Maxim MAX1249BCPE 2.7v to 5.25v, low-power, 4-channel, serial 10-bit adcs in qsop-16 Datasheet

19-1072; Rev 2; 5/98
+2.7V to +5.25V, Low-Power, 4-Channel,
Serial 10-Bit ADCs in QSOP-16
____________________________Features
♦ 4-Channel Single-Ended or 2-Channel
Differential Inputs
♦ Single +2.7V to +5.25V Operation
♦ Internal 2.5V Reference (MAX1248)
♦ Low Power: 1.2mA (133ksps, +3V supply)
54µA (1ksps, +3V supply)
1µA (power-down mode)
♦ SPI/QSPI/MICROWIRE/TMS320-Compatible
4-Wire Serial Interface
♦ Software-Configurable Unipolar or Bipolar Inputs
♦ 16-Pin QSOP Package (same area as 8-pin SO)
________________________Applications
Portable Data Logging
Medical Instruments
Pen Digitizers
_____________Ordering Information
TEMP. RANGE
MAX1248ACPE
0°C to +70°C
16 Plastic DIP
±1/2
MAX1248BCPE
MAX1248ACEE
MAX1248BCEE
0°C to +70°C
0°C to +70°C
0°C to +70°C
16 Plastic DIP
16 QSOP
16 QSOP
±1
±1/2
±1
PIN-PACKAGE
Ordering Information continued at end of data sheet.
factory for availability of alternate surface-mount
packages.
† Contact
__________Typical Operating Circuit
Data Acquisition
Battery-Powered Instruments
System Supervision
+3V
CH0
0V TO
+2.5V
ANALOG
INPUTS
VDD
DGND
C3
0.1µF
VDD
MAX1248 AGND
CH3
SCLK
VREF
C1
4.7µF
DIN
DOUT
REFADJ
CPU
COM
CS
Pin Configuration appears at end of data sheet.
I/O
SCK (SK)
MOSI (SO)
MISO (SI)
SSTRB
SHDN
SPI and QSPI are trademarks of Motorola, Inc.
MICROWIRE is a trademark of National Semiconductor Corp.
INL
(LSB)
PART†
VSS
C2
0.01µF
________________________________________________________________ Maxim Integrated Products
1
For free samples & the latest literature: http://www.maxim-ic.com, or phone 1-800-998-8800.
For small orders, phone 408-737-7600 ext. 3468.
MAX1248/MAX1249
_______________General Description
The MAX1248/MAX1249 10-bit data-acquisition systems combine a 4-channel multiplexer, high-bandwidth
track/hold, and serial interface with high conversion
speed and low power consumption. They operate from
a single +2.7V to +5.25V supply, and their analog
inputs are software configurable for unipolar/bipolar
and single-ended/differential operation.
The 4-wire serial interface connects directly to SPI™/
QSPI™ and MICROWIRE™ devices without external
logic. A serial strobe output allows direct connection
to TMS320-family digital signal processors. The
MAX1248/MAX1249 use either the internal clock or an
external serial-interface clock to perform successiveapproximation analog-to-digital conversions.
The MAX1248 has an internal 2.5V reference, while the
MAX1249 requires an external reference. Both parts
have a reference-buffer amplifier with a ±1.5% voltage
adjustment range.
These devices provide a hard-wired SHDN pin and a
software-selectable power-down, and can be programmed to automatically shut down at the end of a
conversion. Accessing the serial interface automatically
powers up the MAX1248/MAX1249, and the quick
turn-on time allows them to be shut down between all
conversions. This technique can cut supply current to
under 60µA at reduced sampling rates.
The MAX1248/MAX1249 are available in a 16-pin DIP
and a very small QSOP that occupies the same board
area as an 8-pin SO.
For 8-channel versions of these devices, see the
MAX148/MAX149 data sheet.
MAX1248/MAX1249
+2.7V to +5.25V, Low-Power, 4-Channel,
Serial 10-Bit ADCs in QSOP-16
ABSOLUTE MAXIMUM RATINGS
VDD to AGND, DGND .............................................. -0.3V to +6V
AGND to DGND.................................................... -0.3V to +0.3V
CH0–CH3, COM to AGND, DGND ............ -0.3V to (VDD + 0.3V)
VREF to AGND........................................... -0.3V to (VDD + 0.3V)
Digital Inputs to DGND............................................ -0.3V to +6V
Digital Outputs to DGND ........................... -0.3V to (VDD + 0.3V)
Digital Output Sink Current .................................................25mA
Continuous Power Dissipation (TA = +70°C)
Plastic DIP (derate 10.53mW/°C above +70°C) ......... 842mW
QSOP (derate 8.30mW/°C above +70°C) ................... 667mW
CERDIP (derate 10.00mW/°C above +70°C) .............. 800mW
Operating Temperature Ranges
MAX1248_C_E/MAX1249_C_E .......................... 0°C to +70°C
MAX1248_E_E/MAX1249_E_E........................ -40°C to +85°C
MAX1248_MJE/MAX1249_MJE .................... -55°C to +125°C
Storage Temperature Range ............................ -60°C to +150°C
Lead Temperature (soldering, 10sec) ............................ +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
(VDD = +2.7V to +5.25V; COM = 0V; fSCLK = 2.0MHz; external clock (50% duty cycle); 15 clocks/conversion cycle (133ksps);
MAX1248—4.7µF capacitor at VREF pin; MAX1249—external reference, VREF = 2.500V applied to VREF pin; TA = TMIN to TMAX,
unless otherwise noted.)
PARAMETER
SYMBOL
CONDITIONS
DC ACCURACY (Note 1)
Resolution
MIN
TYP
MAX
10
Relative Accuracy (Note 2)
INL
Differential Nonlinearity
DNL
Bits
MAX124_A
MAX124_B
No missing codes over temperature
±0.5
±1.0
±1
MAX124_A
MAX124_B
MAX124_A
MAX124_B
Offset Error
Gain Error (Note 3)
UNITS
±1
±2
±1
±2
LSB
LSB
LSB
LSB
Gain Temperature Coefficient
±0.25
ppm/°C
Channel-to-Channel Offset
Matching
±0.05
LSB
DYNAMIC SPECIFICATIONS (10kHz sine-wave input, 0V to 2.500Vp-p, 133ksps, 2.0MHz external clock, bipolar input mode)
Signal-to-Noise + Distortion Ratio
SINAD
66
dB
Total Harmonic Distortion
THD
Spurious-Free Dynamic Range
SFDR
Up to the 5th harmonic
-70
dB
70
dB
Channel-to-Channel Crosstalk
65kHz, 2.500Vp-p (Note 4)
-75
dB
Small-Signal Bandwidth
-3dB rolloff
2.25
MHz
1.0
MHz
Full-Power Bandwidth
CONVERSION RATE
Conversion Time (Note 5)
Track/Hold Acquisition Time
tCONV
Internal clock, SHDN = FLOAT
5.5
7.5
Internal clock, SHDN = VDD
35
65
µs
External clock = 2MHz, 12 clocks/conversion
6
1.5
µs
tACQ
Aperture Delay
Aperture Jitter
Internal Clock Frequency
External Clock Frequency
2
SHDN = FLOAT
SHDN = VDD
Data transfer only
30
ns
<50
ps
1.8
0.225
MHz
0.1
0
_______________________________________________________________________________________
2.0
2.0
MHz
+2.7V to +5.25V, Low-Power, 4-Channel,
Serial 10-Bit ADCs in QSOP-16
(VDD = +2.7V to +5.25V; COM = 0V; fSCLK = 2.0MHz; external clock (50% duty cycle); 15 clocks/conversion cycle (133ksps);
MAX1248—4.7µF capacitor at VREF pin; MAX1249—external reference, VREF = 2.500V applied to VREF pin; TA = TMIN to TMAX,
unless otherwise noted.)
PARAMETER
SYMBOL
ANALOG/COM INPUTS
Input Voltage Range, SingleEnded and Differential (Note 6)
CONDITIONS
MIN
Unipolar, COM = 0V
Bipolar, COM = VREF / 2
On/off leakage current, VCH_ = 0V or VDD
Multiplexer Leakage Current
TYP
MAX
0 to VREF
±VREF / 2
±0.01
±1
Input Capacitance
16
UNITS
V
µA
pF
INTERNAL REFERENCE (MAX1248 only, reference buffer enabled)
VREF Output Voltage
TA = +25°C (Note 7)
2.470
2.500
VREF Short-Circuit Current
2.530
30
VREF Temperature Coefficient
MAX1248
Load Regulation (Note 8)
0mA to 0.2mA output load
Internal compensation mode
External compensation mode
Capacitive Bypass at VREF
Capacitive Bypass at REFADJ
mA
±30
ppm/°C
0.35
mV
0
4.7
0.01
REFADJ Adjustment Range
V
µF
µF
±1.5
%
EXTERNAL REFERENCE AT VREF (Buffer disabled)
VREF Input Voltage Range
(Note 9)
VDD +
50mV
1.0
VREF Input Current
VREF = 2.500V
VREF Input Resistance
100
18
Shutdown VREF Input Current
150
25
0.01
V
µA
kΩ
10
µA
VDD 0.5
V
Internal compensation mode
External compensation mode
MAX1248
MAX1249
MAX1248
MAX1249
0
4.7
µF
VDD ≤ 3.6V
VDD > 3.6V
2.0
3.0
REFADJ Buffer-Disable Threshold
EXTERNAL REFERENCE AT REFADJ
Capacitive Bypass at VREF
Reference-Buffer Gain
REFADJ Input Current
2.06
2.00
V/V
±50
±10
µA
DIGITAL INPUTS (DIN, SCLK, CS, SHDN)
DIN, SCLK, CS Input High Voltage
DIN, SCLK, CS Input Low Voltage
DIN, SCLK, CS Input Hysteresis
DIN, SCLK, CS Input Leakage
VIH
VIL
VHYST
VIN = 0V or VDD
DIN, SCLK, CS Input Capacitance
CIN
(Note 10)
SHDN Input High Voltage
VSH
VDD - 0.4
SHDN Input Mid Voltage
VSM
1.1
SHDN Input Low Voltage
VSL
IS
0.8
V
±1
µA
15
pF
0.2
IIN
SHDN Input Current
V
SHDN = 0V or VDD
±0.01
V
V
VDD - 1.1
V
0.4
V
±4.0
µA
_______________________________________________________________________________________
3
MAX1248/MAX1249
ELECTRICAL CHARACTERISTICS (continued)
MAX1248/MAX1249
+2.7V to +5.25V, Low-Power, 4-Channel,
Serial 10-Bit ADCs in QSOP-16
ELECTRICAL CHARACTERISTICS (continued)
(VDD = +2.7V to +5.25V; COM = 0V; fSCLK = 2.0MHz, external clock (50% duty cycle); 15 clocks/conversion cycle (133ksps);
MAX1248—4.7µF capacitor at VREF pin; MAX1249—external reference; VREF = 2.500V applied to VREF pin, TA = TMIN to TMAX,
unless otherwise noted.)
PARAMETER
SYMBOL
SHDN Voltage, Floating
VFLT
SHDN Maximum Allowed
Leakage, Mid Input
CONDITIONS
MIN
SHDN = FLOAT
TYP
MAX
VDD / 2
SHDN = FLOAT
UNITS
V
±100
nA
DIGITAL OUTPUTS (DOUT, SSTRB)
Output Voltage Low
VOL
Output Voltage High
VOH
Three-State Leakage Current
Three-State Output Capacitance
POWER REQUIREMENTS
Positive Supply Voltage
IL
COUT
ISINK = 5mA
0.4
ISINK = 16mA
0.8
ISOURCE = 0.5mA
Supply Rejection (Note 12)
4
V
±0.01
CS = VDD (Note 10)
VDD
2.70
Operating mode,
full-scale input (Note 11)
Positive Supply Current
VDD - 0.5
CS = VDD
IDD
Full power-down
±10
µA
15
pF
5.25
V
VDD = 5.25V
1.6
3.0
VDD = 3.6V
1.2
2.0
VDD = 5.25V
3.5
15
VDD = 3.6V
1.2
10
30
70
IDD
Fast power-down (MAX1248) VDD = 5.25V
PSR
VDD = 2.7V to 5.25V, full-scale input,
external reference = 2.500V
V
±0.3
_______________________________________________________________________________________
mA
µA
mV
+2.7V to +5.25V, Low-Power, 4-Channel,
Serial 10-Bit ADCs in QSOP-16
(VDD = +2.7V to +5.25V, TA = TMIN to TMAX, unless otherwise noted.)
PARAMETER
Acquisition Time
SYMBOL
CONDITIONS
MIN
tACQ
1.5
DIN to SCLK Setup
tDS
100
DIN to SCLK Hold
tDH
TYP
MAX
UNITS
µs
ns
0
MAX124_ _C/E
20
200
MAX124_ _M
20
240
ns
SCLK Fall to Output Data Valid
tDO
Figure 1
CS Fall to Output Enable
tDV
Figure 1
240
ns
CS Rise to Output Disable
tTR
Figure 2
240
ns
CS to SCLK Rise Setup
tCSS
CS to SCLK Rise Hold
SCLK Pulse Width High
SCLK Pulse Width Low
SCLK Fall to SSTRB
ns
100
ns
tCSH
0
ns
tCH
200
ns
tCL
200
tSSTRB
ns
Figure 1
240
ns
tSDV
External clock mode only, Figure 1
240
ns
CS Rise to SSTRB Output Disable
tSTR
External clock mode only, Figure 2
240
ns
SSTRB Rise to SCLK Rise
tSCK
Internal clock mode only (Note 10)
CS Fall to SSTRB Output Enable
0
ns
Note 1: Tested at VDD = 2.7V; COM = 0V; unipolar single-ended input mode.
Note 2: Relative accuracy is the deviation of the analog value at any code from its theoretical value after the full-scale range has
been calibrated.
Note 3: MAX1248—internal reference, offset nulled; MAX1249 — external reference (VREF = +2.500V), offset nulled.
Note 4: Ground “on” channel; sine wave applied to all “off” channels.
Note 5: Conversion time defined as the number of clock cycles multiplied by the clock period; clock has 50% duty cycle.
Note 6: The common-mode range for the analog inputs is from AGND to VDD.
Note 7 Sample tested to 0.1% AQL.
Note 8: External load should not change during conversion for specified accuracy.
Note 9: ADC performance is limited by the converter’s noise floor, typically 300µVp-p.
Note 10 Guaranteed by design. Not subject to production testing.
Note 11: The MAX1249 typically draws 400µA less than the values shown.
Note 12: Measured as |VFS(2.7V) - VFS(5.25V)|.
_______________________________________________________________________________________
5
MAX1248/MAX1249
TIMING CHARACTERISTICS
__________________________________________Typical Operating Characteristics
(VDD = 3.0V, VREF = 2.500V, fSCLK = 2.0MHz, CLOAD = 20pF, TA = +25°C, unless otherwise noted.)
SUPPLY CURRENT
vs. SUPPLY VOLTAGE
1.25
CLOAD = 50pF
MAX1249
1.00
CLOAD = 20pF
0.75
2.5
2.0
1.5
1.0
2.5000
0.5
3.25
3.75
4.25
4.75
0
2.25
5.25
2.75
3.25
3.75
SUPPLY VOLTAGE (V)
SUPPLY CURRENT vs. TEMPERATURE
SHUTDOWN CURRENT (µA)
MAX1248
1.2
1.1
1.0
MAX1249
4.75
0.8
-60
-20
20
100
MAX1248-05
0.8
-20
20
60
0.20
100
VDD = 2.7V
2.498
2.497
2.496
2.495
-60 -40 -20 0
140
20 40 60 80 100 120 140
INTEGRAL NONLINEARITY
vs. CODE
0.100
MAX1248-08
MAX1248-07
0.20
VDD = 3.6V
INTEGRAL NONLINEARITY
vs. TEMPERATURE
VDD = 2.7V
0.25
5.25
2.499
TEMPERATURE (°C)
0.30
0.25
VDD = 5.25V
2.500
TEMPERATURE (°C)
TEMPERATURE (°C)
0.30
4.75
2.494
-60
INTERGRAL NONLINEARITY
vs. SUPPLY VOLTAGE
4.25
2.501
1.2
140
3.75
INTERNAL REFERENCE VOLTAGE
vs. TEMPERATURE
0
60
3.25
SHUTDOWN CURRENT
vs. TEMPERATURE
1.6
RLOAD = ∞
CODE = 1010101000
2.75
SUPPLY VOLTAGE (V)
0.4
0.9
2.4975
2.25
5.25
VDD (V)
2.0
MAX1247-04
1.3
4.25
MAX1248-06
2.75
MAX1248-09
0.50
2.25
SUPPLY CURRENT (mA)
MAX1248-09
3.5
3.0
2.5025
INTERNAL REFERENCE VOLTAGE (V)
MAX1248
FULL POWER-DOWN
MAX1248/49-02
CLOAD = 50pF
CLOAD = 20pF
4.0
INTERNAL REFERENCE VOLTAGE
vs. SUPPLY VOLTAGE
INTERNAL REFERENCE VOLTAGE, VREF
SUPPLY CURRENT (mA)
MAX1248-01
RL = ∞
CODE = 10101010
1.75
1.50
SHUTDOWN SUPPLY CURRENT
vs. SUPPLY VOLTAGE
SHUTDOWN SUPPLY CURRENT (µA)
2.00
0.075
0.15
0.10
0.15
0.10
MAX1248
MAX1248
MAX1249
MAX1249
2.25
2.75
3.25
3.75
4.25
SUPPLY VOLTAGE (V)
4.75
5.25
0
-0.050
-0.075
-0.100
0
00
0.025
-0.025
0.05
0.05
6
INL (LSB)
INL (LSB)
0.050
INL (LSB)
MAX1248/MAX1249
+2.7V to +5.25V, Low-Power, 4-Channel,
Serial 10-Bit ADCs in QSOP-16
-60 -40 -20 0
20 40 60 80 100 120 140
0
256
TEMPERATURE (°C)
_______________________________________________________________________________________
512
CODE
768
1024
+2.7V to +5.25V, Low-Power, 4-Channel,
Serial 10-Bit ADCs in QSOP-16
PIN
NAME
FUNCTION
1
VDD
Positive Supply Voltage
2–5
CH0–CH3
Sampling Analog Inputs
6
COM
Ground reference for analog inputs. Sets zero-code voltage in single-ended mode. Must be stable to
±0.5LSB.
7
SHDN
Three-Level Shutdown Input. Pulling SHDN low shuts the MAX1248/MAX1249 down; otherwise, the
devices are fully operational. Pulling SHDN high puts the reference-buffer amplifier in internal compensation mode. Letting SHDN float puts the reference-buffer amplifier in external compensation mode.
8
VREF
Reference-Buffer Output/ADC Reference Input. Reference voltage for analog-to-digital conversion. In
internal reference mode (MAX1248 only), the reference buffer provides a 2.500V nominal output,
externally adjustable at REFADJ. In external reference mode, disable the internal buffer by pulling
REFADJ to VDD.
9
REFADJ
Input to the Reference-Buffer Amplifier. To disable the reference-buffer amplifier, tie REFADJ to VDD.
10
AGND
Analog Ground
11
DGND
Digital Ground
12
DOUT
Serial Data Output. Data is clocked out at SCLK’s falling edge. High impedance when CS is high.
13
SSTRB
Serial Strobe Output. In internal clock mode, SSTRB goes low when the MAX1248/MAX1249 begin the
A/D conversion and goes high when the conversion is completed. In external clock mode, SSTRB
pulses high for one clock period before the MSB decision. High impedance when CS is high (external
clock mode).
14
DIN
Serial Data Input. Data is clocked in at SCLK’s rising edge.
15
CS
Active-Low Chip Select. Data will not be clocked into DIN unless CS is low. When CS is high, DOUT is
high impedance.
16
SCLK
Serial Clock Input. Clocks data in and out of serial interface. In external clock mode, SCLK also sets
the conversion speed. (Duty cycle must be 40% to 60%.)
VDD
DOUT
DOUT
CLOAD
50pF
6k
VDD
DOUT
CLOAD
50pF
DGND
DGND
a) High-Z to VOH and VOL to VOH
b) High-Z to VOL and VOH to VOL
Figure 1. Load Circuits for Enable Time
6k
6k
DOUT
CLOAD
50pF
6k
DGND
a) VOH to High-Z
CLOAD
50pF
DGND
b) VOL to High-Z
Figure 2. Load Circuits for Disable Time
_______________________________________________________________________________________
7
MAX1248/MAX1249
______________________________________________________________Pin Description
MAX1248/MAX1249
+2.7V to +5.25V, Low-Power, 4-Channel,
Serial 10-Bit ADCs in QSOP-16
_______________Detailed Description
The MAX1248/MAX1249 analog-to-digital converters
(ADCs) use a successive-approximation conversion
technique and input track/hold (T/H) circuitry to convert
an analog signal to a 10-bit digital output. A flexible
serial interface provides easy interface to microprocessors (µPs). Figure 3 is a block diagram of the
MAX1248/MAX1249.
Pseudo-Differential Input
The sampling architecture of the ADC’s analog comparator is illustrated in the equivalent input circuit
(Figure 4). In single-ended mode, IN+ is internally
switched to CH0–CH3, and IN- is switched to COM. In
differential mode, IN+ and IN- are selected from two
pairs: CH0/CH1 and CH2/CH3. Configure the channels
with Tables 2 and 3. Please note that the codes for
CH0–CH3 in the MAX1248/MAX1249 correspond to
the codes for CH2–CH5 in the eight-channel
(MAX148/MAX149) versions.
In differential mode, IN- and IN+ are internally switched
to either of the analog inputs. This configuration is
pseudo-differential to the effect that only the signal at
IN+ is sampled. The return side (IN-) must remain stable within ±0.5LSB (±0.1LSB for best results) with
respect to AGND during a conversion. To accomplish
this, connect a 0.1µF capacitor from IN- (the selected
analog input) to AGND.
During the acquisition interval, the channel selected as
the positive input (IN+) charges capacitor CHOLD. The
acquisition interval spans three SCLK cycles and ends
CS
SCLK
on the falling SCLK edge after the last bit of the input
control word has been entered. At the end of the acquisition interval, the T/H switch opens, retaining charge
on CHOLD as a sample of the signal at IN+.
The conversion interval begins with the input multiplexer switching CHOLD from the positive input (IN+) to the
negative input (IN-). In single-ended mode, IN- is simply COM. This unbalances node ZERO at the comparator’s input. The capacitive DAC adjusts during the
remainder of the conversion cycle to restore node
ZERO to 0V within the limits of 10-bit resolution. This
action is equivalent to transferring a charge of 16pF x
[(V IN+) - (V IN-)] from C HOLD to the binary-weighted
capacitive DAC, which in turn forms a digital representation of the analog input signal.
Track/Hold
The T/H enters its tracking mode on the falling clock
edge after the fifth bit of the 8-bit control word has been
shifted in. It enters its hold mode on the falling clock
edge after the eighth bit of the control word has been
shifted in. If the converter is set up for single-ended
inputs, IN- is connected to COM, and the converter
samples the “+” input. If the converter is set up for differential inputs, IN- connects to the “-” input, and the
difference of |IN+ - IN-| is sampled. At the end of the
conversion, the positive input connects back to IN+,
and CHOLD charges to the input signal.
The time required for the T/H to acquire an input signal
is a function of how quickly its input capacitance is
charged. If the input signal’s source impedance is high,
15
16
DIN
14
SHDN
7
CH0
2
CH1
3
CH2
4
CH3
5
CAPACITIVE DAC
INPUT
SHIFT
REGISTER
VREF
INT
CLOCK
CONTROL
LOGIC
CH0
COM
OUTPUT
SHIFT
REGISTER
ANALOG
INPUT
MUX
REFADJ 9
VREF 8
13
SSTRB
RIN
9k
CH1
CSWITCH
CLOCK
IN
SAR
ADC
CH2
OUT
REF
20k
A ≈ 2.06*
CH3
1
11
10
+2.500V
MAX1248
MAX1249
VDD
DGND
AGND
ZERO
16pF
DOUT
T/H
6
+1.21V
REFERENCE
(MAX1248)
12
COMPARATOR
INPUT
CHOLD
MUX –
+
TRACK
T/H
SWITCH
COM
HOLD
AT THE SAMPLING INSTANT,
THE MUX INPUT SWITCHES
FROM THE SELECTED IN+
CHANNEL TO THE SELECTED
IN- CHANNEL.
SINGLE-ENDED MODE: IN+ = CHO–CH3, IN- = COM.
DIFFERENTIAL MODE: IN+ AND IN- SELECTED FROM PAIRS OF
CH0/CH1 AND CH2/CH3.
*A ≈ 2.00 (MAX1249)
Figure 3. Block Diagram
8
Figure 4. Equivalent Input Circuit
_______________________________________________________________________________________
+2.7V to +5.25V, Low-Power, 4-Channel,
Serial 10-Bit ADCs in QSOP-16
MAX1248/MAX1249
Table 1. Control-Byte Format
BIT 7
(MSB)
BIT 6
BIT 5
BIT 4
BIT 3
BIT 2
BIT 1
BIT 0
(LSB)
START
SEL2
SEL1
SEL0
UNI/BIP
SGL/DIF
PD1
PD0
BIT
NAME
DESCRIPTION
7(MSB)
START
The first logic “1” bit after CS goes low defines the beginning of the control byte.
6
5
4
SEL2
SEL1
SEL0
These three bits select which of the four channels are used for the conversion (Tables 2 and 3).
3
UNI/BIP
1 = unipolar, 0 = bipolar. Selects unipolar or bipolar conversion mode. In unipolar mode, an
analog input signal from 0V to VREF can be converted; in bipolar mode, the signal can range
from -VREF / 2 to +VREF / 2.
2
SGL/DIF
1 = single ended, 0 = differential. Selects single-ended or differential conversions. In singleended mode, input signal voltages are referred to COM. In differential mode, the voltage
difference between two channels is measured (Tables 2 and 3).
1
0(LSB)
PD1
PD0
Selects clock and power-down modes.
PD1
PD0
Mode
0
0
Full power-down
0
1
Fast power-down (MAX1248 only)
1
0
Internal clock mode
1
1
External clock mode
the acquisition time lengthens, and more time must be
allowed between conversions. The acquisition time,
tACQ, is the maximum time the device takes to acquire
the signal, and is also the minimum time needed for the
signal to be acquired. It is calculated by:
tACQ = 7.6 x (RS + RIN) x 16pF
where RIN = 9kΩ, RS = the source impedance of the
input signal, and tACQ is never less than 1.5µs. Note
that source impedances below 3kΩ do not significantly
affect the ADC’s AC performance.
Higher source impedances can be used if a 0.01µF
capacitor is connected to the individual analog inputs.
Note that the input capacitor forms an RC filter with the
input source impedance, limiting the ADC’s signal
bandwidth.
Input Bandwidth
The ADC’s input tracking circuitry has a 2.25MHz
small-signal bandwidth, so it is possible to digitize
high-speed transient events and measure periodic signals with bandwidths exceeding the ADC’s sampling
rate by using undersampling techniques. To avoid
high-frequency signals being aliased into the frequency
band of interest, anti-alias filtering is recommended.
Analog Input Protection
Internal protection diodes, which clamp the analog input
to VDD and AGND, allow the channel input pins to swing
from AGND - 0.3V to V DD + 0.3V without damage.
However, for accurate conversions near full scale, the
inputs must not exceed VDD by more than 50mV or be
lower than AGND by 50mV.
If the analog input exceeds 50mV beyond the supplies, do not forward bias the protection diodes of off
channels over 4mA.
How to Start a Conversion
A conversion is started by clocking a control byte into
DIN. With CS low, each rising edge on SCLK clocks a bit
from DIN into the MAX1248/MAX1249’s internal shift register. After CS falls, the first arriving logic “1” bit defines
the control byte’s MSB. Until this first “start” bit arrives,
any number of logic “0” bits can be clocked into DIN with
no effect. Table 1 shows the control-byte format.
The MAX1248/MAX1249 are compatible with SPI/QSPI
and MICROWIRE devices. For SPI, select the correct
clock polarity and sampling edge in the SPI control registers: set CPOL = 0 and CPHA = 0. MICROWIRE, SPI,
and QSPI all transmit a byte and receive a byte at the
same time. Using the Typical Operating Circuit, the sim-
_______________________________________________________________________________________
9
MAX1248/MAX1249
+2.7V to +5.25V, Low-Power, 4-Channel,
Serial 10-Bit ADCs in QSOP-16
DIF = 1)
Table 2. Channel Selection in Single-Ended Mode (SGL/D
SEL2
0
SEL1
0
SEL0
1
1
0
1
0
1
0
1
1
0
CH0
+
CH1
CH2
CH3
COM
–
+
–
+
–
+
–
DIF = 0)
Table 3. Channel Selection in Differential Mode (SGL/D
SEL2
0
SEL1
0
SEL0
1
0
1
0
1
0
1
1
1
0
CH0
+
plest software interface requires only three 8-bit transfers
to perform a conversion (one 8-bit transfer to configure
the ADC, and two more 8-bit transfers to clock out the
10-bit conversion result). See Figure 19 for MAX1248/
MAX1249 QSPI connections.
Simple Software Interface
Make sure the CPU’s serial interface runs in master
mode so the CPU generates the serial clock. Choose a
clock frequency from 100kHz to 2MHz.
1) Set up the control byte for external clock mode and
call it TB1. TB1 should be of the format: 1XXXXX11
binary, where the Xs denote the particular channel
and conversion mode selected.
2) Use a general-purpose I/O line on the CPU to pull
CS low.
3) Transmit TB1 and, simultaneously, receive a byte
and call it RB1. Ignore RB1.
4) Transmit a byte of all zeros ($00 hex) and, simultaneously, receive byte RB2.
5) Transmit a byte of all zeros ($00 hex) and, simultaneously, receive byte RB3.
6) Pull CS high.
Figure 5 shows the timing for this sequence. Bytes RB2
and RB3 contain the result of the conversion padded
with one leading zero, two sub-bits, and three trailing
zeros. The total conversion time is a function of the
10
–
CH1
–
CH2
CH3
+
–
–
+
+
serial-clock frequency and the amount of idle time
between 8-bit transfers. To avoid excessive T/H droop,
make sure the total conversion time does not exceed
120µs.
Digital Output
In unipolar input mode, the output is straight binary
(Figure 16). For bipolar inputs, the output is two’s complement (Figure 17). Data is clocked out at the falling
edge of SCLK in MSB-first format.
Clock Modes
The MAX1248/MAX1249 may use either an external
serial clock or the internal clock to perform the successive-approximation conversion. In both clock modes,
the external clock shifts data in and out of the
MAX1248/MAX1249. The T/H acquires the input signal
as the last three bits of the control byte are clocked into
DIN. Bits PD1 and PD0 of the control byte program the
clock mode. Figures 6–9 show the timing characteristics
common to both modes.
External Clock
In external clock mode, the external clock not only shifts
data in and out, it also drives the analog-to-digital conversion steps. SSTRB pulses high for one clock period
after the last bit of the control byte. Successive-approximation bit decisions are made and appear at DOUT on
each of the next 10 SCLK falling edges (Figure 5).
SSTRB and DOUT go into a high-impedance state when
______________________________________________________________________________________
+2.7V to +5.25V, Low-Power, 4-Channel,
Serial 10-Bit ADCs in QSOP-16
MAX1248/MAX1249
CS
tACQ
SCLK
1
DIN
4
8
SEL2 SEL1 SEL0
UNI/ SGL/
PD1 PD0
BIP DIF
12
16
20
24
START
SSTRB
B9
MSB
A/D STATE
RB3
RB2
RB1
DOUT
B8
B7
B6
ACQUISITION
1.5µs
IDLE
B5
B4
B3
B2
B1
B0
LSB
S1
CONVERSION
S0
FILLED WITH
ZEROS
IDLE
(fCLK = 2MHz)
Figure 5. 24-Clock External Clock Mode Conversion Timing (MICROWIRE and SPI-Compatible, QSPI-Compatible with fSCLK ≤ 2MHz)
•••
CS
tCSH
tCSS
tCL
tCH
SCLK
tCSH
•••
tDS
tDH
•••
DIN
tDV
tDO
tTR
•••
DOUT
Figure 6. Detailed Serial-Interface Timing
CS goes high; after the next CS falling edge, SSTRB will
output a logic low. Figure 7 shows the SSTRB timing in
external clock mode.
The conversion must complete in some minimum time,
or droop on the sample-and-hold capacitors may
degrade conversion results. Use internal clock mode if
the serial-clock frequency is less than 100kHz, or if
serial-clock interruptions could cause the conversion
interval to exceed 120µs.
______________________________________________________________________________________
11
MAX1248/MAX1249
+2.7V to +5.25V, Low-Power, 4-Channel,
Serial 10-Bit ADCs in QSOP-16
•••
CS
•••
tSTR
tSDV
SSTRB
•••
•••
tSSTRB
SCLK
tSSTRB
•• • •
••••
PD0 CLOCKED IN
Figure 7. External Clock Mode SSTRB Detailed Timing
CS
SCLK
1
2
3
4
5
6
7
8
9
10
11
12
18
19
20
21
22
23
24
SGL/
SEL2 SEL1 SEL0 UNI/
BIP DIF PD1 PD0
DIN
START
SSTRB
tCONV
B9
MSB
DOUT
A/D STATE
IDLE
ACQUISITION
1.5µs
CONVERSION
7.5µs MAX
B8
B7
B0
LSB
S1
S0
FILLED WITH
ZEROS
IDLE
(fSCLK = 2MHz) (SHDN = FLOAT)
Figure 8. Internal Clock Mode Timing
Internal Clock
In internal clock mode, the MAX1248/MAX1249 generate their own conversion clocks internally. This frees the
µP from the burden of running the SAR conversion clock
and allows the conversion results to be read back at the
processor’s convenience, at any clock rate from 0MHz
to 2MHz. SSTRB goes low at the start of the conversion
and then goes high when the conversion is complete.
SSTRB is low for a maximum of 7.5µs (SHDN = FLOAT),
during which time SCLK should remain low for best
noise performance.
An internal register stores data when the conversion is
in progress. SCLK clocks the data out of this register at
any time after the conversion is complete. After SSTRB
goes high, the next falling clock edge produces the
12
MSB of the conversion at DOUT, followed by the
remaining bits in MSB-first format (Figure 8). CS does
not need to be held low once a conversion is started.
Pulling CS high prevents data from being clocked into
the MAX1248/MAX1249 and three-states DOUT, but it
does not adversely affect an internal clock mode conversion already in progress. When internal clock mode
is selected, SSTRB does not go into a high-impedance
state when CS goes high.
Figure 9 shows the SSTRB timing in internal clock
mode. In this mode, data can be shifted in and out of
the MAX1248/MAX1249 at clock rates exceeding
2.0MHz if the minimum acquisition time, tACQ, is kept
above 1.5µs.
______________________________________________________________________________________
+2.7V to +5.25V, Low-Power, 4-Channel,
Serial 10-Bit ADCs in QSOP-16
The first high bit clocked into DIN with CS low any
time the converter is idle; e.g., after VDD is applied.
OR
The first high bit clocked into DIN after bit 3 of a conversion in progress is clocked onto the DOUT pin.
If CS is toggled before the current conversion is complete, the next high bit clocked into DIN is recognized as
a start bit; the current conversion is terminated, and a
new one is started.
The fastest the MAX1248/MAX1249 can run with CS
held low between conversions is 15 clocks per conversion. Figure 10a shows the serial-interface timing necessary to perform a conversion every 15 SCLK cycles
in external clock mode. If CS is tied low and SCLK is
continuous, guarantee a start bit by first clocking in 16
zeros.
CS
Most microcontrollers require that conversions occur in
multiples of 8 SCLK clocks; 16 clocks per conversion is
typically the fastest that a microcontroller can drive the
MAX1248/MAX1249. Figure 10b shows the serial-interface timing necessary to perform a conversion every 16
SCLK cycles in external clock mode.
__________ Applications Information
Power-On Reset
When power is first applied, and if SHDN is not pulled
low, internal power-on reset circuitry activates the
MAX1248/MAX1249 in internal clock mode, ready to
convert with SSTRB = high. After the power supplies
have stabilized, the internal reset time is 10µs, and no
conversions should be performed during this phase.
SSTRB is high on power-up and, if CS is low, the first
logical 1 on DIN is interpreted as a start bit. Until a conversion takes place, DOUT shifts out zeros (also see
Table 4).
Reference-Buffer Compensation
In addition to its shutdown function, SHDN selects internal or external compensation. The compensation
affects both power-up time and maximum conversion
speed. The 100kHz minimum clock rate is limited by
droop on the sample-and-hold, and is independent of
the compensation used.
•••
tCONV
tSCK
tCSH
SSTRB
tCSS
•••
tSSTRB
SCLK
•••
tDO
PD0 CLOCK IN
DOUT
•••
NOTE: FOR BEST NOISE PERFORMANCE, KEEP SCLK LOW DURING CONVERSION.
Figure 9. Internal Clock Mode SSTRB Detailed Timing
______________________________________________________________________________________
13
MAX1248/MAX1249
Data Framing
The falling edge of CS does not start a conversion. The
first logic high clocked into DIN is interpreted as a start
bit and defines the first bit of the control byte. A conversion starts on the falling edge of SCLK, after the eighth
bit of the control byte (the PD0 bit) is clocked into DIN.
The start bit is defined as:
MAX1248/MAX1249
+2.7V to +5.25V, Low-Power, 4-Channel,
Serial 10-Bit ADCs in QSOP-16
CS
1
8
1
8
1
SCLK
S
DIN
S
CONTROL BYTE 0
DOUT
S
CONTROL BYTE 1
CONTROL BYTE 2
B9 B8 B7 B6 B5 B4 B3 B2 B1 B0 S1 S0
B9 B8 B7 B6 B5 B4 B3 B2 B1 B0 S1 S0
CONVERSION RESULT 0
CONVERSION RESULT 1
SSTRB
Figure 10a. External Clock Mode, 15 Clocks/Conversion Timing
CS
•••
SCLK
•••
DIN
DOUT
S
S
CONTROL BYTE 0
B9 B8 B7 B6 B5 B4 B3 B2 B1 B0 S1 S0
CONVERSION RESULT 0
•••
CONTROL BYTE 1
B9
B8
B7 B6
•••
CONVERSION RESULT 1
Figure 10b. External Clock Mode, 16 Clocks/Conversion Timing
Float SHDN to select external compensation. The
Typical Operating Circuit uses a 4.7µF capacitor at
VREF. A value of 4.7µF or greater ensures referencebuffer stability and allows converter operation at the
2MHz full clock speed. External compensation increases power-up time (see Choosing Power-Down Mode
and Table 4).
Pull SHDN high to select internal compensation.
Internal compensation requires no external capacitor at
VREF and allows for the shortest power-up times. The
maximum clock rate is 2MHz in internal clock mode
and 400kHz in external clock mode.
Choosing Power-Down Mode
You can save power by placing the converter in a lowcurrent shutdown state between conversions. Select full
power-down or fast power-down mode via bits 1 and 0
of the DIN control byte with SHDN high or floating
(Tables 1 and 5). In both software power-down modes,
the serial interface remains operational, but the ADC
does not convert. Pull SHDN low at any time to shut
down the converter completely. SHDN overrides bits 1
and 0 of the control byte.
14
Full power-down mode turns off all chip functions that
draw quiescent current, reducing supply current typically
to 2µA. Fast power-down mode turns off all circuitry
except the bandgap reference. With fast power-down
mode, the supply current is 30µA. Power-up time can be
shortened to 5µs in internal compensation mode.
Table 4 shows how the choice of reference-buffer compensation and power-down mode affects both power-up
delay and maximum sample rate. In external compensation mode, power-up time is 20ms with a 4.7µF compensation capacitor when the capacitor is initially fully
discharged. From fast power-down, start-up time can be
eliminated by using low-leakage capacitors that do not
discharge more than 1/2LSB while shut down. In powerdown, leakage currents at VREF cause droop on the reference bypass capacitor. Figures 11a and 11b show
the various power-down sequences in both external and
internal clock modes.
______________________________________________________________________________________
+2.7V to +5.25V, Low-Power, 4-Channel,
Serial 10-Bit ADCs in QSOP-16
REFERENCE
BUFFER
REFERENCE-BUFFER
COMPENSATION
MODE
VREF
CAPACITOR
(µF)
POWER-DOWN
MODE
POWER-UP
DELAY
(µs)
MAXIMUM
SAMPLING RATE
(ksps)
Enabled
Internal
—
Fast
5
26
Enabled
Internal
—
Full
300
26
Enabled
External
4.7
Fast
See Figure 13c
133
Enabled
External
4.7
Full
See Figure 13c
133
Disabled
—
—
Fast
2
133
Disabled
—
—
Full
2
133
CLOCK
MODE
EXTERNAL
EXTERNAL
SHDN
SETS SOFTWARE
POWER-DOWN
SETS EXTERNAL
CLOCK MODE
DIN
S X X X X X 1 1
DOUT
SETS EXTERNAL
CLOCK MODE
SX X XX X 0 0
S XX XXX 1 1
VALID
DATA
10+2 DATA BITS
10+2 DATA BITS
POWERED UP
POWERED UP
MODE
SOFTWARE
POWER-DOWN
INVALID
DATA
HARDWARE
POWERDOWN
POWERED UP
Figure 11a. Timing Diagram Power-Down Modes, External Clock
CLOCK
MODE
DIN
INTERNAL
S X X X X X 1 0
SX X XX X 0 0
DOUT
SSTRB
MODE
SETS
POWER-DOWN
SETS INTERNAL
CLOCK MODE
S
DATA VALID
DATA VALID
CONVERSION
CONVERSION
POWERED UP
POWER-DOWN
POWERED UP
Figure 11b. Timing Diagram Power-Down Modes, Internal Clock
______________________________________________________________________________________
15
MAX1248/MAX1249
Table 4. Typical Power-Up Delay Times
Table 5. Software Power-Down and
Clock Mode
PD1
PD0
DEVICE
0
0
Full Power-Down
0
1
Fast Power-Down
1
0
Internal Clock
1
1
External Clock
10,000
100
4 CHANNELS
10
Table 6. Hardware Power-Down and
Internal Clock Frequency
1 CHANNEL
1
SHDN
STATE
DEVICE
MODE
REFERENCEBUFFER
COMPENSATION
INTERNAL
CLOCK
FREQUENCY
1
Enabled
Internal
225kHz
Floating
Enabled
External
1.8MHz
0
PowerDown
N/A
N/A
Software Power-Down
Software power-down is activated using bits PD1 and PD0
of the control byte. As shown in Table 5, PD1 and PD0
also specify the clock mode. When software shutdown is
asserted, the ADC operates in the last specified clock
mode until the conversion is complete. Then the ADC
powers down into a low quiescent-current state. In internal
clock mode, the interface remains active, and conversion
results may be clocked out after the MAX1248/MAX1249
enter a software power-down.
The first logical 1 on DIN is interpreted as a start bit
and powers up the MAX1248/MAX1249. Following the
start bit, the data input word or control byte also determines clock mode and power-down states. For example, if the DIN word contains PD1 = 1, then the chip
remains powered up. If PD0 = PD1 = 0, a power-down
resumes after one conversion.
Hardware Power-Down
Pulling SHDN low places the converter in hardware
power-down (Table 6). Unlike software power-down
mode, the conversion is not completed; it stops coincidentally with SHDN being brought low. SHDN also controls the clock frequency in internal clock mode. Letting
SHDN float sets the internal clock frequency to 1.8MHz.
When returning to normal operation with SHDN floating,
there is a tRC delay of approximately 2MΩ x CL, where
CL is the capacitive loading on the SHDN pin. Pulling
SHDN high sets the internal clock frequency to 225kHz.
16
VREF = VDD = 3.0V
RLOAD = ∞
CODE = 1010101000
1000
IDD (µA)
MAX1248/MAX1249
+2.7V to +5.25V, Low-Power, 4-Channel,
Serial 10-Bit ADCs in QSOP-16
0.1
0.1
1
10
100
1k
10k
100k
1M
CONVERSION RATE (Hz)
Figure 12. Average Supply Current vs. Conversion Rate
with External Reference
This feature eases the settling-time requirement for the
reference voltage. With an external reference, the
MAX1248/MAX1249 can be considered fully powered
up within 2µs of actively pulling SHDN high.
Power-Down Sequencing
The MAX1248/MAX1249 auto power-down modes can
save considerable power when operating at less than
maximum sample rates. Figures 12, 13a, and 13b show
the average supply current as a function of the sampling
rate. The following discussion illustrates the various
power-down sequences.
Lowest Power at up to 500
Conversions/Channel/Second
The following examples illustrate two different powerdown sequences. Other combinations of clock rates,
compensation modes, and power-down modes may
give lowest power consumption in other applications.
Figure 13a depicts the MAX1248 power consumption
for one or eight channel conversions, utilizing full
power-down mode and internal-reference compensation. A 0.01µF bypass capacitor at REFADJ forms an
RC filter with the internal 20kΩ reference resistor with a
0.2ms time constant. To achieve full 10-bit accuracy, 8
time constants or 1.6ms are required after power-up.
Waiting 1.6ms in FASTPD mode instead of in full powerup can reduce the power consumption by a factor of 10
or more. This is achieved by using the sequence shown
in Figure 14.
______________________________________________________________________________________
+2.7V to +5.25V, Low-Power, 4-Channel,
Serial 10-Bit ADCs in QSOP-16
10,000
RLOAD = ∞
CODE = 1010101000
10
AVERAGE SUPPLY CURRENT (µA)
AVERAGE SUPPLY CURRENT (µA)
MAX1248/MAX1249
100
4 CHANNELS
RLOAD = ∞
CODE = 1010101000
1000
100
4 CHANNELS
1 CHANNEL
10
1 CHANNEL
1
1
0.01
0.1
1
10
100
1k
0.1
CONVERSION RATE (Hz)
Lowest Power at Higher Throughputs
Figure 13b shows the power consumption with
external-reference compensation in fast power-down,
with one and four channels converted. The external
4.7µF compensation requires a 75µs wait after power-up
with one dummy conversion. This circuit combines fast
multi-channel conversion with lowest power consumption
possible. Full power-down mode may provide increased
power savings in applications where the
MAX1248/MAX1249 are inactive for long periods of time,
but where intermittent bursts of high-speed conversions
are required.
Internal and External References
The MAX1248 can be used with an internal or external
reference voltage, whereas an external reference is
required for the MAX1249. An external reference can
be connected directly at VREF or at the REFADJ pin.
An internal buffer is designed to provide 2.5V at VREF
for both the MAX1248 and the MAX1249. The
MAX1248’s internally trimmed 1.21V reference is
buffered with a gain of 2.06. The MAX1249’s REFADJ
pin is also buffered with a gain of 2.06 to scale an
external 1.25V reference at REFADJ to 2.5V at VREF.
10
100
1k
10k
100k
1M
CONVERSION RATE (Hz)
Figure 13b. MAX1248 Supply Current vs. Conversion Rate,
FASTPD
3.0
2.5
POWER-UP DELAY (ms)
Figure 13a. MAX1248 Supply Current vs. Conversion Rate,
FULLPD
1
2.0
1.5
1.0
0.5
0
0
0.01
0.1
1
10
TIME IN SHUTDOWN (sec)
Figure 13c. Typical Reference-Buffer Power-Up Delay vs. Time
in Shutdown
Internal Reference (MAX1248)
The MAX1248’s full-scale range with the internal reference is 2.5V with unipolar inputs and ±1.25V with bipolar inputs. The internal-reference voltage is adjustable
to ±1.5% with the circuit of Figure 15.
______________________________________________________________________________________
17
MAX1248/MAX1249
+2.7V to +5.25V, Low-Power, 4-Channel,
Serial 10-Bit ADCs in QSOP-16
COMPLETE CONVERSION SEQUENCE
1.6ms WAIT
DIN
CH1
(ZEROS)
1
00
FULLPD
1
01
FASTPD
(ZEROS)
CH7
1
11
1
NOPD
00
1
FULLPD
01
FASTPD
1.21V
REFADJ
0V
2.50V
τ = RC = 20kΩ x CREFADJ
VREF
0V
tBUFFEN ≈ 75µs
Figure 14. MAX1248 FULLPD/FASTPD Power-Up Sequence
External Reference
With both the MAX1248 and MAX1249, an external reference can be placed at either the input (REFADJ) or
the output (VREF) of the internal reference-buffer amplifier. The REFADJ input impedance is typically 20kΩ for
the MAX1248 and higher than 100kΩ for the MAX1249,
where the internal reference is omitted. At VREF, the
DC input resistance is a minimum of 18kΩ. During conversion, an external reference at VREF must deliver
up to 350µA DC load current and have an output
impedance of 10Ω or less. If the reference has higher
output impedance or is noisy, bypass it close to the
VREF pin with a 4.7µF capacitor.
Using the REFADJ input makes buffering the external
reference unnecessary. To use the direct VREF input,
disable the internal buffer by tying REFADJ to VDD. In
power-down, the input bias current to REFADJ can be
as much as 25µA with REFADJ tied to V DD . Pull
REFADJ to AGND to minimize the input bias current in
power-down.
+3.3V
24k
MAX1248
510k
9
100k
REFADJ
0.01µF
Figure 15. MAX1248 Reference-Adjust Circuit
OUTPUT CODE
FULL-SCALE
TRANSITION
11 . . . 111
11 . . . 110
11 . . . 101
Transfer Function
Table 7 shows the full-scale voltage ranges for unipolar
and bipolar modes.
The external reference must have a temperature coefficient of 20ppm/°C or less to achieve accuracy to within
1LSB over the commercial temperature range of 0°C to
+70°C.
Figure 16 depicts the nominal, unipolar input/output
(I/O) transfer function, and Figure 17 shows the bipolar
input/output transfer function. Code transitions occur
halfway between successive-integer LSB values.
Output coding is binary, with 1LSB = 2.44mV (2.500V /
1024) for unipolar operation and 1LSB = 2.44mV
[(2.500V / 2 - -2.500V / 2) / 1024] for bipolar operation.
18
FS = VREF + COM
ZS = COM
VREF
1024
1LSB =
00 . . . 011
00 . . . 010
00 . . . 001
00 . . . 000
0 1
(COM)
2
3
INPUT VOLTAGE (LSBs)
FS
FS - 3/2LSB
Figure 16. Unipolar Transfer Function, Full Scale (FS) = VREF
+ COM, Zero Scale (ZS) = COM
______________________________________________________________________________________
+2.7V to +5.25V, Low-Power, 4-Channel,
Serial 10-Bit ADCs in QSOP-16
MAX1248/MAX1249
Table 7. Full Scale and Zero Scale
UNIPOLAR MODE
BIPOLAR MODE
Full Scale
Zero Scale
VREF + COM
COM
Positive
Zero
Negative
Full Scale
Scale
Full Scale
VREF / 2
+ COM
COM
-VREF / 2
+ COM
OUTPUT CODE
011 . . . 111
FS = VREF + COM
2
011 . . . 110
ZS = COM
000 . . . 010
000 . . . 001
000 . . . 000
SUPPLIES
-VREF
+ COM
2
VREF
1LSB =
1024
-FS =
+3V
+3V
GND
+3V
DGND
R* = 10Ω
111 . . . 111
111 . . . 110
111 . . . 101
VDD
AGND
COM DGND
100 . . . 001
100 . . . 000
COM*
- FS
MAX1248
MAX1249
+FS - 1LSB
INPUT VOLTAGE (LSB)
DIGITAL
CIRCUITRY
* OPTIONAL
*COM ≥ VREF / 2
Figure 17. Bipolar Transfer Function, Zero Scale (ZS) = COM,
Full Scale (FS) = VREF / 2 + COM
Layout, Grounding, and Bypassing
For best performance, use printed circuit boards.
Wire-wrap boards are not recommended. Board layout
should ensure that digital and analog signal lines are
separated from each other. Do not run analog and digital (especially clock) lines parallel to one another, or
digital lines underneath the ADC package.
Figure 18 shows the recommended system ground
connections. Establish a single-point analog ground
(star ground point) at AGND, separate from the logic
ground. Connect all other analog grounds and DGND
to the star ground. No other digital system ground
should be connected to this ground. For lowest noise
operation, the ground return to the star ground’s power
supply should be low impedance and as short as possible.
Figure 18. Power-Supply Grounding Connection
High-frequency noise in the VDD power supply may
affect the ADC’s high-speed comparator. Bypass the
supply to the star ground with 0.1µF and 1µF capacitors close to pin 1 of the MAX1248/MAX1249. Minimize
capacitor lead lengths for best supply-noise rejection.
If the +3V power supply is very noisy, a 10Ω resistor
can be connected as a lowpass filter (Figure 18).
High-Speed Digital Interfacing with QSPI
The MAX1248/MAX1249 can interface with QSPI using
the circuit in Figure 19 (fSCLK = 2.0MHz, CPOL = 0,
CPHA = 0). This QSPI circuit can be programmed to do a
conversion on each of the four channels. The result is
stored in memory without taxing the CPU, since QSPI
incorporates its own micro-sequencer.
The MAX1248/MAX1249 are QSPI compatible up to their
maximum external clock frequency of 2MHz.
______________________________________________________________________________________
19
MAX1248/MAX1249
+2.7V to +5.25V, Low-Power, 4-Channel,
Serial 10-Bit ADCs in QSOP-16
+3V
+3V
1µF
0.1µF
ANALOG
INPUTS
+2.5V
1
VDD
2
CH0
SCLK 16
3
CS 15
MAX1248
CH1 MAX1249 DIN 14
4
CH2
SSTRB 13
5
CH3
DOUT 12
6
COM
DGND 11
7
SHDN
AGND 10
8
VREF
REFADJ
SCK
PCS0
MC683XX
MOSI
MISO
(GND)
9
CLOCK CONNECTIONS NOT SHOWN
0.1µF
Figure 19. MAX1248/MAX1249 QSPI Connections External Reference
TMS320LC3x Interface
Figure 20 shows an application circuit to interface the
MAX1248/MAX1249 to the TMS320 in external clock
mode. The timing diagram for this interface circuit is
shown in Figure 21.
Use the following steps to initiate a conversion in the
MAX1248/MAX1249 and to read the results:
1) The TMS320 should be configured with CLKX
(transmit clock) as an active-high output clock and
CLKR (TMS320 receive clock) as an active-high
input clock. CLKX and CLKR on the TMS320 are
tied together with the MAX1248/MAX1249’s SCLK
input.
2) The MAX1248/MAX1249’s CS pin is driven low by
the TMS320’s XF_ I/O port, to enable data to be
clocked into the MAX1248/MAX1249’s DIN.
3) An 8-bit word (1XXXXX11) should be written to the
MAX1248/MAX1249 to initiate a conversion and
place the device into external clock mode. Refer to
Table 1 to select the proper XXXXX bit values for
your specific application.
4) The MAX1248/MAX1249’s SSTRB output is monitored via the TMS320’s FSR input. A falling edge on
the SSTRB output indicates that the conversion is in
progress and data is ready to be received from the
MAX1248/MAX1249.
20
XF
CLKX
CS
SCLK
TMS320LC3x
MAX1249
CLKR
DX
DIN
DR
DOUT
FSR
SSTRB
Figure 20. MAX1248/MAX1249-to-TMS320 Serial Interface
5) The TMS320 reads in one data bit on each of the
next 16 rising edges of SCLK. These data bits represent the 10 + 2-bit conversion result followed by
four trailing bits, which should be ignored.
6) Pull CS high to disable the MAX1248/MAX1249 until
the next conversion is initiated.
______________________________________________________________________________________
+2.7V to +5.25V, Low-Power, 4-Channel,
Serial 10-Bit ADCs in QSOP-16
MAX1248/MAX1249
CS
SCLK
DIN
START
SEL2
SEL1
SEL0
UNI/BIP SGL/DIF
PD1
PD0
HIGH
IMPEDANCE
SSTRB
DOUT
MSB
B8
B0
LSB
S1
S0
HIGH
IMPEDANCE
Figure 21. TMS320 Serial-Interface Timing Diagram
___________________________________________Ordering Information (continued)
INL
(LSB)
PART†
16 Plastic DIP
±1/2
16 Plastic DIP
±1
-40°C to +85°C
-40°C to +85°C
-55°C to +125°C
16 QSOP
16 QSOP
16 CERDIP*
±1/2
±1
±1/2
MAX1249ACEE
MAX1249BCEE
MAX1249AEPE
0°C to +70°C
0°C to +70°C
-40°C to +85°C
16 QSOP
16 QSOP
16 Plastic DIP
MAX1249BEPE
-40°C to +85°C
16 Plastic DIP
±1
MAX1249AEEE
-40°C to +85°C
16 QSOP
±1/2
PART†
TEMP. RANGE
PIN-PACKAGE
MAX1248AEPE
-40°C to +85°C
MAX1248BEPE
-40°C to +85°C
MAX1248AEEE
MAX1248BEEE
MAX1248AMJE
TEMP. RANGE
PIN-PACKAGE
INL
(LSB)
±1/2
±1
±1/2
MAX1248BMJE
-55°C to +125°C
16 CERDIP*
±1
MAX1249BEEE
-40°C to +85°C
16 QSOP
±1
MAX1249ACPE
0°C to +70°C
16 Plastic DIP
±1/2
MAX1249AMJE
-55°C to +125°C
16 CERDIP*
±1/2
MAX1249BCPE
0°C to +70°C
16 Plastic DIP
±1
MAX1249BMJE
-55°C to +125°C
16 CERDIP*
±1
Contact factory for availability of alternate surface-mount packages.
* Contact factory for availability of CERDIP package, and for processing to MIL-STD-883B.
†
______________________________________________________________________________________
21
_________________Pin Configuration
___________________Chip Information
TRANSISTOR COUNT: 2554
TOP VIEW
VDD 1
16 SCLK
CH0 2
15 CS
14 DIN
CH1 3
CH2 4
CH3 5
MAX1248
MAX1249
13 SSTRB
12 DOUT
COM 6
11 DGND
SHDN 7
10 AGND
VREF 8
9
REFADJ
DIP/QSOP
________________________________________________________Package Information
QSOP.EPS
MAX1248/MAX1249
+2.7V to +5.25V, Low-Power, 4-Channel,
Serial 10-Bit ADCs in QSOP-16
22
______________________________________________________________________________________
+2.7V to +5.25V, Low-Power, 4-Channel,
Serial 10-Bit ADCs in QSOP-16
PDIPN.EPS
CDIPS.EPS
______________________________________________________________________________________
23
MAX1248/MAX1249
___________________________________________Package Information (continued)
MAX1248/MAX1249
+2.7V to +5.25V, Low-Power, 4-Channel,
Serial 10-Bit ADCs in QSOP-16
NOTES
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.
24 ____________________Maxim Integrated Products, 120 San Gabriel Drive, Sunnyvale, CA 94086 408-737-7600
© 1998 Maxim Integrated Products
Printed USA
is a registered trademark of Maxim Integrated Products.
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