19-1900; Rev 1; 5/06
12-Bit, 40Msps, 3.3V, Low-Power ADC
with Internal Reference
Ordering Information
PART*
TEMP RANGE
Functional Diagram appears at end of data sheet.
PINPACKAGE
PKG
CODE
MAX1421CCM
0°C to +70°C
48 TQFP
C48-2
MAX1421ECM
-40°C to +85°C
48 TQFP
C48-2
MAX1421ECM+
-40°C to +85°C
48 TQFP
C48-2
+Denotes lead-free package.
D10
37
38
39
40
41
REFIN
AVDD
AGND
PD
OE
D11
42
43
44
45
46
AGND
AVDD
CML
REFN
REFP
47
48
Pin Configuration
AGND
AVDD
AVDD
AGND
AGND
1
36
2
35
3
34
4
33
5
32
INP
INN
AGND
AGND
AVDD
AVDD
AGND
6
31
MAX1421
7
30
8
29
D9
D8
D7
D6
DVDD
DVDD
24
DGND
DGND
D5
D4
D3
D2
AGND
AVDD
DVDD
DGND
D0
D1
23
25
22
26
12
21
27
11
20
28
19
9
10
AGND
AVDD
IF and Baseband Digitization
♦ Space-Saving 48-Pin TQFP Package
18
Radar
♦ Power-Down Modes
180mW (Reference Shutdown Mode)
10µW (Shutdown Mode)
17
Data Acquisition
♦ Differential, Wideband Input T/H Amplifier
16
CCD Pixel Processing
♦ Internal, 2.048V Precision Bandgap Reference
15
Medical Ultrasound Imaging
♦ 66dB SNR at fIN = 15MHz
AVDD
AGND
CLK
CLK
________________________Applications
♦ 67dB SNR at fIN = 5MHz
14
In addition to low operating power, the MAX1421 features two power-down modes, a reference power-down
and a shutdown mode. In reference power-down, the
internal bandgap reference is deactivated, resulting in
a typical 2mA supply current reduction. For idle periods, a full shutdown mode is available to maximize
power savings.
The MAX1421 provides parallel, offset binary, CMOScompatible three-state outputs.
The MAX1421 is available in a 7mm x 7mm, 48-pin
TQFP package, and is specified over the commercial
(0°C to +70°C) and the extended industrial (-40°C to
+85°C) temperature ranges.
Pin-compatible higher- and lower-speed versions of the
MAX1421 are also available. Please refer to the
MAX1420 data sheet for a frequency of 60Msps and
the MAX1422 data sheet for a frequency of 20Msps.
♦ Single 3.3V Power Supply
13
The MAX1421 is a 3.3V, 12-bit analog-to-digital converter (ADC), featuring a fully-differential input,
pipelined, 12-stage ADC architecture with wideband
track-and-hold (T/H) and digital error correction incorporating a fully-differential signal path. The MAX1421 is
optimized for low-power, high-dynamic performance
applications in imaging and digital communications.
The converter operates from a single 3.3V supply, consuming only 188mW while delivering a typical signal-tonoise ratio (SNR) of 66dB at an input frequency of
15MHz and a sampling frequency of 40Msps. The fullydifferential input stage has a small signal -3dB bandwidth of 400MHz and may be operated with
single-ended inputs.
An internal 2.048V precision bandgap reference sets
the full-scale range of the ADC. A flexible reference
structure accommodates an internal or externally
applied buffered or unbuffered reference for applications requiring increased accuracy or a different input
voltage range.
Features
48-TQFP
________________________________________________________________ Maxim Integrated Products
For pricing, delivery, and ordering information, please contact Maxim/Dallas Direct! at
1-888-629-4642, or visit Maxim’s website at www.maxim-ic.com.
1
MAX1421
General Description
MAX1421
12-Bit, 40Msps, 3.3V, Low-Power ADC
with Internal Reference
ABSOLUTE MAXIMUM RATINGS
AVDD, DVDD to AGND ..............................................-0.3V to +4V
DVDD, AVDD to DGND..............................................-0.3V to +4V
DGND to AGND.....................................................-0.3V to +0.3V
INP, INN, REFP, REFN, REFIN,
CML, CLK, CLK,....................(AGND - 0.3V) to (AVDD + 0.3V)
D0–D11, OE, PD .......................(DGND - 0.3V) to (DVDD + 0.3V)
Continuous Power Dissipation (TA = +70°C)
48-Pin TQFP (derate 12.5mW/°C above +70°C)........1000mW
Maximum Junction Temperature .....................................+150°C
Operating Temperature Ranges
MAX1421CCM ...................................................0°C to +70°C
MAX1421ECM ................................................-40°C to +85°C
Storage Temperature Range .............................-65°C to +150°C
Lead Temperature (soldering, 10s) .................................+300°C
Stresses beyond those listed under “Absolute Maximum Ratings” may cause permanent damage to the device. These are stress ratings only, and functional
operation of the device at these or any other conditions beyond those indicated in the operational sections of the specifications is not implied. Exposure to
absolute maximum rating conditions for extended periods may affect device reliability.
ELECTRICAL CHARACTERISTICS
(V AVDD = V DVDD = 3.3V, AGND = DGND = 0, V IN = ±1.024V, differential input voltage at -0.5dB FS, internal reference,
fCLK = 40MHz (50% duty cycle), digital output load CL ≈ 10pF, TA ≥ +25°C guaranteed by production test, TA < +25°C guarnateed
by design and characterization. Typical values are at TA = +25°C.)
PARAMETER
SYMBOL
CONDITIONS
MIN
TYP
MAX
UNITS
DC ACCURACY
Resolution
RES
Differential Nonlinearity
DNL
Integral Nonlinearity
INL
Midscale Offset
Midscale Offset Temperature
Coefficient
Gain Error
12
TA = +25°C, no missing codes
±0.5
TA = TMIN to TMAX
±2
MSO
-3
GETC
±.75
+3
%FSR
%/°C
Internal reference (Note 1)
-5
+0.1
+5
External reference applied to REFIN (Note 2)
-5
±3
+5
-1.5
±0.5
+1.5
External reference applied to REFP, CML,
and REFN (Note 3)
LSB
LSB
3 ✕ 10-4
MSOTC
GE
Bits
+1
TA = TMIN to TMAX
External reference applied to REFP, CML,
and REFN (Note 3)
Gain-Error Temperature
Coefficient
-1
15 ✕ 10-6
%FSR
%/°C
DYNAMIC PERFORMANCE (fCLK = 40MHz, 4096-point FFT)
2
Signal-to-Noise Ratio
SNR
Spurious-Free Dynamic Range
SFDR
Total Harmonic Distortion
THD
Signal-To-Noise Plus Distortion
SINAD
Effective Number of Bits
ENOB
Two-Tone Intermodulation
Distortion
IMDTT
fIN = 5MHz
fIN = 15MHz, TA = +25°C
67
62
66
64
70
fIN = 5MHz
fIN = 15MHz, TA = +25°C
73
fIN = 5MHz
-74
fIN = 15MHz, TA = +25°C
-69
fIN = 5MHz
fIN = 15MHz, TA = +25°C
66
60
fIN = 5MHz
fIN = 15MHz, TA = +25°C
fIN1 = 11.569MHz, fIN2 = 13.445MHz
(Note 4)
dB
63.5
10.7
60
10.3
-80
_______________________________________________________________________________________
dBc
-62
dBc
dB
Bits
dBc
12-Bit, 40Msps, 3.3V, Low-Power ADC
with Internal Reference
(V AVDD = V DVDD = 3.3V, AGND = DGND = 0, VIN = ±1.024V, differential input voltage at -0.5dB FS, internal reference,
fCLK = 40MHz (50% duty cycle), digital output load CL ≈ 10pF, TA ≥ +25°C guaranteed by production test, TA < +25°C guarnateed
by design and characterization. Typical values are at TA = +25°C.)
PARAMETER
SYMBOL
CONDITIONS
MIN
TYP
MAX
UNITS
Differential Gain
DG
±1
%
Differential Phase
DP
±0.25
degrees
32.5
kΩ
ANALOG INPUTS (INP, INN, CML)
Input Resistance
RIN
Either input to ground
Input Capacitance
CIN
Either input to ground
Common-Mode Input Level
(Note 5)
Common-Mode Input Voltage
Range (Note 5)
Differential Input Range
4
pF
VCML
VAVDD
× 0.5
V
VCMVR
VCML
±5%
V
±VDIFF
V
VIN
VINP - VINN (Note 6)
Small-Signal Bandwidth
BW-3dB
(Note 7)
400
MHz
Large-Signal Bandwidth
FPBW-3dB
(Note 7)
150
MHz
1
Clock
Cycle
Overvoltage Recovery
OVR
1.5 ✕ FS input
INTERNAL REFERENCE (REFIN bypassed with 0.22µF in parallel with 1nF)
Common-Mode Reference Input
Voltage
VCML
At CML
VAVDD
✕ 0.5
V
Positive Reference Voltage
Range
VREFP
At REFP
VCML
+ 0.512
V
Negative Reference Voltage
Range
VREFN
At REFN
VCML
- 0.512
V
Differential Reference Voltage
Range
VDIFF
(Note 6)
1.024
±5%
V
±100
ppm/°C
Differential Reference
Temperature Coefficient
REFTC
EXTERNAL REFERENCE (VREFIN = 2.048V)
REFIN Input Resistance
RIN
REFIN Input Capacitance
CIN
10
pF
VREFIN
2.048
±10%
V
1.08 ✕
0.92 ✕
VREFIN /
VREFIN /
VREFIN /
2
2
2
V
REFIN Reference Input Voltage
Differential Reference Voltage
VDIFF
(Note 8)
(Note 6)
5
kΩ
EXTERNAL REFERENCE (VREFIN = AGND, reference voltage applied to REFP, REFN, and CML)
REFP, REFN, CML Input Current
IIN
REFP, REFN, CML Input
Capacitance
CIN
-200
+200
15
µA
pF
_______________________________________________________________________________________
3
MAX1421
ELECTRICAL CHARACTERISTICS (continued)
MAX1421
12-Bit, 40Msps, 3.3V, Low-Power ADC
with Internal Reference
ELECTRICAL CHARACTERISTICS (continued)
(V AVDD = V DVDD = 3.3V, AGND = DGND = 0, VIN = ±1.024V, differential input voltage at -0.5dB FS, internal reference,
fCLK = 40MHz (50% duty cycle), digital output load CL ≈ 10pF, TA ≥ +25°C guaranteed by production test, TA < +25°C guarnateed
by design and characterization. Typical values are at TA = +25°C.)
PARAMETER
SYMBOL
Differential Reference Voltage
Range
VDIFF
CML Input Voltage Range
CONDITIONS
MIN
TYP
MAX
UNITS
1.024
±10%
V
VCML
1.65
±10%
V
REFP Input Voltage Range
VREFP
VCML +
VDIFF / 2
V
REFN Input Voltage Range
VREFN
VCML VDIFF / 2
V
(Note 6)
DIGITAL INPUTS (CLK, CLK, OE, PD)
Input Logic-High
VIH
Input Logic-Low
VIL
0.7 ✕
VDVDD
0.3 ✕
VDVDD
V
+20
µA
±330
CLK, CLK
Input Current
V
PD
-20
OE
-20
Input Capacitance
+20
10
pF
DIGITAL OUTPUTS (D0–D11)
Output Logic-High
VOH
IOH = 200µA
VDVDD
- 0.5
VDVDD
V
Output Logic-Low
VOL
IOL = -200µA
0
0.5
V
+10
µA
Three-State Leakage
-10
Three-State Capacitance
2
pF
POWER REQUIREMENTS
Analog Supply Voltage
VAVDD
3.135
3.3
3.465
Digital Supply Voltage
VDVDD
2.7
3.3
3.6
V
Analog Supply Current
IAVDD
52
65
mA
50
63
mA
20
µA
Analog Supply Current with
Internal Reference in Shutdown
REFIN = AGND
Analog Shutdown Current
PD = DVDD
Digital Supply Current
IDVDD
Digital Shutdown Current
5.5
PD = DVDD
V
mA
20
µA
214
mW
Power Dissipation
PDISS
Analog power
188
Power-Supply Rejection Ratio
PSRR
(Note 9)
±1
Clock Frequency
fCLK
Figure 5
Clock High
tCH
Figure 5, clock period 25ns
12.5
ns
Clock Low
tCL
Figure 5, clock period 25ns
12.5
ns
mV/V
TIMING CHARACTERISTICS
4
0.1
40.0
_______________________________________________________________________________________
MHz
12-Bit, 40Msps, 3.3V, Low-Power ADC
with Internal Reference
(V AVDD = V DVDD = 3.3V, AGND = DGND = 0, VIN = ±1.024V, differential input voltage at -0.5dB FS, internal reference,
fCLK = 40MHz (50% duty cycle), digital output load CL ≈ 10pF, TA ≥ +25°C guaranteed by production test, TA < +25°C guarnateed
by design and characterization. Typical values are at TA = +25°C.)
PARAMETER
SYMBOL
Pipeline Delay (Latency)
CONDITIONS
MIN
TYP
Figure 5
7
MAX
UNITS
fCLK
cycles
Aperture Delay
tAD
Figure 9
2
ns
Aperture Jitter
tAJ
Figure 9
2
ps
Data Output Delay
tOD
Figure 5
Bus Enable Time
tBE
Figure 4
5
ns
Bus Disable Time
tBD
Figure 4
5
ns
Note 1:
Note 2:
Note 3:
Note 4:
Note 5:
Note 6:
Note 7:
Note 8:
Note 9:
5
10
14
ns
Internal reference, REFIN bypassed to AGND with a combination of 0.22µF in parallel with 1nF capacitor.
External 2.048V reference applied to REFIN.
Internal reference disabled. VREFIN = 0, VREFP = 2.162V, VCML = 1.65V, and VREFN = 1.138V.
IMD is measured with respect to either of the fundamental tones.
Specifies the common-mode range of the differential input signal supplied to the MAX1421.
VDIFF = VREFP - VREFN.
Input bandwidth is measured at a 3dB level.
VREFIN is internally biased to 2.048V through a 10kΩ resistor.
Measured as the ratio of the change in mid-scale offset voltage for a ± 5% change in VAVDD using the internal reference.
Typical Operating Characteristics
(VAVDD = VDVDD = 3.3V, AGND = DGND = 0, VIN = ±1.024V, differential input voltage, fCLK = 40MHz (50% duty cycle), digital output
load CL = 10pF, TA = TMIN to TMAX, unless otherwise noted. Typical values are at TA = +25°C.)
-60
HD3
HD2
-80
-100
-40
HD2
HD3
-60
-80
-100
-120
5
10
15
ANALOG INPUT FREQUENCY (MHz)
20
fIN = 38.54MHz
AIN = -0.49dB FS
-20
-40
HD2
-60
HD3
-80
-100
-120
0
0
MAX1421 toc03
fIN
FFT PLOT, 4096-POINT RECORD,
DIFFERENTIAL INPUT
AMPLITUDE (dB)
AMPLITUDE (dB)
-40
fIN = 19.90MHz
AIN = -0.50dB FS
-20
AMPLITUDE (dB)
fIN = 7.54MHz
AIN = -0.45dB FS
-20
0
MAX1421 toc01
0
FFT PLOT, 4096-POINT RECORD,
DIFFERENTIAL INPUT
MAX1421 toc02
FFT PLOT, 4096-POINT RECORD,
DIFFERENTIAL INPUT
-120
0
5
10
15
ANALOG INPUT FREQUENCY (MHz)
20
0
5
10
15
20
ANALOG INPUT FREQUENCY (MHz)
_______________________________________________________________________________________
5
MAX1421
ELECTRICAL CHARACTERISTICS (continued)
Typical Operating Characteristics (continued)
(VAVDD = VDVDD = 3.3V, AGND = DGND = 0, VIN = ±1.024V, differential input voltage, fCLK = 40MHz (50% duty cycle), digital output
load CL = 10pF, TA = TMIN to TMAX, unless otherwise noted. Typical values are at TA = +25°C.)
SPURIOUS-FREE DYNAMIC RANGE
vs. ANALOG INPUT FREQUENCY
77
66
-40
-60
IM3
69
61
IM2
-80
53
-100
0
2
4
6
8
50
0
10 12 14 16 18 20
58
54
45
-120
62
5 10 15 20 25 30 35 40 45 50 55
ANALOG INPUT FREQUENCY (MHz)
ANALOG INPUT FREQUENCY (MHz)
0
SPURIOUS-FREE DYNAMIC RANGE
vs. ANALOG INPUT POWER (fIN = 15MHz)
SIGNAL-TO-NOISE PLUS DISTORTION
vs. ANALOG INPUT FREQUENCY
TOTAL HARMONIC DISTORTION
vs. ANALOG INPUT FREQUENCY
-56
80
MAX1421 toc08
70
MAX1421 toc07
-50
5 10 15 20 25 30 35 40 45 50 55
ANALOG INPUT FREQUENCY (MHz)
66
MAX1421 toc09
fIN2
SNR (dB)
fIN1
SFDR (dBc)
AMPLITUDE (dB)
70
MAX1421 toc05
fIN1 = 11.51MHz
fIN2 = 13.51MHz
AIN1 = AIN2 = -6.5dB FS
-20
85
MAX1421 toc04
0
SIGNAL-TO-NOISE RATIO
vs. ANALOG INPUT FREQUENCY
MAX1421 toc06
TWO-TONE IMD, 8192-POINT RECORD,
DIFFERENTIAL INPUT
70
-68
62
SFDR (dBc)
-62
SINAD (dB)
THD (dBc)
60
58
50
40
54
30
50
-80
SIGNAL-TO-NOISE RATIO
vs. ANALOG INPUT POWER (fIN = 15MHz)
-30
-40
60
THD (dBc)
50
40
30
-50
-40
-30
-20
-10
ANALOG INPUT POWER (dBFS)
0
SIGNAL-TO-NOISE PLUS DISTORTION
vs. ANALOG INPUT POWER (fIN = 15MHz)
80
70
60
-50
-60
20
50
40
30
20
-70
10
10
0
0
-80
-60
6
-60
SINAD (dB)
70
5 10 15 20 25 30 35 40 45 50 55
ANALOG INPUT FREQUENCY (MHz)
TOTAL HARMONIC DISTORTION
vs. ANALOG INPUT POWER (fIN = 15MHz)
MAX1421 toc10
80
20
0
5 10 15 20 25 30 35 40 45 50 55
ANALOG INPUT FREQUENCY (MHz)
MAX1421 toc11
0
MAX1421 toc12
-74
SNR (dB)
MAX1421
12-Bit, 40Msps, 3.3V, Low-Power ADC
with Internal Reference
-50
-40
-30
-20
-10
ANALOG INPUT POWER (dBFS)
0
-60
-50
-40
-30
-20
-10
ANALOG INPUT POWER (dBFS)
0
-60
-50
-40
-30
-20
-10
ANALOG INPUT POWER (dBFS)
_______________________________________________________________________________________
0
12-Bit, 40Msps, 3.3V, Low-Power ADC
with Internal Reference
fIN = 5.52MHz
68
66
64
68
62
64
60
85
-80
-40
-15
TEMPERATURE (°C)
fIN = 5.52MHz
60
64
62
60
-15
10
35
-15
INTEGRAL NONLINEARITY
NONLINEARITY
vs.INTEGRAL
DIGITAL OUTPUT
CODE
vs. DIGITAL OUTPUT CODE
2.0
2.0
1.5
1.5
1.0
1.0
0.5
0.5
0
0
-0.5
-0.5
-1.0
-1.0
-1.5
-1.5
-2.0
-2.00
60
35
60
DIFFERENTIAL NONLINEARITY
vs. DIGITAL OUTPUT CODE
0.500
0.375
0.125
0
-0.125
-0.250
-0.375
1024
2048
3072
1024
2048 CODE 3072
DIGITAL OUTPUT
4096
4096
-0.500
0
1024
2048
3072
TEMPERATURE (°C)
DIGITAL OUTPUT CODE
DIGITAL OUTPUT CODE
GAIN ERROR vs. TEMPERATURE, EXTERNAL
REFERENCE (VREFIN = 2.048V)
ANALOG SUPPLY CURRENT
vs. TEMPERATURE
DIGITAL SUPPLY CURRENT
vs. TEMPERATURE
0.25
12
MAX1421 toc20
70
MAX1421 toc19
0.50
CL ≈ 10pF
-0.25
50
40
-0.50
-15
10
35
TEMPERATURE (°C)
60
85
8
6
30
-40
4096
10
IAVDD (mA)
IAVDD (mA)
60
0
85
0.250
0
85
10
TEMPERATURE (°C)
DNL (LSB)
66
-40
-40
85
INL (LSB)
INL (LSB)
68
MAX1421 toc16
70
SINAD (dB)
35
TEMPERATURE (°C)
SIGNAL-TO-NOISE PLUS DISTORTION
vs. TEMPERATURE
GAIN ERROR (%FSR)
10
MAX1421 toc18
35
MAX1421 toc17
10
MAX1421 toc17
-15
-76
-78
60
-40
-74
MAX1421 toc21
72
fIN = 5.52MHz
-72
THD (dBc)
SNR (dB)
SFDR (dBc)
76
-70
MAX1421 toc14
fIN = 5.52MHz
80
70
MAX1421 toc13
84
TOTAL HARMONIC DISTORTION
vs. TEMPERATURE
SIGNAL-TO-NOISE RATIO
vs. TEMPERATURE
MAX1421 toc15
SPURIOUS-FREE DYNAMIC RANGE
vs. TEMPERATURE
4
-40
-15
10
35
TEMPERATURE (°C)
60
85
-40
-15
10
35
60
85
TEMPERATURE (°C)
________________________________________________________________________________________
7
MAX1421
Typical Operating Characteristics (continued)
(VAVDD = VDVDD = 3.3V, AGND = DGND = 0, VIN = ±1.024V, differential input voltage, fCLK = 40MHz (50% duty cycle), digital output
load CL = 10pF, TA = TMIN to TMAX, unless otherwise noted. Typical values are at TA = +25°C.)
Typical Operating Characteristics (continued)
(VAVDD = VDVDD = 3.3V, AGND = DGND = 0, VIN = ±1.024V, differential input voltage, fCLK = 40MHz (50% duty cycle), digital output
load CL = 10pF, TA = TMIN to TMAX, unless otherwise noted. Typical values are at TA = +25°C.)
INTERNAL REFERENCE VOLTAGE
vs. ANALOG SUPPLY VOLTAGE
SNR/SINAD, -THD/SFDR
vs. CLOCK FREQUENCY
SFDR
75
2.0750
70
MAX1421 toc23
-THD
MAX1421 toc22
2.0625
65
SNR
60
VREFIN (V)
SNR/SINAD, -THD/SFDR (dB, dBc)
80
SINAD
2.0500
55
50
2.0375
45
fIN = 5MHz
2.0250
40
15
20
25
30
35
3.1
40
3.2
3.3
3.4
3.5
VDD (V)
CLOCK FREQUENCY (MHz)
INTERNAL REFERENCE VOLTAGE
vs. TEMPERATURE
OUTPUT NOISE HISTOGRAM (DC-INPUT)
50,000
MAX1421 toc24
2.10
2.08
MAX1421 toc25
10
43707
45,000
40,000
COUNTS
35,000
VREFIN (V)
MAX1421
12-Bit, 40Msps, 3.3V, Low-Power ADC
with Internal Reference
2.06
30,000
25,000
20,000
2.04
15,000
5000
0
2.00
-40
-15
10
35
TEMPERATURE (°C)
8
10824
10709
10,000
2.02
60
85
1
N-3
179
N-2 N-1
N
N+1
116
0
N+2
N+3
DIGITAL OUTPUT NOISE
_______________________________________________________________________________________
12-Bit, 40Msps, 3.3V, Low-Power ADC
with Internal Reference
PIN
NAME
FUNCTION
1, 4, 5, 8,
9, 12, 13,
16, 19, 41,
48
AGND
Analog Ground. Connect all return paths for analog signals to AGND.
2, 3, 10,
11, 14, 15,
20, 42, 47
AVDD
Analog Supply Voltage. For optimum performance bypass each pin to the closest AGND with a
parallel combination of a 0.1µF and a 1nF capacitor. Connect a single 10µF and 1µF capacitor
combination between AVDD and AGND.
6
INP
Positive Analog Signal Input
7
INN
Negative Analog Signal Input
17
CLK
Clock Frequency Input. Clock frequency input ranges from 100kHz to 40MHz.
18
CLK
Complementary Clock Frequency Input. This input is used for differential clock inputs. If the ADC is
driven with a single-ended clock, bypass CLK with a 0.1µF capacitor to AGND.
21, 31, 32,
DVDD
Digital Supply Voltage. For optimum performance bypass each pin to the closest DGND with a
parallel combination of a 0.1µF and a 1nF capacitor. Connect a single 10µF and 1µF capacitor
combination between DVDD and DGND.
22, 29, 30
DGND
Digital Ground
23–28
D0–D5
Digital Data Outputs. Data bits D0 through D5, where D0 represents the LSB.
33–38
D6–D11
Digital Data Outputs. D6 through D11, where D11 represents the MSB.
39
OE
Output Enable Input. A logic “1” on OE places the outputs D0–D11 into a high-impedance state. A
logic “0” allows for the data bits to be read from the outputs.
40
PD
Shutdown Input. A logic “1” on PD places the ADC into shutdown mode.
43
REFIN
External Reference Input. Bypass to AGND with a capacitor combination of 0.22µF in parallel with
1nF. REFIN can be biased externally to adjust reference levels and calibrate full-scale errors. To
disable the internal reference, connect REFIN to AGND.
44
REFP
Positive Reference I/O. Bypass to AGND with a capacitor combination of 0.22µF in parallel with 1nF.
With the internal reference disabled (REFIN = AGND), REFP should be biased to VCML + VDIFF / 2.
45
REFN
Negative Reference I/O. Bypass to AGND with a capacitor combination of 0.22µF in parallel with 1nF.
With the internal reference disabled (REFIN = AGND), REFN should be biased to VCML - VDIFF / 2.
46
CML
Common-Mode Level Input. Bypass to AGND with a capacitor combination of 0.22µF in parallel
with 1nF. With the internal reference disabled (REFIN = AGND).
_______________________________________________________________________________________
9
MAX1421
Pin Description
MAX1421
12-Bit, 40Msps, 3.3V, Low-Power ADC
with Internal Reference
Detailed Description
The MAX1421 uses a 12-stage, fully-differential, pipelined architecture (Figure 1) that allows for high-speed
conversion while minimizing power consumption. Each
sample moves through a pipeline stage every halfclock-cycle. Including the delay through the output latch,
the latency is seven clock cycles.
A 2-bit (2-comparator) flash ADC converts the heldinput voltage into a digital code. The following digital-toanalog converter (DAC) converts the digitized result
back into an analog voltage, which is then subtracted
from the original held-input signal. The resulting error
signal is then multiplied by two, and the product is
passed along to the next pipeline stage. This process is
repeated until the signal has been processed by all 12
stages. Each stage provides a 1-bit resolution. Digital
error correction compensates for ADC comparator offsets in each pipeline stage and ensures no missing
codes.
Input Track-and-Hold Circuit
Figure 2 displays a simplified functional diagram of the
input T/H circuit in both track-and-hold modes. In track
mode, switches S1, S2a, S2b, S4a, S4b, S5a, and S5b
are closed. The fully differential circuit passes the input
signal to the two capacitors (C2a and C2b) throughswitches (S4a and S4b). Switches S2a and S2b set the
common mode for the transconductance amplifier
(OTA) input, and open simultaneously with S1, sampling the input waveform. The resulting differential voltage is held on capacitors C2a and C2b. Switches S4a
and S4b are then opened before switches S3a and S3b,
connecting capacitors C1a and C1b to the output of the
amplifier, and switch S4c is closed. The OTA is used to
charge capacitors C1a and C1b to the same values
originally held on C2a and C2b. This value is then presented to the first-stage quantizer and isolates the
pipeline from the fast-changing input. The wide-input
bandwidth, T/H amplifier allows the MAX1421 to track
and sample/hold analog inputs of high frequencies
beyond Nyquist. The analog inputs INP and INN can be
driven either differentially or single-ended. Match the
impedance of INP and INN and set the common-mode
voltage to midsupply (AVDD / 2) for optimum performance.
Analog Input and Reference Configuration
The full-scale range of the MAX1421 is determined by
the internally generated voltage difference between
REFP (AVDD / 2 + VREFIN / 4) and REFN (AVDD / 2 V REFIN / 4). The MAX1421’s full-scale range is
adjustable through REFIN, which provides a high input
impedance for this purpose. REFP, CML (AVDD / 2), and
REFN are internally buffered low impedance outputs.
MDAC
VIN
INTERNAL
BIAS
Σ
T/H
x2
S5a
S2a
C1a
TO NEXT
STAGE
FLASH
ADC
CML
VOUT
S3a
DAC
S4a
2 BITS
OUT
C2a
S4c
VIN
STAGE 1
STAGE 2
STAGE 12
S1
OTA
OUT
S4b
C2b
C1b
S3b
DIGITAL CORRECTION LOGIC
Figure 1. Pipelined Architecture
10
12
S2b
D11–D0
INTERNAL
BIAS
Figure 2. Internal Track-and-Hold Circuit
______________________________________________________________________________________
S5b
CML
12-Bit, 40Msps, 3.3V, Low-Power ADC
with Internal Reference
MAX1421
AVDD
50Ω
(AV2DD)
CML
R
0.22µF
1nF
AVDD
2
50Ω
R
REFP
MAX4284
R
R
AVDD
2
0.22µF
(AV2DD + 1V)
1nF
MAX1421
AVDD
4
R
50Ω
R
(
)
REFN AVDD + 1V
2
MAX4284
0.22µF
1nF
R
AVDD
4
REFIN
R
AGND
+1V
Figure 3. Unbuffered External Reference Drive—Internal Reference Disabled
The MAX1421 provides three modes of reference operation:
• Internal reference mode
•
Buffered external reference mode
•
Unbuffered external reference mode
In internal reference mode, the on-chip +2.048V
bandgap reference is active and REFIN, REFP, CML,
and REFN are left floating. For stability purposes,
bypass REFIN, REFP, REFN, and CML with a capacitor
network of 0.22µF, in parallel with a 1nF capacitor to
AGND.
In buffered external reference mode, the reference voltage levels can be adjusted externally by applying a
stable and accurate voltage at REFIN.
In unbuffered external reference mode, REFIN is connected to AGND, which deactivates the on-chip buffers
of REFP, CML, and REFN. With their buffers shut down,
these nodes become high impedance and can be driven by external reference sources, as shown in Figure 3.
Clock Inputs (CLK, CLK)
The MAX1421’s CLK and CLK inputs accept both single-ended and differential input operation, and accept
CMOS-compatible clock signals. If CLK is driven with a
single-ended clock signal, bypass CLK with a 0.1µF
capacitor to AGND. Since the interstage conversion of
the device depends on the repeatability of the rising
and falling edges of the external clock, use a clock with
low jitter and fast rise and fall times (< 2ns). In particular, sampling occurs on the rising edge of the clock signal, requiring this edge to have the lowest possible
jitter. Any significant aperture jitter limits the SNR performance of the ADC according to the following relationship:
1
SNRdB = 20 × log10
2π × ƒIN × t AJ
where fIN represents the analog input frequency and
tAJ is the aperture jitter.
Clock jitter is especially critical for high input frequency
applications. The clock input should always be considered as an analog input and routed away from any analog or digital signal lines.
The MAX1421 clock input operates with a voltage
threshold set to AVDD / 2. Clock inputs must meet the
specifications for high and low periods, as stated in the
Electrical Characteristics.
______________________________________________________________________________________
11
MAX1421
12-Bit, 40Msps, 3.3V, Low-Power ADC
with Internal Reference
OE
INP
D11–D0
ADC
INN
tBD
tBE
AVDD
OUTPUT
DATA D11–D0
10kΩ
HIGH-Z
VALID DATA
10kΩ
Figure 5. Output Enable Timing
CLK
10kΩ
10kΩ
CLK
MAX1421
AGND
Figure 4. Simplified Clock Input Circuit
Figure 4 shows a simplified model of the clock input circuit. This circuit consists of two 10kΩ resistors to bias
the common-mode level of each input. This circuit may
be used to AC-couple the system clock signal to the
MAX1421 clock input.
Output Enable (OE), Power-Down (PD), and
Output Data (D0–D11)
With OE high, the digital outputs enter a high-impedance state. If OE is held low with PD high, the outputs
are latched at the last value prior to the power-down. All
data outputs, D0 (LSB) through D11 (MSB), are
TTL/CMOS-logic compatible. There is a seven clock-
Table 1. MAX1421 Output Code for
Differential Inputs
DIFFERENTIAL
INPUT VOLTAGE*
DIFFERENTIAL
INPUT
OFFSET
BINARY
VREF × 2047/2048
+FULL SCALE 1LSB
1111 1111 1111
VREF × 2046/2048
+FULL SCALE 2LSB
1111 1111 1110
VREF × 1/2048
+ 1 LSB
1000 0000 0001
0
Bipolar Zero
1000 0000 0000
-VREF × 1/2048
- 1 LSB
0111 1111 1111
-VREF × 2046/2048
-FULL SCALE +
1 LSB
0000 0000 0001
-VREF × 2047/2048
-FULL SCALE
0000 0000 0000
cycle latency between any particular sample and its
valid output data. The output coding is in offset binary
format (Table 1).
The capacitive load on the digital outputs D0 through
D11 should be kept as low as possible (≤ 10pF), to avoid
large digital currents that could feed back into the analog
portion of the MAX1421, thereby degrading its dynamic
performance. The use of digital buffers (e.g.,
74LVCH16244) on the digital outputs of the ADC can further isolate the digital outputs from heavy capacitive
loads. To further improve the dynamic performance of
the MAX1421, add small-series resistors of 100Ω to the
digital output paths, close to the ADC. Figure 5 displays
the timing relationship between output enable and data
output.
System Timing Requirements
Figure 6 depicts the relationship between the clock
input, analog input, and data output. The MAX1421
samples at the rising edge of CLK (falling edge of CLK)
and output data is valid seven clock cycles (latency)
later. Figure 6 also displays the relationship between
the input clock parameters and the valid output data.
Applications Information
Figure 7 depicts a typical application circuit containing
a single-ended to differential converter. The internal reference provides an AVDD / 2 output voltage for levelshifting purposes. The input is buffered and then split to
a voltage follower and inverter. A lowpass filter at the
input suppresses some of the wideband noise associated with high-speed op amps. Select the RISO and CIN
values to optimize the filter performance and to suit a
particular application. For the application in Figure 7, a
RISO of 50Ω is placed before the capacitive load to prevent ringing and oscillation. The 22pF CIN capacitor
acts as a small bypassing capacitor.
Connecting CIN from INN to INP may further improve
dynamic performance.
*VREF = VREFP - VREFN
12
HIGH-Z
______________________________________________________________________________________
12-Bit, 40Msps, 3.3V, Low-Power ADC
with Internal Reference
MAX1421
7 CLOCK-CYCLE LATENCY
N
N+1
N+2
N+3
N+4
N+5
N+6
ANALOG INPUT
CLK
tDO
tCH
tCL
CLK
DATA OUTPUT
N-7
N-6
N-5
N-4
N-3
N-2
N-1
N
Figure 6. System and Output Timing Diagram
Using Transformer Coupling
An RF transformer (Figure 8) provides an excellent solution to convert a single-ended signal to a fully differential signal, required by the MAX1421 for optimum
performance. Connecting the center tap of the transformer to CML provides an AVDD / 2 DC level shift to
the input. Although a 1:1 transformer is shown, a 1:2 or
1:4 step-up transformer may be selected to reduce the
drive requirements.
In general, the MAX1421 provides better SFDR and
THD with fully differential input signals over singleended input signals, especially for very high input frequencies. In differential input mode, even-order
harmonics are suppressed and each of the inputs
requires only half the signal swing compared to singleended mode.
Single-Ended AC-Coupled Input Signal
Figure 9 shows an AC-coupled, single-ended application, using a MAX4108 op amp. This configuration provides high-speed, high-bandwidth, low noise, and low
distortion to maintain the integrity of the input signal.
Grounding, Bypassing, and
Board Layout
The MAX1421 requires high-speed board layout design
techniques. Locate all bypass capacitors as close to
the device as possible, preferably on the same side of
the board as the ADC, using surface-mount devices for
minimum inductance. Bypass REFP, REFN, REFIN, and
CML with a parallel network of 0.22µF capacitors and
1nF to AGND. AVDD should be bypassed with a similar
network of a 10µF bipolar capacitor in parallel with two
ceramic capacitors of 1nF and 0.1µF. Follow the same
rules to bypass the digital supply DV DD to DGND.
Multilayer boards with separate ground and power
planes produce the highest level of signal integrity.
Consider the use of a split ground plane arrangement
to match the physical location of the analog ground
(AGND) and the digital output driver ground (DGND) on
the ADCs package. The two ground planes should be
joined at a single point so that the noisy digital ground
currents do not interfere with the analog ground plane.
Alternatively, all ground pins could share the same
ground plane, if the ground plane is sufficiently isolated
from any noisy, digital systems ground plane (e.g.,
downstream output buffer, DSP ground plane). Route
high-speed digital signal traces away from sensitive
analog traces and remove digital ground and power
planes from underneath digital outputs. Keep all signal
lines short and free of 90 degree turns.
Static Parameter Definitions
Integral Nonlinearity (INL)
Integral nonlinearity is the deviation of the values on an
actual transfer function from a straight line. This straightline can be either a best straight-line fit or a line drawn
between the endpoints of the transfer function, once offset and gain errors have been nullified. The static linearity parameters for the MAX1421 are measured using
the best straight-line fit method.
Differential Nonlinearity (DNL)
Differential nonlinearity is the difference between an
actual step-width and the ideal value of 1LSB. A DNL
______________________________________________________________________________________
13
MAX1421
12-Bit, 40Msps, 3.3V, Low-Power ADC
with Internal Reference
+5V
0.1µF
LOWPASS FILTER
INP
MAX4108
RISO
50Ω
0.1µF
300Ω
CIN*
22pF
0.1µF
-5V
MAX1421
600Ω
0.1µF
44pF*
600Ω
300Ω
CML
0.1µF
0.22µF
+5V
1nF
+5V
0.1µF
600Ω
INPUT
0.1µF
LOWPASS FILTER
MAX4108
300Ω
-5V
0.1µF
INN
MAX4108
RISO
50Ω
300Ω
-5V
CIN*
22pF
0.1µF
300Ω
300Ω
300Ω
*TWO CIN (22pF) CAPS MAY BE REPLACED BY
ONE 44pF CAP, TO IMPROVE PERFORMANCE.
Figure 7. Typical Application Circuit for Single-Ended to Differential Conversion
error specification of less than 1LSB guarantees no
missing codes.
Dynamic Parameter Definitions
Aperture Jitter
Figure 10 depicts the aperture jitter (tAJ), which is the
sample-to-sample variation in the aperture delay.
Aperture Delay
Aperture delay (tAD) is the time defined between the
falling edge of the sampling clock and the instant when
an actual sample is taken (Figure 10).
Signal-to-Noise Ratio (SNR)
For a waveform perfectly reconstructed from digital
samples, the theoretical maximum SNR is the ratio of
14
the full-scale analog input (RMS value) to the RMS
quantization error (residual error). The ideal theoretical
minimum analog-to-digital noise is caused by quantization error only and results directly from the ADCs resolution (N-bits):
SNR(MAX) = (6.02 ✕ N + 1.76)dB
In reality, there are other noise sources besides quantization noise e.g., thermal noise, reference noise, clock
jitter, etc. SNR is computed by taking the ratio of the
RMS signal to the RMS noise, which includes all spectral components minus the fundamental, the first four
harmonics, and the DC offset.
______________________________________________________________________________________
12-Bit, 40Msps, 3.3V, Low-Power ADC
with Internal Reference
MAX1421
25Ω
INP
22pF *
0.1µF
1
VIN
T1
2
N.C.
3
MAX1421
44pF *
6
5
4
CML
0.22µF
1nF
MINICIRCUITS
T1–1T–KK81
25Ω
INN
22pF *
*REPLACE BOTH 22pF CAPS WITH 44pF BETWEEN
INP AND INN TO IMPROVE DYNAMIC PERFORMANCE.
Figure 8. Using a Transformer for AC-Coupling
VIN
RISO
50Ω
0.1µF
MAX4108
100Ω
1kΩ
CIN
22pF
0.22µF
1nF
INP
MAX1421
CML
RISO
50Ω
INN
100Ω
CIN
22pF
Figure 9. Single-Ended AC-Coupled Input Signal
Signal-to-Noise Plus Distortion (SINAD)
SINAD is computed by taking the ratio of the RMS signal to all spectral components minus the fundamental
and the DC offset.
CLK
CLK
ANALOG
INPUT
Effective Number of Bits (ENOB)
ENOB specifies the dynamic performance of an ADC at
a specific input frequency and sampling rate. An ideal
ADC’s error consists of quantization noise only. ENOB
is computed from:
tAD
tAJ
SAMPLED
DATA (T/H)
ENOB =
T/H
TRACK
HOLD
TRACK
SINAD - 1.76
6.02
Figure 10. Track-and-Hold Aperture Timing
______________________________________________________________________________________
15
MAX1421
12-Bit, 40Msps, 3.3V, Low-Power ADC
with Internal Reference
Total Harmonic Distortion (THD)
THD is typically the ratio of the RMS sum of the first four
harmonics of the input signal to the fundamental itself.
This is expressed as:
THD = 20 × log10
(V
2
2
2
2
+ V3 + V4 + V5
V1
2
)
where V1 is the fundamental amplitude, and V2 through
V5 are the amplitudes of the 2nd- through 5th-order harmonics.
Spurious-Free Dynamic Range (SFDR)
SFDR is the ratio expressed in decibels of the RMS
amplitude of the fundamental (maximum signal component) to the RMS value of the next largest spurious
component, excluding DC offset.
Functional Diagram
CLK
CLK
AGND
INTERFACE
INP
T/H
PIPELINE ADC
INN
PD
BANDGAP
REFERENCE
OUTPUT
DRIVERS
REFIN REFP CML REFN
______________________________________________________________________________________
D11–D0
DVDD
REF SYSTEM +
BIAS
Intermodulation Distortion (IMD)
The two-tone IMD is the ratio expressed in decibels of
either input tone to the worst 3rd-order (or higher) intermodulation products. The individual input tone levels
are at -6.5dB full scale and their envelope is at -0.5dB
full scale.
16
AVDD
MAX1421
DGND
OE
12-Bit, 40Msps, 3.3V, Low-Power ADC
with Internal Reference
32L/48L,TQFP.EPS
PACKAGE OUTLINE, 32/48L LQFP, 7x7x1.4mm
21-0054
F
1
2
PACKAGE OUTLINE, 32/48L LQFP, 7x7x1.4mm
21-0054
F
2
2
Maxim cannot assume responsibility for use of any circuitry other than circuitry entirely embodied in a Maxim product. No circuit patent licenses are
implied. Maxim reserves the right to change the circuitry and specifications without notice at any time.
17 ____________________Maxim Integrated Products, 120 San Gabriel Drive, Sunnyvale, CA 94086 408-737-7600
© 2006 Maxim Integrated Products
is a registered trademark of Maxim Integrated Products, Inc.
MAX1421
Package Information
(The package drawing(s) in this data sheet may not reflect the most current specifications. For the latest package outline information
go to www.maxim-ic.com/packages.)
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