19-1342; Rev 1; 8/98
IT
TION K
VALUA
E
BLE
AVAILA
0.1%-Accurate Signal Conditioner
for Piezoresistive Sensor Compensation
________________General Description
The MAX1457 is a highly integrated analog-sensor sig-
nal processor optimized for piezoresistive sensor cali-
bration and compensation. It includes a programmable
current source for sensor excitation, a 3-bit program-
mable-gain amplifier (PGA), a 12-bit ADC, five 16-bit
DACs, and an uncommitted op amp. Achieving a total
error factor within 0.1% of the sensor’s repeatability
errors, the MAX1457 compensates offset, full-span out-
put (FSO), offset TC, FSO TC, and full-span output non-
linearity of silicon piezoresistive sensors.
The MAX1457 calibrates and compensates first-order
temperature errors by adjusting the offset and span of
the input signal via digital-to-analog converters (DACs),
thereby eliminating quantization noise. If needed, resid-
ual higher-order errors are then compensated using lin-
ear interpolation of the first-order coefficients stored in
a look-up table (in external EEPROM).
The MAX1457 integrates three traditional sensor-
manufacturing 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 external EEPROM) of calibration and compensation
coefficients determined from transducer pretest data.
•
Final Test: Verification of transducer calibration and
compensation, without removal from a pretest socket.
Analog outputs are provided for both pressure and tem-
perature. A general-purpose, uncommitted op amp is also
included on-chip to increase the overall circuit gain, or to
facilitate the implementation of a 2-wire, 4–20mA transmit-
ter. The serial interface is compatible with MicroWire™
and SPI™, and directly connects to an external EEPROM.
Additionally, built-in testability features of the MAX1457
facilitate manufacturing and calibration of multiple sensor
modules, thus lowering manufacturing cost.
Although optimized for use with piezoresistive sensors,
the MAX1457 may also be used with other resistive
sensor types (i.e., accelerometers and strain gauges)
with the addition of a few external components.
V
DD
ISRC
____________________________Features
o
High Accuracy (within ±0.1% of sensor’s
repeatable errors)
o
Compensates Offset, Offset TC, FSO, FSO TC,
Temperature/Pressure Nonlinearity
o
Rail-to-Rail
®
Analog Output for Calibrated,
Temperature-Compensated Pressure
Measurements
o
Programmable Sensor Excitation Current
o
SPI/MicroWire-Compatible Serial Interface
o
Fast Signal-Path Settling Time (<1ms)
o
Accepts Sensor Outputs from 5mV/V to 30mV/V
o
Pin-Compatible with MCA7707
MAX1457
_______________Ordering Information
PART
TEMP. RANGE
PIN-PACKAGE
MAX1457CWI
0°C to +70°C
28 Wide SO
MAX1457CCJ
0°C to +70°C
32 TQFP
MAX1457C/D
0°C to +70°C
Dice*
Ordering Information continued at end of data sheet.
Note:
Contact the factory for customized solutions.
*Dice
are tested at T
A
= +25°C.
Pin Configurations appear at end of data sheet.
Functional Diagram
V
DD
BIAS
GENERATOR
NBIAS
FADJ
FOUT
VOUT
LINDAC
FSOTCDAC
OTCDAC
OFSTDAC
FSODAC
A=1
LINOUT
A=1
FSOTCOUT
V
BDRIVE
A=1
VBBUF
MAX1457
OSCILLATOR
BDRIVE
INP
PGA
INM
V
DD
AGND
16-BIT DAC - FSO
16-BIT DAC - OFFSET
16-BIT DAC - OFFSET TC
16-BIT DAC - FSO TC
16-BIT DAC - FSO LINEARITY
V
DD
V
SS
12-BIT ADC
_______________________Customization
Maxim can customize the MAX1457 for unique require-
ments. With a dedicated cell library of more than 90
sensor-specific functional blocks, Maxim can quickly pro-
vide customized MAX1457 solutions. Contact Maxim for
additional information.
Rail-to-Rail is a registered trademark of Nippon Motorola, Ltd.
SPI is a trademark of Motorola, Inc.
MicroWire is a trademark of National Semiconductor Corp.
MCS
ECS
ECLK
EDI
EDO
LINDACREF
AMP+
AMP-
SERIAL
EEPROM
INTERFACE
AMPOUT
V
SS
________________________________________________________________
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.
0.1%-Accurate Signal Conditioner
for Piezoresistive Sensor Compensation
MAX1457
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)
Continuous Power Dissipation (T
A
= +70°C)
28-Pin Wide SO (derate 12.50mW/°C above +70°C) ..........1W
32-Pin TQFP (derate 11.1mW/°C above +70°C)...........889mW
Operating Temperature Ranges
MAX1457C_ _ ......................................................0°C to +70°C
MAX1457A_ _ .................................................-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
= 0V, 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 Time
Common-Mode Rejection Ratio
Input-Referred Adjustable
Offset Range
Input-Referred Adjustable
Full-Span Output Range
ANALOG OUTPUT (PGA)
Differential Signal Gain Range
Minimum Differential Signal Gain
Differential Signal Gain Tempco
Output Voltage Swing
Output Current Range
Output Noise
CURRENT SOURCE
Bridge Current Range
Bridge Voltage Swing
Current-Source Reference Input
Voltage Range
DAC Voltage Resolution
Differential Nonlinearity
DAC Resolution
2
_______________________________________________________________________________________
I
BR
V
BR
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
5kΩ load to V
SS
or V
DD
No load
V
OUT
= (V
SS
+ 0.25V) to (V
DD
- 0.25V)
Gain = 54, DC to 10Hz, sensor impedance =
5kΩ, full-span output = 4V
V
SS
+ 0.25
V
SS
+ 0.02
-1.0
(sink)
0.0025
T
A
= T
MIN
to T
MAX
49
54 to 306
54
±50
V
DD
- 0.25
V
DD
- 0.02
1.0
(source)
60
V/V
V/V
ppm/°C
V
mA
%FSO
CMRR
f
CLK
= 100kHz, to 63% of final value
From V
SS
to V
DD
(Note 4)
(Note 5)
R
IN
(Notes 2, 3)
V
DD
I
DD
R
BIAS
= 400kΩ, f
CLK
= 100kHz (Note 1)
4.5
5
2.0
1
1
±0.5
0.01
1
90
±100
5 to 30
5.5
2.6
V
mA
MΩ
MΩ
µV/°C
%V
DD
ms
dB
mV
mV/V
SYMBOL
CONDITIONS
MIN
TYP
MAX
UNITS
DIGITAL-TO-ANALOG CONVERTERS
Reference voltage = 5.000V
Output filter capacitor = 0.1µF, f
CLK
= 100kHz
200
2
16
µV
LSB
Bits
0.1%-Accurate Signal Conditioner
for Piezoresistive Sensor Compensation
ELECTRICAL CHARACTERISTICS (continued)
(V
DD
= +5V, V
SS
= 0V, T
A
= +25°C, unless otherwise noted.)
PARAMETER
SYMBOL
CONDITIONS
MIN
TYP
MAX
UNITS
MAX1457
ANALOG-TO-DIGITAL CONVERTER
ADC Differential Nonlinearity
Conversion Time
ADC Resolution
OUTPUTS
(LINDAC, FSOTCDAC)
Voltage Swing
Current Drive
Offset Voltage
UNCOMMITTED OP AMP
Input Common-Mode
Voltage Range
Open-Loop Gain
Offset Voltage (as unity-gain
follower)
Output Voltage Swing
Output Current Range
CMR
A
V
R
BIAS
= 400kΩ
R
BIAS
= 400kΩ, V
IN
= 2.5V (no load)
5kΩ load to V
SS
or V
DD
No load
V
OUT
= (V
SS
+ 0.25V) to (V
DD
- 0.25V)
-20
V
SS
+ 0.25
V
SS
+ 0.02
-1.0
(sink)
V
SS
+ 1.3
60
20
V
DD
- 0.25
V
DD
- 0.02
1.0
(source)
V
DD
- 1.2
V
dB
mV
V
mA
V
OFS
R
BIAS
= 400kΩ (no load)
R
BIAS
= 400kΩ, V
IN
= 2.5V,
V
OUT
= 2.5V ±20mV
(V
IN
- V
OUT
) at V
IN
= 2.5V,
R
BIAS
= 400kΩ (no load)
V
SS
+ 1.3
-50
-20
V
DD
- 1.3
50
20
V
µA
mV
V
BR
= 2.5V to 3.5V, f
CLK
= 100kHz
f
CLK
= 100kHz
2
160
12
LSB
ms
Bits
Note 1:
Circuit of Figure 5 with current source turned off. This value is adjustable through a bias resistor and represents the IC cur-
rent consumption. This excludes the 93C66 EEPROM average current, which is approximately 13µA at a refresh rate of 3Hz
(f
CLK
= 100kHz).
Note 2:
Temperature errors for the entire range are compensated together with the sensor errors.
Note 3:
The sensor and the MAX1457 must always be at the same temperature during calibration and use.
Note 4:
This is the maximum allowable sensor offset at minimum gain (54V/V).
Note 5:
This is the sensor’s sensitivity normalized to its drive voltage, assuming a desired full-span output of 4V and a bridge volt-
age of 2.5V. Lower sensitivities can be accommodated by using the auxiliary op amp. Higher sensitivities can be accommo-
dated by operating at lower bridge voltages.
_______________________________________________________________________________________
3
0.1%-Accurate Signal Conditioner
for Piezoresistive Sensor Compensation
MAX1457
______________________________________________________________Pin Description
PIN
SO
1
2
3
4
5
6
7
—
8
9
10
11
12
13
14
15
16
17
18
19
TQFP
28
29
30
31
1
2
3
4, 16,
22, 32
5
6
7
8
9
10
11
12
13
14
15
17
NAME
INP
INM
AMP+
AMP-
AMPOUT
BDRIVE
VOUT
N.C.
ISRC
FSOTCOUT
VBBUF
LINOUT
LINDACREF
LINDAC
V
SS
OTCDAC
FSODAC
FSOTCDAC
OFSTDAC
EDO
FUNCTION
Positive Sensor Input. Input impedance >1MΩ. Rail-to-rail input range.
Negative Sensor Input. Input impedance >1MΩ. Rail-to-rail input range.
Positive Input of General-Purpose Operational Amplifier
Negative Input of General-Purpose Operational Amplifier
Output of General-Purpose Operational Amplifier. High impedance when MCS is low.
Sensor Excitation Current. This pin drives a nominal 0.5mA through the sensor.
PGA Output Voltage. Connect a 0.1µF capacitor from VOUT to V
SS
. High impedance when
MCS is low.
Not internally connected.
Current-Source Reference. Connect a 50kΩ resistor from ISRC to V
SS
.
Buffered FSO TC DAC Output. Tie to ISRC with a resistor (R
STC
≥
50kΩ).
Buffered Bridge Voltage (the voltage at BDRIVE). Leave unconnected if unused.
Buffered FSO Linearity DAC Output. Use a resistor (R
LIN
) greater than 100kΩ, from LINOUT
to ISRC to correct second order FSO nonlinearity errors. Leave unconnected if not
correcting second order FSO nonlinearity errors.
Reference Input to FSO Linearity DAC. Normally tied to VOUT.
FSO Linearity DAC Output Voltage. Connect 0.1µF capacitor from LINDAC to V
SS
.
Negative Power Supply Input
OFFSET TC DAC Output Voltage. Connect a 0.1µF capacitor from OTCDAC to V
SS
.
FSO DAC Output Voltage. Connect a 0.1µF capacitor from FSODAC to V
SS
.
FSO TC DAC Output Voltage. Connect a 0.1µF capacitor from FSOTCDAC to V
SS
.
OFFSET DAC Output Voltage. Connect a 0.1µF capacitor from OFSTDAC to V
SS
.
Serial Input (data from EEPROM), active high. CMOS logic-level input pin through which the
MAX1457’s internal registers are updated with EEPROM coefficients. Disabled when MCS is
low.
Serial Output (data to EEPROM), active high. CMOS logic-level output pin through which
the MAX1457 gives external commands to the EEPROM. Temperature-compensation data
is available through this pin. Becomes high impedance when MCS is low.
CMOS Logic-Level Clock Output for external EEPROM. High impedance when MCS is low.
Chip-Select Output for external EEPROM. CMOS logic-level output pin through which the
MAX1457 enables/disables EEPROM operation. High impedance when MCS is low.
Frequency Output. Internal oscillator output signal. Normally left open.
Frequency Adjust. Connect to V
SS
with a 1.5MΩ resistor (R
OSC
) to set internal oscillator fre-
quency to 100kHz. Connect a 0.1µF bypass capacitor from FADJ to V
SS
.
Master Chip Select. The MAX1457 is selected when MCS is high. Leave unconnected for
normal operation (internally pulled up to V
DD
with 1MΩ resistor). External 5kΩ pull-up may
be required in noisy environments.
Bias Setting Pin. Connect to V
DD
with a 400kΩ resistor (R
BIAS
). Connect a 0.1µF bypass
capacitor from NBIAS to V
SS
.
Mid-Supply Reference for Analog Circuitry. Connect a 0.1µF capacitor from V
SS
to AGND.
Positive Power-Supply Input. Connect a 0.1µF capacitor from V
DD
to V
SS
.
20
21
22
23
24
18
19
20
21
23
EDI
ECLK
ECS
FOUT
FADJ
25
24
MCS
26
27
28
25
26
27
NBIAS
AGND
V
DD
4
_______________________________________________________________________________________
0.1%-Accurate Signal Conditioner
for Piezoresistive Sensor Compensation
MAX1457
_______________Detailed Description
The MAX1457 provides an analog amplification path for
the sensor signal and a digital path for calibration and
temperature correction. Calibration and correction are
achieved by varying the offset and gain of a program-
mable-gain amplifier (PGA) and by varying the sensor
bridge current. The PGA utilizes a switched-capacitor
CMOS technology, with an input-referred offset trim-
ming range of ±100mV (20mV/V) and an approximate
3µV (input referred, at minimum gain of 54V/V) resolu-
tion (16 bits). The PGA provides eight gain values from
54V/V to 306V/V. The bridge current source is program-
mable from 0.1mA to 2mA, with a 15nA step size.
The MAX1457 uses five 16-bit DACs with calibration
coefficients stored in a low-cost external EEPROM. This
memory (an external 4096-bit EEPROM) contains the
following calibration coefficients as 16-bit words:
•
FSO (full-span output)
•
FSO TC (including nonlinearities)
•
Offset
•
Offset TC (including nonlinearities)
•
Pressure nonlinearity
Figure 1 shows a typical pressure-sensor output and
defines the offset, full-scale, and full-span output values
as a function of voltage.
PRESSURE
OFFSET
Offset Correction
Initial offset calibration is accomplished by reading a
16-bit word (coefficient) from the EEPROM and writing it
to the OFFSET DAC. The resulting voltage (OFSTDAC)
is fed into a summing junction at the PGA output for
compensating the sensor offset with a resolution of
±0.2mV (±0.005% FSO).
VOLTAGE
FULL-SPAN OUTPUT (FSO)
FULL-SCALE (FS)
Figure 1. Typical Pressure-Sensor Output
TO/FROM
EXTERNAL EEPROM
V
DD
R
STC
ECS
TEMPERATURE-
DEPENDENT VOLTAGE
ECLK EDO
EDI
DAC REFERENCE VOLTAGE
V
BR
EEPROM
INTERFACE
I
BR
BDRIVE
V
BR
PGA
T
ADC
12
16
FSO TC
DAC
16
OFFSET TC
OUTPUT
Σ
A=1
Figure 2. Simplified Diagram of Temperature Error Correction
_______________________________________________________________________________________
5
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