
–
REFIN
5k
5
OVDD
14
OUTPUT
DRIVERS
INTERNAL
CLOCK
BUSY
OTR
10µF
REFCOM2
+
6
10µF
7, 8, 9
X1.62/
X1.15
AGND
11
+
5V OR 3V
•
•
•
D0
2k
REFCOM1
29
OGND 28
D13
12
5k
+
DVP
AIN–
22µF*
4
AVP
AIN+
25
27
26
CONTROL LOGIC
AVM
36
SLP
35
NAP
34
PGA0
33
PGA1
32
CONVST
31
DGND
1411 TA01
*A 22µF CAPACITOR IS NEEDED IF REFOUT IS USED TO DRIVE AIN–
1411f
7
LTC1411
TEST CIRCUITS
Load Circuits for Access Timing
Load Circuits for Output Float Delay
5V
5V
1k
1k
DN
DN
DN
1k
DN
1k
CL
CL
(A) VOH TO Hi-Z
(B) Hi-Z TO VOL AND VOH TO VOL
(A) Hi-Z TO VOH AND VOL TO VOH
CL
CL
(B) VOL TO Hi-Z
1411 TC02
1411 TC01
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APPLICATIO S I FOR ATIO
CONVERSION DETAILS
The LTC1411 uses a successive approximation algorithm
and an internal sample-and-hold circuit to convert an
analog signal to a 14-bit parallel output. The ADC is
complete with a precision reference, internal clock and a
programmable input range. The device is easy to interface
with microprocessors and DSPs. (Please refer to the
Digital Interface section for the data format.)
Conversions are started by a falling edge on the CONVST
input. Once a conversion cycle has begun, it cannot be
restarted. Between conversions, the ADC acquires the
analog input in preparation for the next conversion. In the
acquire phase, a minimum time of 100ns will provide
enough time for the sample-and-hold capacitors to acquire the analog signal.
10
1
2
AIN+
AIN
AVP
30
DVP
OVDD
–
OGND 28
D13
+
14-BIT
ADC
–
14
OUTPUT
DRIVERS
INTERNAL
CLOCK
D0
OTR
CONTROL LOGIC
SLP
35
NAP
34
PGA0
33
PGA1
32
CONVST
31
12
25
27
26
DGND
Figure 1. Simplified Block Diagram
DYNAMIC PERFORMANCE
The LTC1411 has excellent high speed sampling capability. FFT (Fast Fourier Transform) test techniques are used
to test the ADC’s frequency response, distortion and noise
at the rated throughput. By applying a low distortion sine
wave and analyzing the digital output using an FFT algorithm, the ADC’s spectral content can be examined for
frequencies outside the fundamental. Figure 2a shows a
typical LTC1411 FFT plot.
Signal-to-Noise
•
•
•
BUSY
36
29
During the conversion, the internal differential 14-bit
capacitive DAC output is sequenced by the SAR from the
most significant bit (MSB) to the least significant bit
(LSB). The input is successively compared with the binary
weighted charges supplied by the differential capacitive
DAC. Bit decisions are made by a high speed comparator.
At the end of a conversion, the DAC output balances the
analog input (AIN+ – AIN–). The SAR contents (a 14-bit
data word) which represents the difference of AIN+ and
AIN– are loaded into the 14-bit output latches.
1411 F01
The signal-to-(noise + distortion) ratio [S/N + D)] is the
ratio between the RMS amplitude of the fundamental input
frequency to the RMS amplitude of all other frequency
components at the A/D output. The output is band limited
to frequencies from the above DC and below half the
sampling frequency. Figure 2a shows a typical spectral
content with a 2.5MHz sampling rate and a 100kHz input.
The dynamic performance holds well to higher input
frequencies (see Figure 2b).
1411f
8
LTC1411
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APPLICATIO S I FOR ATIO
0
SINAD = 78.8dB
SFDR = 95dB
fSAMPLE = 2.5MHz
fIN = 100kHz
S/(N + D) (dB)
–40
–60
–80
–100
14
80
13
74
12
68
11
62
10
56
50
44
38
EFFECTIVE BITS
AMPLITUDE (dB)
–20
86
32
–120
26
–140
14
20
0
250
500
750
1000
INPUT FREQUENCY (kHz)
1250
10
100
1000
INPUT FREQUENCY (kHz)
1411 TA02
1411 G14
Figure 2a. LTC1411 Nonaveraged, 4096 Point FFT,
Input Frequency = 100kHz
Figure 3. Effective Bits and Signal/(Noise + Distortion)
vs Input Frequency
0
0
SINAD = 75dB
SFDR = 81dB
fSAMPLE = 2.5MHz
fIN = 1MHz
–20
–10
–20
–30
–40
DISTORTION (dB)
AMPLITUDE (dB)
10000
–60
–80
–40
–50
–60
–70
–80
–100
2ND
THD
–90
–120
3RD
–100
–110
–140
0
250
500
750
1000
1250
FREQUENCY (kHz)
10
100
1000
INPUT FREQUENCY (kHz)
1411 G03
1411 G15
Figure 2b. LTC1411 4096 Point FFT,
Input Frequency = 1MHz
Effective Number of Bits
The effective number of bits (ENOBs) is a measurement of
the resolution of an ADC and is directly related to the
S/(N + D) by the equation:
ENOBS = [S/(N + D) – 1.76]/6.02
where S/(N + D) is expressed in dB. At the maximum
sampling rate of 2.5MHz the LTC1411 maintains good
ENOBs up to the Nyquist input frequency of 1.25MHz.
Refer to Figure␣ 3.
Total Harmonic Distortion
10000
Figure 4. Distortion vs Input Frequency
itself. The out-of-band harmonics alias into the frequency
band between DC and half the sampling frequency. THD is
expressed as:
2
2
2
2
V + V3 + V4 + … VN
THD = 20 log 2
V1
where V1 is the RMS amplitude of the fundamental frequency and V2 through VN are the amplitudes of the
second through Nth harmonics. THD vs input frequency is
shown in Figure 4. The LTC1411 has good distortion
performance up to the Nyquist frequency and beyond.
Total harmonic distortion (THD) is the ratio of the RMS
sum of all harmonics of the input signal to the fundamental
1411f
9
LTC1411
U
W
U U
APPLICATIO S I FOR ATIO
100
The peak harmonic or spurious noise is the largest spectral component excluding the input signal and DC. This
value is expressed in dB relative to the RMS value of a fullscale input signal.
Full-Power and Full-Linear Bandwidth
The full-power bandwidth is that input frequency at which
the amplitude of the reconstructed fundamental is reduced by 3db for a full-scale input signal.
The full-linear bandwidth is the input frequency at which
the S/(N + D) has dropped to 74dB (12 effective bits). The
LTC1411 has been designed to optimize input bandwidth,
allowing the ADC to undersample input signals with frequencies above the converter’s Nyquist frequency. The
noise floor stays very low at high frequencies; S/(N + D)
becomes dominated by distortion at frequencies far beyond Nyquist.
Driving the Analog Input
The differential analog inputs of the LTC1411 are easy to
drive. The inputs may be driven differentially or as a singleended input (i.e., the AIN– input is tied to a fixed DC voltage
such as the REFOUT pin of the LTC1411 or an external
source). Figure 1 shows a simplified block diagram for the
analog inputs of the LTC1411. The AIN+ and AIN– are
sampled at the same instant. Any unwanted signal that is
common mode to both inputs will be reduced by the
common mode rejection of the sample-and-hold circuit.
The inputs draw only one small current spike while charging the sample-and-hold capacitors at the end of conversion. During conversion, the analog inputs draw only a
small leakage current. If the source impedance of the
driving circuits is low, then the LTC1411 inputs can be
driven directly. More acquisition time should be allowed
for a higher impedance source. Figure 5 shows the acquisition time versus source resistance.
Choosing an Input Amplifier
Choosing an input amplifier is easy if a few requirements
are taken into consideration. First, to limit the magnitude
of the voltage spike seen by the amplifier from charging
ACQUISITION TIME (µs)
Peak Harmonic or Spurious Noise
10
1
0.1
0.01
1
10
1000 10000 100000
100
SOURCE RESISTANCE (Ω)
1411 G16
Figure 5. Acquisition Time vs Source Resistance
the sampling capacitor, choose an amplifier that has a low
output impedance (
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