19-1538; Rev 0; 9/99
KIT
ATION
EVALU
BLE
AVAILA
1% Accurate, Digitally Trimmed,
Rail-to-Rail Sensor Signal Conditioner
General Description
Features
o
Medium Accuracy (±1%), Single-Chip Sensor
Signal Conditioning
o
Rail-to-Rail
®
Output
o
Sensor Errors Trimmed Using Correction
Coefficients Stored in Internal EEPROM—
Eliminates Laser Trimming and Potentiometers
o
Compensates Offset, Offset TC, FSO, FSO TC,
and FSO Linearity
o
Programmable Current Source (0.1mA to 2.0mA)
for Sensor Excitation
o
Fast Signal-Path Settling Time (<1ms)
o
+5V Single Supply
o
Accepts Sensor Outputs from +10mV/V to
+40mV/V
o
Fully Analog Signal Path
MAX1478
The MAX1478 highly integrated, analog sensor signal
processor is optimized for piezoresistive sensor calibra-
tion and compensation without any external compo-
nents. It includes a programmable current source for
sensor excitation, a 3-bit programmable-gain amplifier
(PGA), a 128-bit internal EEPROM, and four 12-bit digi-
tal-to-analog converters (DACs). Achieving a total error
factor within 1% of the sensor’s repeatability errors, the
MAX1478 compensates offset, offset temperature coeffi-
cient, full-span output (FSO), FSO temperature coeffi-
cient (FSO TC), and FSO nonlinearity of silicon
piezoresistive sensors.
The MAX1478 calibrates and compensates first-order
temperature errors by adjusting the offset and span of
the input signal via DACs, thereby eliminating the quan-
tization noise associated with digital signal path solu-
tions. Built-in testability features on the MAX1478 result
in the integration of three traditional sensor-manufactur-
ing operations into one automated process:
•
Pretest:
Data acquisition of sensor performance
under the control of a host test computer.
•
Calibration and compensation:
Computation and
storage (in an internal EEPROM) of calibration and
compensation coefficients computed by the test
computer and downloaded to the MAX1478.
•
Final test operation:
Verification of transducer cali-
bration and compensation without removal from the
pretest socket.
Although optimized for use with piezoresistive sensors,
the MAX1478 may also be used with other resistive
sensors (i.e., accelerometers and strain gauges) with
some additional external components.
Pilot Production System
To simplify your pressure sensor design, Maxim has
developed a fully automated pilot production system
that will smooth the difficult transition from prototype to
production. Details appear at the end of this data sheet.
Ordering Information
PART
MAX1478C/D
MAX1478AAE
TEMP. RANGE
0°C to +70°C
-40°C to +125°C
PIN-PACKAGE
Dice*
16 SSOP
*Dice
are tested at T
A
= +25°C, DC parameters only.
Functional Diagram appears at end of data sheet.
______________________Customization
For high-volume applications, Maxim can customize the
MAX1478 for unique requirements. With a dedicated
cell library consisting of more than 90 sensor-specific
functional blocks, Maxim can quickly provide cus-
tomized MAX1478 solutions.
TOP VIEW
SCLK 1
CS 2
I.C. 3
TEMP 4
FSOTC 5
DIO 6
WE 7
V
SS
8
Pin Configuration
16 I.C.
15 V
DD
14 INM
________________________Applications
Piezoresistive Pressure and Acceleration
Transducers and Transmitters
Manifold Absolute Pressure (MAP) Sensors
Automotive Systems
Hydraulic Systems
Industrial Pressure Sensors
Strain-Gauge Sensors
Industrial Temperature Sensors
MAX1478
13 BDRIVE
12 INP
11 I.C.
10 OUT
9
ISRC
SSOP
Rail-to-Rail is a registered trademark of Nippon Motorola, Ltd.
1
________________________________________________________________
Maxim Integrated Products
For free samples & the latest literature: http://www.maxim-ic.com, or phone 1-800-998-8800.
For small orders, phone 1-800-835-8769.
1% Accurate, Digitally Trimmed,
Rail-to-Rail Sensor Signal Conditioner
MAX1478
ABSOLUTE MAXIMUM RATINGS
Supply Voltage, V
DD
to V
SS
......................................-0.3V to +6V
All Other Pins ...................................(V
SS
- 0.3V) to (V
DD
+ 0.3V)
Short-Circuit Duration, FSOTC, OUT, BDRIVE ...........Continuous
Continuous Power Dissipation (T
A
= +70°C)
16-Pin SSOP (derate 8.00mW/°C above +70°C) ..........640mW
Operating Temperature Range
MAX1478AAE .................................................-40°C to +125°C
Storage Temperature Range .............................-65°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
(V
DD
= +5V, V
SS
= 0, T
A
= +25°C, unless otherwise noted.)
PARAMETER
GENERAL CHARACTERISTICS
Supply Voltage
Supply Current
ANALOG INPUT
(PGA)
Input Impedance
Input-Referred Offset Tempco
Amplifier Gain Nonlinearity
Output Step Response
Common-Mode Rejection Ratio
Input-Referred Adjustable Offset
Range
Input-Referred Adjustable FSO
Range
ANALOG OUTPUT
(PGA)
Differential-Signal Gain Range
Minimum Differential Signal Gain
Differential-Signal Gain Tempco
Output Voltage Swing
Output Current Range
Output Noise
No load
V
OUT
= (V
SS
+ 0.25V) to (V
DD
- 0.25V)
DC to 10Hz (gain = 41, source
impedance = 5kΩ)
V
SS
+ 0.05
-0.45
(sink)
500
Selectable in eight steps
T
A
= T
MIN
to T
MAX
36
41 to 230
41
±50
V
DD -
0.05
0.45
(source)
45
V/V
V/V
ppm/°C
V
mA
µV
RMS
CMRR
63% of final value
From V
SS
to V
DD
At minimum gain (Note 4)
(Note 5)
R
IN
(Notes 2, 3)
1
±0.5
0.01
1
90
±150
10 to 40
MΩ
µV/°C
%V
DD
ms
dB
mV
mV/V
V
DD
I
DD
(Note 1)
4.5
5.0
3
5.5
6
V
mA
SYMBOL
CONDITIONS
MIN
TYP
MAX
UNITS
2
_______________________________________________________________________________________
1% Accurate, Digitally Trimmed,
Rail-to-Rail Sensor Signal Conditioner
ELECTRICAL CHARACTERISTICS (continued)
(V
DD
= +5V, V
SS
= 0, T
A
= +25°C, unless otherwise noted.)
PARAMETER
CURRENT SOURCE
Bridge Current Range
Bridge Voltage Swing
Reference Input Voltage Range
(ISRC)
DAC Resolution
Differential Nonlinearity
Offset DAC Bit Weight
Offset TC DAC Bit Weight
FSO DAC Bit Weight
FSO TC DAC Bit Weight
IRO DAC
DAC Resolution
DAC Bit Weight
FSOTC BUFFER
Output Voltage Swing
Current Drive
INTERNAL RESISTORS
Current-Source Reference
Resistor
FSO Trim Resistor
Temperature-Dependent
Resistor
Note 1:
Note 2:
Note 3:
Note 4:
Note 5:
R
ISRC
R
FTC
R
TEMP
Typically 4600ppm/°C tempco
75
75
100
kΩ
kΩ
kΩ
No load
V
FSOTC
= 2.5V
V
SS
+ 0.3
-20
V
DD
- 1.3
20
V
µA
Input referred, V
DD
= 5V (Note 6)
3
9
Bits
mV/bit
DNL
∆V
OUT
∆Code
∆V
OUT
∆Code
∆V
ISRC
∆Code
∆V
FSOTC
∆Code
DAC reference = V
DD
= 5.0V
DAC reference = V
BDRIVE
= 2.5V
DAC reference = V
DD
= 5.0V
DAC reference = V
BDRIVE
= 2.5V
±1.5
2.8
1.4
1.22
0.6
I
BDRIVE
V
BDRIVE
V
ISRC
0.1
V
SS
+ 1.3
V
SS
+ 1.3
0.5
2.0
V
DD
- 1.3
V
DD
- 1.3
mA
V
V
SYMBOL
CONDITIONS
MIN
TYP
MAX
UNITS
MAX1478
DIGITAL-TO-ANALOG CONVERTERS
12
Bits
LSB
mV/bit
mV/bit
mV/bit
mV/bit
Excludes the sensor or load current.
All electronics temperature errors are compensated together with sensor errors.
The sensor and the MAX1478 must always be at the same temperature during calibration and use.
This is the maximum allowable sensor offset.
This is the sensor’s sensitivity normalized to its drive voltage, assuming a desired full-span output of 4V and a bridge
voltage of 2.5V.
Note 6:
Bit weight is ratiometric to V
DD
.
_______________________________________________________________________________________
3
1% Accurate, Digitally Trimmed,
Rail-to-Rail Sensor Signal Conditioner
MAX1478
Pin Description
PIN
1
2
3, 11,
16
4
5
6
NAME
SCLK
CS
I.C.
TEMP
FSOTC
DIO
FUNCTION
Data Clock Input. Used only during programming/testing. Internally pulled to V
SS
with a 1MΩ (typical) resis-
tor. Data is clocked in on the rising edge of the clock. The maximum SCLK frequency is 10kHz.
Chip-Select Input. The MAX1478 is selected when this pin is high. When low, OUT and DIO become high
impedance. Internally pulled to V
DD
with a 1MΩ (typical) resistor. Leave unconnected for normal operation.
Internally Connected. Leave unconnected.
Temperature Sensor Output. An internal temperature sensor (a 100kΩ, 4600ppm/°C TC resistor) that can
provide a temperature-dependent voltage.
Buffered FSOTC DAC Output. An internal 75kΩ resistor (R
FTC
) connects FSOTC to ISRC (see
Functional
Diagram).
Optionally, external resistors can be used in place of or in parallel with R
FTC
and R
ISRC
.
Data Input/Output. Used only during programming/testing. Internally pulled to V
SS
with a 1MΩ (typical)
resistor. High impedance when CS is low.
Dual-Function Input Pin. Used to enable EEPROM erase/write operations. Also used to set the DAC refresh-
rate mode. Internally pulled to V
DD
with a 1MΩ (typical) resistor. See the
Chip-Select (CS) and Write-Enable
(WE)
section.
Negative Power-Supply Input
Current-Source Reference. An internal 75kΩ resistor (R
ISRC
) connects ISRC to V
SS
(see
Functional
Diagram).
Optionally, external resistors can be used in place of or in parallel with R
FTC
and R
ISRC
.
PGA Output Voltage
Positive Sensor Input. Input impedance >1MΩ. Rail-to-rail input range.
Sensor Excitation Current Output. This current source drives the bridge.
Negative Sensor Input. Input impedance >1MΩ. Rail-to-rail input range.
Positive Power-Supply Input. Connect a 0.1µF capacitor from V
DD
to V
SS.
7
8
9
10
12
13
14
15
WE
V
SS
ISRC
OUT
INP
BDRIVE
INM
V
DD
_______________Detailed Description
The MAX1478 provides an analog amplification path for
the sensor signal. Calibration and temperature com-
pensation are achieved by varying the offset and gain
of a programmable-gain amplifier (PGA) and by varying
the sensor bridge current. The PGA uses a switched-
capacitor CMOS technology, with an input-referred
coarse offset trimming range of approximately ±63mV
(9mV steps). An additional output-referred fine offset
trim is provided by the Offset DAC (approximately
2.8mV steps). The PGA provides eight gain values from
+41V/V to +230V/V. The bridge current source is pro-
grammable from 0.1mA to 2mA.
The MAX1478 uses four 12-bit DACs and one 3-bit
DAC, with calibration coefficients stored by the user in
an internal 128-bit EEPROM. This memory contains the
following information as 12-bit-wide words:
•
Configuration register
•
Offset calibration coefficient
•
Offset temperature error-compensation coefficient
•
FSO (full-span output) calibration coefficient
•
FSO temperature error-compensation coefficient
•
24 user-defined bits for customer programming of
manufacturing data (e.g., serial number and date)
Figure 1 shows a typical pressure-sensor output and
defines the offset, full-scale, and FSO values as a func-
tion of voltage.
4
_______________________________________________________________________________________
1% Accurate, Digitally Trimmed,
Rail-to-Rail Sensor Signal Conditioner
FSO TC Compensation
Silicon piezoresistive transducers (PRTs) exhibit a large
positive input resistance tempco (TCR) so that, while
under constant current excitation, the bridge voltage
(V
BDRIVE
) increases with temperature. This depen-
dence of V
BDRIVE
on the sensor temperature can be
used to compensate the sensor temperature errors.
PRTs also have a large negative full-span output sensi-
tivity tempco (TCS) so that, with constant voltage exci-
tation, FSO will decrease with temperature, causing a
full-span output temperature coefficient (FSO TC) error.
However, if the bridge voltage can be made to increase
with temperature at the same rate that TCS decreases
with temperature, the FSO will remain constant.
FSO TC compensation is accomplished by resistor
R
FTC
and the FSOTC DAC, which modulate the excita-
tion reference current at ISRC as a function of tempera-
ture (Figure 3). FSO DAC sets V
ISRC
and remains
constant with temperature, while the voltage at FSOTC
varies with temperature. FSOTC is the buffered output
of the FSOTC DAC. The reference DAC voltage is
V
BDRIVE
, which is temperature dependent. The FSOTC
DAC alters the tempco of the current source. When the
tempco of the bridge voltage is equal in magnitude and
opposite in polarity to the TCS, the FSO TC errors are
compensated and FSO will be constant with tempera-
ture.
junction to correct the error. Use the Offset TC DAC to
adjust the amount of BDRIVE voltage that is added to
the output summing junction (Figure 2).
MAX1478
Analog Signal Path
The fully differential analog signal path consists of four
stages:
•
Front-end summing junction for coarse offset correction
•
3-bit PGA with eight selectable gains ranging from
41 through 230
•
Three-input-channel summing junction
•
Differential to single-ended output buffer (Figure 2)
Coarse Offset Correction
The sensor output is first fed into a differential summing
junction (INM (negative input) and INP (positive input))
with a CMRR >90dB, an input impedance of approxi-
mately 1MΩ, and a common-mode input voltage range
from V
SS
to V
DD
. At this summing junction, a coarse off-
set-correction voltage is added, and the resultant volt-
age is fed into the PGA. The 3-bit (plus sign)
input-referred Offset DAC (IRO DAC) generates the
coarse offset-correction voltage. The DAC voltage ref-
erence is 1.25% of V
DD
; thus, a V
DD
of 5V results in a
front-end offset-correction voltage ranging from -63mV
to +63mV, in 9mV steps (Table 1). To add an offset to
the input signal, set the IRO sign bit high; to subtract an
offset from the input signal, set the IRO sign bit low.
The IRO DAC bits (C2, C1, C0, and IRO sign bit) are
programmed in the configuration register (see
Internal
EEPROM
section).
Offset TC Compensation
Compensating offset TC errors involves first measuring
the uncompensated offset TC error, then determining
the percentage of the temperature-dependent voltage
V
BDRIVE
that must be added to the output summing
1.25% V
DD
4.5
IRO
DAC
BDRIVE
OFFTC
DAC
A2 A1 A0
SOTC
A = 2.3
±
VOLTAGE (V)
FULL-SPAN OUTPUT (FSO)
INP
FULL SCALE (FS)
0.5
OFFSET
P
MIN
P
MAX
PRESSURE
V
DD
OFFSET
DAC
SOFF
INM
Σ
PGA
Σ
A = 2.3
±
A=1
OUT
Figure 1. Typical Pressure-Sensor Output
Figure 2. Signal-Path Block Diagram
5
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