__________________General Description
The MAX187/MAX189 serial 12-bit analog-to-digital
converters (ADCs) operate from a single +5V supply
and accept a 0V to 5V analog input. Both parts feature
an 8.5µs successive-approximation ADC, a fast
track/hold (1.5µs), an on-chip clock, and a high-speed
3-wire serial interface.
The MAX187/MAX189 digitize signals at a 75ksps
throughput rate. An external clock accesses data from
the interface, which communicates without external
hardware to most digital signal processors and microcontrollers. The interface is compatible with SPI™,
QSPI™, and Microwire™.
The MAX187 has an on-chip buffered reference, and
the MAX189 requires an external reference. Both the
MAX187 and MAX189 save space with 8-pin DIP and
16-pin SO packages. Power consumption is 7.5mW
and reduces to only 10µW in shutdown.
Excellent AC characteristics and very low power consumption combined with ease of use and small package size make these converters ideal for remote DSP
and sensor applications, or for circuits where power
consumption and space are crucial.
___________________________Applications
Portable Data Logging
Remote Digital Signal Processing
Isolated Data Acquisition
High-Accuracy Process Control
________________________________Features
♦ 12-Bit Resolution
♦ ±
1
⁄2 LSB Integral Nonlinearity (MAX187A/MAX189A)
♦ Internal Track/Hold, 75kHz Sampling Rate
♦ Single +5V Operation
♦ Low Power: 2µA Shutdown Current
1.5mA Operating Current
♦ Internal 4.096V Buffered Reference (MAX187)
♦ 3-Wire Serial Interface, Compatible with SPI,
QSPI, and Microwire
♦ Small-Footprint 8-Pin DIP and 16-Pin SO
_________________Ordering Information
MAX187/MAX189
+5V, Low-Power, 12-Bit Serial ADCs
________________________________________________________________
Maxim Integrated Products
1
Call toll free 1-800-998-8800 for free samples or literature.
19-0196; Rev 0; 10/93
PART TEMP. RANGE PIN-PACKAGE
ERROR
(LSB)
MAX187ACPA 0°C to +70°C 8 Plastic DIP ±
1
⁄2
MAX187BCPA 0°C to +70°C 8 Plastic DIP ±1
MAX187CCPA 0°C to +70°C 8 Plastic DIP ±2
MAX187ACWE 0°C to +70°C 16 Wide SO ±
1
⁄2
MAX187BCWE 0°C to +70°C 16 Wide SO ±1
MAX187CCWE 0°C to +70°C 16 Wide SO ±2
MAX187BC/D 0°C to +70°C Dice* ±1
™ SPI and QSPI are trademarks of Motorola. Microwire is a trademark of National Semiconductor.
Ordering Information continued on last page.
* Dice are specified at T
A
= +25°C, DC parameters only.
** Contact factory for availability and processing to MIL-STD-883.
________________Functional Diagram
GND
5
REF
4
AIN
2
V
DD
1
T/H
BANDGAP
REFERENCE
+2.5V
(MAX187 ONLY)
10k
AV = 1.638
(4.096V)
REF-
REF+
DAC
OUTPUT
SHIFT
REGISTER
DOUT
6
SCLK
8
CS
7
SHDN
3
CONTROL
AND
TIMING
12-BIT
SAR
COMPARATOR
BUFFER ENABLE/DISABLE
NOTE: PIN NUMBERS SHOWN ARE FOR 8-PIN DIPs ONLY.
MAX187
MAX189
TOP VIEW
1
2
3
4
8
7
6
5
SCLK
CS
DOUT
GND
REF
SHDN
AIN
V
DD
DIP
MAX187
MAX189
Pin Configurations continued on last page.
_________________Pin Configurations
EVALUATION KIT MANUAL
FOLLOWS DATA SHEET
MAX187/MAX189
+5V, Low-Power, 12-Bit Serial ADCs
2 _______________________________________________________________________________________
ELECTRICAL CHARACTERISTICS
(VDD= +5V ±5%; GND = 0V; unipolar input mode; 75ksps, f
CLK
= 4.0MHz, external clock (50% duty cycle); MAX187—internal
reference: V
REF
= 4.096V, 4.7µF capacitor at REF pin, or MAX189—external reference: V
REF
= 4.096V applied to REF pin, 4.7µF
capacitor at REF pin; T
A
= T
MIN
to T
MAX
; unless otherwise noted.)
PARAMETER SYMBOL CONDITIONS MIN TYP MAX UNITS
DC ACCURACY (Note 1)
Resolution 12 Bits
MAX18_A ±
1
⁄2
Relative Accuracy (Note 2) MAX18_B ±1 LSB
MAX18_C ±2
Differential Nonlinearity DNL No missing codes over temperature ±1 LSB
Offset Error
MAX18_A ±1
1
⁄2
LSB
MAX18_B/C ±3
Gain Error (Note 3)
MAX187 ±3
LSB
MAX189A ±1
MAX189B/C ±3
Gain Temperature Coefficient External reference, 4.096V ±0.8 ppm/°C
DYNAMIC SPECIFICATIONS
(10kHz sine wave input, 0V to 4.096V
p-p
, 75ksps)
Signal-to-Noise plus
Distortion Ratio
SINAD 70 dB
Total Harmonic Distortion
(up to the 5th harmonic)
THD -80 dB
Spurious-Free Dynamic Range SFDR 80 dB
Small-Signal Bandwidth Rolloff -3dB 4.5 MHz
Full-Power Bandwidth 0.8 MHz
V
DD
to GND.............................................................-0.3V to +6V
AIN to GND................................................-0.3V to (V
DD
+ 0.3V)
REF to GND...............................................-0.3V to (V
DD
+ 0.3V)
Digital Inputs to GND.................................-0.3V to (V
DD
+ 0.3V)
Digital Outputs to GND..............................-0.3V to (V
DD
+ 0.3V)
SHDN
to GND.............................................-0.3V to (V
DD
+ 0.3V)
REF Load Current (MAX187) .........................4.0mA Continuous
REF Short-Circuit Duration (MAX187)................................20sec
DOUT Current..................................................................±20mA
Continuous Power Dissipation (T
A
= +70°C)
8-Pin Plastic DIP (derate 9.09mW/°C above +70°C)..500mW
16-Pin Wide SO (derate 8.70mW/°C above +70°C)...478mW
8-Pin CERDIP (derate 8.00mW/°C above +70°C) ......440mW
Operating Temperature Ranges:
MAX187_C_ _/MAX189_C_ _.............................0°C to +70°C
MAX187_E_ _/MAX189_E_ _..........................-40°C to +85°C
MAX187_MJA/MAX189_MJA .......................-55°C to +125°C
Storage Temperature Range............................-60°C to +150°C
Lead Temperature (soldering, 10sec)............................+300°C
ABSOLUTE MAXIMUM RATINGS
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.
PARAMETER SYMBOL CONDITIONS MIN TYP MAX UNITS
CONVERSION RATE
Conversion Time t
CONV
5.5 8.5 µs
Track/Hold Acquisition Time t
ACQ
1.5 µs
Throughput Rate External clock, 4MHz, 13 clocks 75 ksps
Aperture Delay t
APR
10 ns
Aperture Jitter <50 ps
ANALOG INPUT
Input Voltage Range 0 to V
REF
V
Input Capacitance (Note 4) 16 pF
INTERNAL REFERENCE (MAX187 only, reference buffer enabled)
REF Output Voltage V
REF
TA= +25°C 4.076 4.096 4.116
MAX187_C 4.060 4.132
TA= T
MIN
to T
MAX
MAX187_E 4.050 4.140
V
MAX187_M 4.040 4.150
REF Short-Circuit Current 30 mA
MAX187AC/BC ±30 ±50
REF Tempco
MAX187AE/BE ±30 ±60
ppm/°C
MAX187AM/BM ±30 ±80
MAX187C ±30
Load Regulation (Note 5) 0mA to 0.6mA output load 1 mV
EXTERNAL REFERENCE AT REF (Buffer disabled, V
REF
= 4.096V)
Input Voltage Range 2.50 V
DD
+ 50mV V
Input Current 200 350 µA
Input Resistance 12 20 kΩ
Shutdown REF Input Current 1.5 10 µA
MAX187/MAX189
+5V, Low-Power, 12-Bit Serial ADCs
_______________________________________________________________________________________ 3
ELECTRICAL CHARACTERISTICS (continued)
(VDD= +5V ±5%; GND = 0V; unipolar input mode; 75ksps, f
CLK
= 4.0MHz, external clock (50% duty cycle); MAX187—internal
reference: V
REF
= 4.096V, 4.7µF capacitor at REF pin, or MAX189—external reference: V
REF
= 4.096V applied to REF pin, 4.7µF
capacitor at REF pin; T
A
= T
MIN
to T
MAX
; unless otherwise noted.)
MAX187/MAX189
+5V, Low-Power, 12-Bit Serial ADCs
4 _______________________________________________________________________________________
ELECTRICAL CHARACTERISTICS (continued)
(VDD= +5V ±5%; GND = 0V; unipolar input mode; 75ksps, f
CLK
= 4.0MHz, external clock (50% duty cycle); MAX187—internal
reference: V
REF
= 4.096V, 4.7µF capacitor at REF pin, or MAX189—external reference: V
REF
= 4.096V applied to REF pin, 4.7µF
capacitor at REF pin; T
A
= T
MIN
to T
MAX
; unless otherwise noted.)
PARAMETER SYMBOL CONDITIONS MIN TYP MAX UNITS
DIGITAL INPUTS (SCLK, CS,
SHDN
)
SCLK, CSInput High Voltage V
INH
2.4 V
SCLK, CSInput Low Voltage V
INL
0.8 V
SCLK, CSInput Hysteresis V
HYST
0.15 V
SCLK, CSInput Leakage I
IN
VIN= 0V or V
DD
±1 µA
SCLK, CSInput Capacitance C
IN
(Note 4) 15 pF
SHDN
Input High Voltage V
INSH
V
DD
- 0.5 V
SHDN
Input Low Voltage V
INSL
0.5 V
SHDN
Input Current I
INS
SHDN
= VDDor 0V ±4.0 µA
SHDN
Input Mid Voltage V
IM
1.5 VDD-1.5 V
SHDN
Voltage, Floating V
FLT
SHDN
= open 2.75 V
SHDN
Maximum Allowed
SHDN
= open -100 100 nA
Leakage, Mid Input
DIGITAL OUTPUT (DOUT)
Output Voltage Low V
OL
I
SINK
= 5mA 0.4
V
I
SINK
= 16mA 0.3
Output Voltage High V
OHISOURCE
= 1mA 4 V
Three-State Leakage Current I
L
CS
= 5V ±10 µA
Three-State Output
C
OUT
CS
= 5V (Note 4) 15 pF
Capacitance
POWER REQUIREMENTS
Supply Voltage V
DD
4.75 5.25 V
Supply Current
Operating mode
MAX187 1.5 2.5
mA
I
DD
MAX189 1.0 2.0
Power-down mode 2 10 µA
VDD= +5V, ±5%; external reference, 4.096V;
Power-Supply Rejection PSR
full-scale input (Note 6)
±0.06 ±0.5 mV
MAX187/MAX189
+5V, Low-Power, 12-Bit Serial ADCs
_______________________________________________________________________________________ 5
TIMING CHARACTERISTICS
(VDD= +5.0V ±5%, TA= T
MIN
to T
MAX
, unless otherwise noted.)
PARAMETER SYMBOL CONDITIONS MIN TYP MAX UNITS
Track/Hold Acquisition Time t
ACQ
CS
= high (Note 7) 1.5 µs
SCLK Fall to Output Data Valid t
DO
C
LOAD
= 100pF
MAX18_ _C/E 20 150
ns
MAX18_ _M 20 200
CS
Fall to Output Enable t
DV
C
LOAD
= 100pF 100 ns
CS
Rise to Output Disable t
TR
C
LOAD
= 100pF 100 ns
SCLK Clock Frequency f
SCLK
5 MHz
SCLK Pulse Width High t
CH
100 ns
SCLK Pulse Width Low t
CL
100 ns
SCLK Low to
CS
Fall
t
CSO
50 ns
Setup Time
CS
Pulse Width t
CS
500 ns
Note 1: Tested at VDD= +5V.
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: MAX187—internal reference, offset nulled; MAX189–external +4.096V reference, offset nulled. Excludes reference errors.
Note 4: Guaranteed by design. Not subject to production testing.
Note 5: External load should not change during conversion for specified ADC accuracy.
Note 6: DC test, measured at 4.75V and 5.25V only.
Note 7: To guarantee acquisition time, t
ACQ
is the maximum time the device takes to acquire the signal, and is also the minimum
time needed for the signal to be acquired.
MAX187/MAX189
+5V, Low-Power, 12-Bit Serial ADCs
6 _______________________________________________________________________________________
0.12
0.10
0.08
0.06
0.04
0.02
0
-60 -20 20 60 100 140
TEMPERATURE (°C)
POWER-SUPPLY REJECTION (mV)
POWER-SUPPLY REJECTION vs.
TEMPERATURE
0.14
0.16
4.090
4.087
4.084
4.081
-60 -20 20 60 140
TEMPERATURE (°C)
INTERNAL REFERENCE VOLTAGE (V)
V
REF
vs. TEMPERATURE
100
4.089
4.088
4.086
4.085
4.083
4.082
4.080
1.8
1.4
1.0
0.6
0.2
-60 -20 20 60 100 140
TEMPERATURE (°C)
SUPPLY CURRENT (mA)
MAX187
MAX189
SUPPLY CURRENT vs. TEMPERATURE
2.2
________________________________________________T ypical Operating Characteristics
5
4
3
2
1
0
-60 -20 20 60 100 140
TEMPERATURE (°C)
SHUTDOWN SUPPLY CURRENT (µA)
SHUTDOWN SUPPLY CURRENT vs.
TEMPERATURE
6
7
MAX187/MAX189
+5V, Low-Power, 12-Bit Serial ADCs
_______________________________________________________________________________________ 7
PIN
DIP WIDE SO
NAME FUNCTION
11VDDSupply voltage, +5V ±5%
2 3 AIN Sampling analog input, 0V to V
REF
range
36SHDN
Three-level shutdown input. Pulling
SHDN
low shuts the MAX187/MAX189
down to 10µA (max) supply current. Both MAX187 and MAX189 are fully operational with either
SHDN
high or floating. For the MAX187, pulling
SHDN
high
enables the internal reference, and letting
SHDN
float disables the internal
reference and allows for the use of an external reference.
4 8 REF
Reference voltage—sets analog voltage range and functions as a 4.096V output
for the MAX187 with enabled internal reference. REF also serves as a +2.5V to
V
DD
input for a precision reference for both MAX187 (disabled internal reference)
and MAX189. Bypass with 4.7µF if internal reference is used, and with 0.1µF if an
external reference is applied.
5 — GND Analog and digital ground
— 10 AGND Analog ground
— 11 DGND Digital ground
6 12 DOUT Serial data output. Data changes state at SCLK’s falling edge.
715
CS
Active-low chip select initiates conversions on the falling edge. When CSis high,
DOUT is high impedance.
8 16 SCLK Serial clock input. Clocks data out with rates up to 5MHz.
— 2,4,5,7,9,13,14 N.C. Not internally connected. Connect to AGND for best noise performance.
_______________________________________________________________________Pin Description
_______________Detailed Description
Converter Operation
The MAX187/MAX189 use input track/hold (T/H) and
successive approximation register (SAR) circuitry to
convert an analog input signal to a digital 12-bit output.
No external hold capacitor is needed for the T/H.
Figures 3a and 3b show the MAX187/MAX189 in their
simplest configuration. The MAX187/MAX189 convert
input signals in the 0V to V
REF
range in 10µs, including
T/H acquisition time. The MAX187’s internal reference
is trimmed to 4.096V, while the MAX189 requires an
external reference. Both devices accept external reference voltages from +2.5V to VDD. The serial interface
requires only three digital lines, SCLK,
CS,
and DOUT,
and provides easy interface to microprocessors (µPs).
Both converters have two modes: normal and shut-
down. Pulling
SHDN
low shuts the device down and
reduces supply current to below 10µA, while pulling
SHDN
high or leaving it floating puts the device into the
operational mode. A conversion is initiated by
CS
falling. The conversion result is available at DOUT in
unipolar serial format. A high bit, signaling the end of
conversion (EOC), followed by the data bits (MSB first),
make up the serial data stream.
The MAX187 operates in one of two states: (1) internal
reference and (2) external reference. Select internal
reference operation by forcing
SHDN
high, and external
reference operation by floating
SHDN
.
Analog Input
Figure 4 illustrates the sampling architecture of the
ADC’s analog comparator. The full-scale input voltage
depends on the voltage at REF.
REFERENCE
ZERO FULL
SCALE SCALE
Internal Reference
0V +4.096V
(MAX187 only)
External Reference 0V V
REF
For specified accuracy, the external reference voltage
range spans from +2.5V to VDD.
MAX187/MAX189
+5V, Low-Power, 12-Bit Serial ADCs
8 _______________________________________________________________________________________
DOUT DOUT
3k
DGND
C
LOAD
= 100pF C
LOAD
= 100pF
3k
DGND
+5V
b. High-Z to VOL and VOH to V
OL
a. High-Z to VOH and VOL to V
OH
DOUT DOUT
3k
DGND
C
LOAD
= 100pF C
LOAD
= 100pF
3k
DGND
+5V
b. VOLto High-Za. V
OH
to High-Z
Figure 1. Load Circuits for DOUT Enable Time
Figure 2. Load Circuits for DOUT Disable Time
Track/Hold
In track mode, the analog signal is acquired and stored
in the internal hold capacitor. In hold mode, the T/H
switch opens and maintains a constant input to the
ADC’s SAR section.
During acquisition, the analog input AIN charges
capacitor C
HOLD
. Bringing CSlow ends the acquisition
interval. At this instant, the T/H switches the input side
of C
HOLD
to GND. The retained charge on C
HOLD
represents a sample of the input, unbalancing the node
ZERO at the comparator’s input.
In hold mode, the capacitive DAC adjusts during the
remainder of the conversion cycle to restore node
ZERO to 0V within the limits of a 12-bit resolution. This
action is equivalent to transferring a charge from
C
HOLD
to the binary-weighted capacitive DAC, which in
turn forms a digital representation of the analog input
signal. At the conversion’s end, the input side of C
HOLD
switches back to AIN, and C
HOLD
charges to the input
signal again.
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, the acquisition time lengthens and more time
must be allowed between conversions. Acquisition time
is calculated by:
t
ACQ
= 9 (RS+ RIN) 16pF,
where RIN= 5kΩ, RS= the source impedance of the
input signal, and t
ACQ
is never less than 1.5µs. Source
impedances below 5kΩ do not significantly affect the
AC performance of the ADC.
MAX187/MAX189
+5V, Low-Power, 12-Bit Serial ADCs
_______________________________________________________________________________________ 9
AIN
TRACK
INPUT
HOLD
C
PACKAGE
GND
TRACK
HOLD
5k
R
IN
C
HOLD
16pF
-+
C
SWITCH
COMPARATOR
ZERO
REF
12-BIT CAPACITIVE DAC
AT THE SAMPLING INSTANT,
THE INPUT SWITCHES FROM
AIN TO GND.
ON
OFF
SHUTDOWN
INPUT
ANALOG INPUT
0V TO +5V
+5V
1
2
3
4
V
DD
AIN
SHDN
REF
8
7
6
5
SCLK
CS
DOUT
GND
SERIAL
INTERFACE
0.1µF
4.7µF
0.1µF
REFERENCE
INPUT
MAX189
ON
OFF
SHUTDOWN
INPUT
ANALOG INPUT
0V TO +5V
+5V
1
2
3
4
V
DD
AIN
SHDN
REF
8
7
6
5
SCLK
CS
DOUT
GND
SERIAL
INTERFACE
4.7µF
4.7µF
0.1µF
MAX187
Figure 3a. MAX187 Operational Diagram
Figure 3b. MAX189 Operational Diagram
Figure 4. Equivalent Input Circuit
MAX187/MAX189
+5V, Low-Power, 12-Bit Serial ADCs
10 ______________________________________________________________________________________
COMPLETE CONVERSION SEQUENCE
t
WAKE
POWERED UPPOWERED DOWNPOWERED UP
CONVERSION 0 CONVERSION 1
DOUT
CS
SHDN
Input Bandwidth
The ADCs’ input tracking circuitry has a 4.5MHz smallsignal bandwidth, and an 8V/µs slew rate. 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 aliasing of unwanted high-frequency signals into
the frequency band of interest, an anti-alias filter is recommended. See the MAX274/MAX275 continuous-time
filters data sheet.
Input Protection
Internal protection diodes that clamp the analog input
allow the input to swing from GND - 0.3V to VDD+ 0.3V
without damage. However, for accurate conversions
near full scale, the input must not exceed VDDby more
than 50mV, or be lower than GND by 50mV.
If the analog input exceeds the supplies by more than
50mV beyond the supplies, limit the input current to
2mA, since larger currents degrade conversion
accuracy.
Driving the Analog Input
The input lines to AIN and GND should be kept as short
as possible to minimize noise pickup. Shield longer
leads. Also see the
Input Protection
section
.
Because the MAX187/MAX189 incorporate a T/H, the
drive requirements of the op amp driving AIN are less
stringent than those for a successive-approximation
ADC without a T/H. The typical input capacitance is
16pF. The amplifier bandwidth should be sufficient to
handle the frequency of the input signal. The MAX400
and OP07 work well at lower frequencies. For higherfrequency operation, the MAX427 and OP27 are practical choices. The allowed input frequency range is limit-
ed by the 75ksps sample rate of the MAX187/MAX189.
Therefore, the maximum sinusoidal input frequency
allowed is 37.5kHz. Higher-frequency signals cause
aliasing problems unless undersampling techniques
are used.
Reference
The MAX187 can be used with an internal or external reference, while the MAX189 requires an external reference.
Internal Reference
The MAX187 has an on-chip reference with a buffered
temperature-compensated bandgap diode, lasertrimmed to +4.096V ±0.5%. Its output is connected to
REF and also drives the internal DAC. The output can
be used as a reference voltage source for other components and can source up to 0.6mA. Decouple REF
with a 4.7µF capacitor. The internal reference is
enabled by pulling the
SHDN
pin high. Letting
SHDN
float disables the internal reference, which allows the
use of an external reference, as described in the
External Reference
section.
External Reference
The MAX189 operates with an external reference at the
REF pin. To use the MAX187 with an external reference,
disable the internal reference by letting
SHDN
float. Stay
within the voltage range +2.5V to VDDto achieve specified accuracy. The minimum input impedance is 12kΩ
for DC currents. During conversion, the external reference must be able to deliver up to 350µA DC load current and have an output impedance of 10Ω or less. The
recommended minimum value for the bypass capacitor
is 0.1µF. If the reference has higher output impedance
or is noisy, bypass it close to the REF pin with a 4.7µF
capacitor.
Figure 5. MAX187/MAX189 Shutdown Sequence
____________________Serial Interface
Initialization After Power-Up and
Starting a Conversion
When power is first applied, it takes the fully discharged 4.7µF reference bypass capacitor up to 20ms
to provide adequate charge for specified accuracy.
With
SHDN
not pulled low, the MAX187/MAX189 are
now ready to convert.
To start a conversion, pull CSlow. At
CS’s
falling edge,
the T/H enters its hold mode and a conversion is initiated. After an internally timed 8.5µs conversion period,
the end of conversion is signaled by DOUT pulling
high. Data can then be shifted out serially with the
external clock.
Using
SHDN
to Reduce Supply Current
Power consumption can be reduced significantly by
shutting down the MAX187/MAX189 between conversions. This is shown in Figure 6, a plot of average supply current vs. conversion rate. Because the MAX189
uses an external reference voltage (assumed to be present continuously), it "wakes up" from shutdown more
quickly, and therefore provides lower average supply
currents. The wakeup-time, t
WAKE,
is the time from
SHDN deasserted to the time when a conversion may
be initiated. For the MAX187, this time is 2µs. For the
MAX189, this time depends on the time in shutdown
(see Figure 7) because the external 4.7µF reference
bypass capacitor loses charge slowly during shutdown
(see the specifications for shutdown, REF Input Current
= 10µA max).
External Clock
The actual conversion does not require the external
clock. This frees the µP from the burden of running the
SAR conversion clock, and allows the conversion result
to be read back at the µP’s convenience at any clock
rate from 0MHz to 5MHz. The clock duty cycle is unrestricted if each clock phase is at least 100ns. Do not
run the clock while a conversion is in progress.
Timing and Control
Conversion-start and data-read operations are controlled by the CSand SCLK digital inputs. The timing
diagrams of Figures 8 and 9 outline the operation of the
serial interface.
A CSfalling edge initiates a conversion sequence: The
T/H stage holds input voltage, the ADC begins to convert, and DOUT changes from high impedance to logic
low. SCLK must be kept inactive during the conversion.
An internal register stores the data when the conversion
is in progress.
End of conversion (EOC) is signaled by DOUT going
high. DOUT’s rising edge can be used as a framing
signal. SCLK shifts the data out of this register any time
after the conversion is complete. DOUT transitions on
SCLK’s falling edge. The next falling clock edge produces the MSB of the conversion at DOUT, followed by
the remaining bits. Since there are 12 data bits and one
leading high bit, at least 13 falling clock edges are
needed to shift out these bits. Extra clock pulses occurring after the conversion result has been clocked out,
and prior to a rising edge of CS, produce trailing 0s at
DOUT and have no effect on converter operation.
+5V, Low-Power, 12-Bit Serial ADCs
______________________________________________________________________________________ 11
10000
1000
100
10
1
0.1 1 10 100 1000 10000
CONVERSIONS PER SECOND
SUPPLY CURRENT (µA)
100000
MAX187
MAX189*
*REF CONNECTED TO V
DD
3.0
2.5
2.0
1.5
1.0
0.5
0
0.0001 0.001 0.01 0.1 1 10
TIME IN SHUTDOWN (sec)
t
WAKE
(ms)
MAX187/MAX189
Figure 6. Average Supply Current vs. Conversion Rate
Figure 7. t
WAKE
vs. Time in Shutdown (MAX187 only)
MAX187/MAX189
+5V, Low-Power, 12-Bit Serial ADCs
12 ______________________________________________________________________________________
CS
SCLK
DOUT
INTERFACE IDLE
CONVERSION
IN PROGRESS
EOC
EOC
A/D
STATE
TRACK
CONVERSION
0
0µs8.5µs (t
CONV
)
TRAILING
ZEROS
IDLE
0µs
0.5µs
(tCS)
TOTAL = 12.25µs
CONV. 1
CLOCK OUTPUT DATA
TRACK
12 × 0.250µs = 3.25µs
B11 B10 B9 B8 B7 B6 B5 B4 B3 B2 B1 B0
14 8 12
MINIMUM
CYCLE TIME
…
…
…
…
CS
SCLK
DOUT
INTERNAL
T/H
(TRACK)
t
CS0
t
CONV
t
DV
t
APR
(HOLD) (TRACK)
B2 B1 B0
t
CH
t
DO
t
CL
t
TR
t
CS
Figure 8. MAX187/MAX189 Interface Timing Sequence
Figure 9. MAX187/MAX189 Detailed Serial-Interface Timing
Minimum cycle time is accomplished by using DOUT’s
rising edge as the EOC signal. Clock out the data with
13 clock cycles at full speed. Raise CSafter the conversion’s LSB has been read. After the specified minimum
time, t
ACQ
, CScan be pulled low again to initiate the
next conversion.
Output Coding and Transfer Function
The data output from the MAX187/MAX189 is binary,
and Figure 10 depicts the nominal transfer function.
Code transitions occur halfway between successive
integer LSB values. If V
REF
= +4.096V, then
1 LSB = 1.00mV or 4.096V/4096.
_____________Dynamic Performance
High-speed sampling capability and a 75ksps throughput make the MAX187/MAX189 ideal for wideband signal processing. To support these and other related
applications, Fast Fourier Transform (FFT) test techniques are used to guarantee the ADC’s dynamic frequency response, distortion, and noise at the rated
throughput. Specifically, this involves applying a lowdistortion sine wave to the ADC input and recording the
digital conversion results for a specified time. The data
is then analyzed using an FFT algorithm that determines its spectral content. Conversion errors are then
seen as spectral elements outside of the fundamental
input frequency. ADCs have traditionally been evaluated by specifications such as Zero and Full-Scale Error,
Integral Nonlinearity (INL), and Differential Nonlinearity
(DNL). Such parameters are widely accepted for specifying performance with DC and slowly varying signals,
but are less useful in signal-processing applications,
where the ADC’s impact on the system transfer function
is the main concern. The significance of various DC
errors does not translate well to the dynamic case, so
different tests are required.
Signal-to-Noise Ratio and
Effective Number of Bits
Signal-to-noise plus distortion (SINAD) is the ratio of the
fundamental input frequency’s RMS amplitude to the
RMS amplitude of all other ADC output signals. The
input bandwidth is limited to frequencies above DC and
below one-half the ADC sample (conversion) rate.
The theoretical minimum ADC noise is caused by quantization error and is a direct result of the ADC’s resolution: SINAD = (6.02N + 1.76)dB, where N is the number
of bits of resolution. An ideal 12-bit ADC can, therefore,
do no better than 74dB. An FFT plot of the output
shows the output level in various spectral bands. Figure
11 shows the result of sampling a pure 10kHz sine
wave at a 75ksps rate with the MAX187/MAX189.
MAX187/MAX189
+5V, Low-Power, 12-Bit Serial ADCs
______________________________________________________________________________________ 13
11…111
11…110
11…101
00…011
00…010
00…001
00…000
012 FS
OUTPUT CODE
FS - 3/2LSBINPUT VOLTAGE (LSBs)
FS = +4.096V
1LSB =
FS
4096
FULL-SCALE
TRANSITION
3
AMPLITUDE (dB)
20
0
-20
-40
-60
-80
-100
-120
-140
0 18.75 37.5
fS = 75ksps
f
T
= 10kHz
T
A
= +25°C
FREQUENCY (kHz)
Figure 11. MAX187/MAX189 FFT plot
Figure 10. MAX187/MAX189 Unipolar Transfer Function,
4.096V = Full Scale
MAX187/MAX189
+5V, Low-Power, 12-Bit Serial ADCs
14 ______________________________________________________________________________________
11.8
11.6
11.4
11.2
11.0
10.8
10.6
10.4
10.2
1 10 100 1000
(UNDERSAMPLED)
EFFECTIVE BITS
INPUT FREQUENCY (kHz)
12.0
12.2
MAX187
MAX189
CS
SCLK
DOUT
I/O
SCK
MISO
+5V
SS
a. SPI
MAX187
MAX189
CS
SCLK
DOUT
CS
SCK
MISO
+5V
SS
b. QSPI
MAX187
MAX189
CS
SCLK
DOUT
I/O
SK
SI
c. MICROWIRE
Figure 13. Common Serial-Interface Connections to the
MAX187/MAX189
Figure 12. Effective Bits vs. Input Frequency
The effective resolution (effective number of bits) the
ADC provides can be determined by transposing the
above equation and substituting in the measured
SINAD: N = (SINAD - 1.76)/6.02. Figure 12 shows the
effective number of bits as a function of the input frequency for the MAX187/MAX189.
Total Harmonic Distortion
If a pure sine wave is sampled by an ADC at greater
than the Nyquist frequency, the nonlinearities in the
ADC’s transfer function create harmonics of the input
frequency present in the sampled output data.
Total Harmonic Distortion (THD) is the ratio of the RMS
sum of all the harmonics (in the frequency band above
DC and below one-half the sample rate, but not including the DC component) to the RMS amplitude of the
fundamental frequency. This is expressed as follows:
THD = 20log
√ V
2
2
+ V
3
2
+ V
4
2
+ … V
N
2
V
1
where V1is the fundamental RMS amplitude, and V
2
through VNare the amplitudes of the 2nd through Nth
harmonics. The THD specification in the
Electrical
Characteristics
includes the 2nd through 5th
harmonics.
____________Applications Information
Connection to Standard Interfaces
The MAX187/MAX189 serial interface is fully compatible
with SPI, QSPI, and Microwire standard serial
interfaces.
If a serial interface is available, set the CPU’s serial
interface in master mode so the CPU generates the serial clock. Choose a clock frequency up to 2.5MHz.
1. Use a general-purpose I/O line on the CPU to pull
CS
low. Keep SCLK low.
2. Wait the for the maximum conversion time specified
before activating SCLK. Alternatively, look for a
DOUT rising edge to determine the end of
conversion.
3. Activate SCLK for a minimum of 13 clock cycles. The
first falling clock edge will produce the MSB of the
DOUT conversion. DOUT output data transitions on
SCLK’s falling edge and is available in MSB-first format. Observe the SCLK to DOUT valid timing characteristic. Data can be clocked into the µP on SCLK’s
rising edge.
4. Pull
CS
high at or after the 13th falling clock edge. If
CS
remains low, trailing zeros are clocked out after
the LSB.
5. With
CS
= high, wait the minimum specified time, tCS,
before launching a new conversion by pulling
CS
low. If a conversion is aborted by pulling CShigh
before the conversions end, wait for the minimum
acquisition time, t
ACQ
, before starting a new
conversion.
Data can be output in 1-byte chunks or continuously, as
shown in Figure 8. The bytes will contain the result of
the conversion padded with one leading 1, and trailing
0s if SCLK is still active with CS kept low.
MAX187/MAX189
+5V, Low-Power, 12-Bit Serial ADCs
______________________________________________________________________________________ 15
MSB D10 D9 D8 D7 D6 D5 D4 D3 D2 D1 LSB
HI-Z
t
CONV
DOUT
CS
SCLK
HI-Z
1ST BYTE READ 2ND BYTE READ
EOC
MSB D10 D9 D8 D7 D6 D5 D4 D3 D2 D1 LSB
HI-Z
t
CONV
DOUT
CS
SCLK
HI-Z
EOC
Figure 14. SPI/Microwire Serial Interface Timing (CPOL = CPHA = 0)
Figure 15. QSPI Serial Interface Timing (CPOL = CPHA = 0)
MAX187/MAX189
SPI and Microwire
When using SPI or QSPI, set CPOL = 0 and CPHA = 0.
Conversion begins with a CSfalling edge. DOUT goes
low, indicating a conversion in progress. Wait until
DOUT goes high or the maximum specified 8.5µs conversion time. Two consecutive 1-byte reads are
required to get the full 12 bits from the ADC. DOUT output data transitions on SCLK’s falling edge and is
clocked into the µP on SCLK’s rising edge.
The first byte contains a leading 1 and 7 bits of conversion result. The second byte contains the remaining 5
bits and 3 trailing 0s. See Figure 13 for connections
and Figure 14 for timing.
QSPI
Set CPOL = CPHA = 0. Unlike SPI, which requires two
1-byte reads to acquire the 12 bits of data from the
ADC, QSPI allows the minimum number of clock cycles
necessary to clock in the data. The MAX187/MAX189
require 13 clock cycles from the µP to clock out the
12 bits of data with no trailing 0s (Figure 15). The maximum clock frequency to ensure compatibility with QSPI
is 2.77MHz.
Opto-Isolated Interface,
Serial-to-Parallel Conversion
Many industrial applications require electrical isolation
to separate the control electronics from hazardous
electrical conditions, provide noise immunity, or prevent excessive current flow where ground disparities
exist between the ADC and the rest of the system.
Isolation amplifiers typically used to accomplish these
tasks are expensive. In cases where the signal is eventually converted to a digital form, it is cost effective to
isolate the input using opto-couplers in a serial link.
The MAX187 is ideal in this application because it
includes both T/H amplifier and voltage reference,
operates from a single supply, and consumes very little
power (Figure 16).
+5V, Low-Power, 12-Bit Serial ADCs
16 ______________________________________________________________________________________
MAX187
ANALOG
INPUT
SIGNAL
GROUND
4.7µF5
4
2
1
6
8
7
3
10µF
0.1µF
GND
REF
AIN
V
DD
DOUT
SCLK
CS
SHDN
+5V
3k
3k
470Ω
8
7
6
5
1
2
3
4
200Ω
200Ω
+5V
8
7
6
5
1
2
3
4
1
2
3
4
8
7
6
5
+5V ON THIS SIDE OF
BARRIER MUST BE ISOLATED POWER
6N136
6N136
6N136
74HC04
74HC04
8.2k
+5V
14
11
12
10
7
6
5
4
3
2
1
15
16
D11 (MSB)
D10
D9
D8
+5V
0.1µF
CS/START
SCLK/INPUT CLOCK
SER
SCK
RCK
SCLR
QH
QG
QF
QE
QD
QC
QB
QA
13 8
9
7
6
5
4
3
2
1
15
16
+5V
0.1µF
13 8
14
11
12
10
74HC595
74HC595
SER
SCK
RCK
SCLR
QH
QG
QF
QE
QD
QC
QB
QA
QH′
D0(LSB)
D1
D2
D3
D4
D5
D6
D7
Figure 16. 12-Bit Isolated ADC
MAX187/MAX189
+5V, Low-Power, 12-Bit Serial ADCs
______________________________________________________________________________________ 17
SUPPLIES
+5V GND
DGND+5V
DIGITAL
CIRCUITRY
DGNDAGNDV
DD
MAX187
MAX189
*OPTIONAL
R* = 10Ω
4.7µF
0.01µF
The ADC results are transmitted across a 1500V isolation barrier provided by three 6N136 opto-isolators.
Isolated power must be supplied to the converter and
the isolated side of the opto-couplers. 74HC595 threestate shift registers are used to construct a 12-bit parallel data output. The timing sequence is identical to the
timing shown in Figure 8. Conversion speed is limited
by the delay through the opto-isolators. With a 140kHz
clock, conversion time is 100µs.
The universal 12-bit parallel data output can also be
used without the isolation stage when a parallel interface is required. Clock frequencies up to 2.9MHz are
possible without violating the 20ns shift-register setup
time. Delay or invert the clock signal to the shift registers beyond 2.9MHz.
Layout, Grounding, Bypassing
For best performance, use printed circuit boards. Wirewrap 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 17 shows the recommended system ground
connections. A single-point analog ground (“star”
ground point) should be established at GND, separate
from the logic ground. All other analog grounds should
be connected to this ground. The 16-pin versions also
have a dedicated DGND pin available. Connect DGND
to this star ground point for further noise reduction. No
other digital system ground should be connected to
this single-point analog ground. The ground return to
the power supply for this ground should be low impedance and as short as possible for noise-free operation.
High-frequency noise in the VDDpower supply may
affect the ADC’s high-speed comparator. Bypass this
supply to the single-point analog ground with 0.01µF
and 4.7µF bypass capacitors. Minimize capacitor lead
lengths for best supply-noise rejection. If the +5V
power supply is very noisy, a 10Ω resistor can be connected as a lowpass filter to attenuate supply noise
(Figure 17).
Figure 17. Power-Supply Grounding Condition
MAX187/MAX189
+5V, Low-Power, 12-Bit Serial ADCs
18 ______________________________________________________________________________________
__Ordering Information (continued)
PART TEMP. RANGE PIN-PACKAGE
ERROR
(LSB)
MAX187AEPA -40°C to +85°C 8 Plastic DIP ±
1
⁄2
MAX187BEPA -40°C to +85°C 8 Plastic DIP ±1
MAX187CEPA -40°C to +85°C 8 Plastic DIP ±2
MAX187AEWE -40°C to +85°C 16 Wide SO ±
1
⁄2
MAX187BEWE -40°C to +85°C 16 Wide SO ±1
MAX187CEWE -40°C to +85°C 16 Wide SO ±2
MAX187AMJA -55°C to +125°C 8 CERDIP** ±
1
⁄2
MAX187BMJA -55°C to +125°C 8 CERDIP** ±1
MAX189ACPA 0°C to +70°C 8 Plastic DIP ±
1
⁄2
MAX189BCPA 0°C to +70°C 8 Plastic DIP ±1
MAX189CCPA 0°C to +70°C 8 Plastic DIP ±2
MAX189ACWE 0°C to +70°C 16 Wide SO ±
1
⁄2
MAX189BCWE 0°C to +70°C 16 Wide SO ±1
MAX189CCWE 0°C to +70°C 16 Wide SO ±2
MAX189BC/D 0°C to +70°C Dice* ±1
MAX189AEPA -40°C to +85°C 8 Plastic DIP ±
1
⁄2
MAX189BEPA -40°C to +85°C 8 Plastic DIP ±1
MAX189CEPA -40°C to +85°C 8 Plastic DIP ±2
MAX189AEWE -40°C to +85°C 16 Wide SO ±
1
⁄2
MAX189BEWE -40°C to +85°C 16 Wide SO ±1
MAX189CEWE -40°C to +85°C 16 Wide SO ±2
MAX189AMJA -55°C to +125°C 8 CERDIP** ±
1
⁄2
MAX189BMJA -55°C to +125°C 8 CERDIP** ±1
* Dice are specified at TA= +25°C, DC parameters only.
**Contact factory for availability and processing to MIL-STD-883.
____Pin Configurations (continued)
16
15
14
13
12
11
10
9
1
2
3
4
5
6
7
8
SCLK
CS
N.C.
N.C.
N.C.
AIN
N.C.
V
DD
MAX187
MAX189
DOUT
DGND
AGND
N.C.
REF
N.C.
SHDN
N.C.
Wide SO
MAX187/MAX189
+5V, Low-Power, 12-Bit Serial ADCs
______________________________________________________________________________________ 19
___________________Chip Topography
DGND
AIN
AGND
0.151"
(3.84mm)
0.117"
(2.97mm)
REF
AGND
DOUT
V
DD
SCLK
SHDN
CS
TRANSISTOR COUNT: 2278;
SUBSTRATE CONNECTED TO VDD.
MAX187/MAX189
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.
20
__________________Maxim Integrated Products, 120 San Gabriel Drive, Sunnyvale, CA 94086 (408) 737-7600
© 1993 Maxim Integrated Products Printed USA is a registered trademark of Maxim Integrated Products.
MAX187/MAX189
+5V, Low-Power, 12-Bit Serial ADCs
________________________________________________________________Package Information
DIM
A
A1
B
C
E
e
H
L
MIN
0.093
0.004
0.014
0.009
0.291
0.394
0.016
MAX
0.104
0.012
0.019
0.013
0.299
0.419
0.050
MIN
2.35
0.10
0.35
0.23
7.40
10.00
0.40
MAX
2.65
0.30
0.49
0.32
7.60
10.65
1.27
INCHES MILLIMETERS
21-0042A
W PACKAGE
SMALL
OUTLINE
DIM
D
D
D
D
D
MIN
0.398
0.447
0.496
0.598
0.697
MAX
0.413
0.463
0.512
0.614
0.713
MIN
10.10
11.35
12.60
15.20
17.70
MAX
10.50
11.75
13.00
15.60
18.10
INCHES MILLIMETERS
PINS
16
18
20
24
28
1.27
0.050
L
HE
D
e
A
A1
C
0°- 8°
0.101mm
0.005in.
B
DIM
A
A1
A2
A3
B
B1
C
D1
E
E1
e
eA
eB
L
MIN
–
0.015
0.125
0.055
0.016
0.045
0.008
0.050
0.600
0.525
0.100
0.600
–
0.120
MAX
0.200
–
0.175
0.080
0.020
0.065
0.012
0.090
0.625
0.575
–
–
0.700
0.150
MIN
–
0.38
3.18
1.40
0.41
1.14
0.20
1.27
15.24
13.34
2.54
15.24
–
3.05
MAX
5.08
–
4.45
2.03
0.51
1.65
0.30
2.29
15.88
14.61
–
–
17.78
3.81
INCHES MILLIMETERS
P PACKAGE
PLASTIC
DUAL-IN-LINE
DIM
D
D
D
MIN
1.230
1.430
2.025
MAX
1.270
1.470
2.075
MIN
31.24
36.32
51.44
MAX
32.26
37.34
52.71
INCHES MILLIMETERS
PINS
24
28
40
C
A
A2
E1
D
E
eA
eB
A3
B1
0°-15°
B
A1
L
D1
e
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