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General Description

The MAX1450 sensor signal conditioner is optimized for
piezoresistive sensor calibration and temperature com-
pensation. It includes an adjustable current source for
sensor excitation and a 3-bit programmable-gain amplifi-
er (PGA). Achieving a total typical error factor within
1% of the sensor's inherent repeatability errors, the
MAX1450 compensates offset, full-span output (FSO), off-
set tempco, FSO tempco, and FSO nonlinearity of silicon
piezoresistive sensors via external trimmable resistors,
potentiometers, or digital-to-analog converters (DACs).

The MAX1450 is capable of compensating sensors that
display close error distributions with a single tempera-
ture point, making it ideal for low-cost, medium-accuracy
applications. Although optimized for use with popular
piezoresistive sensors, it may also be used with other
resistive sensor types such as strain gauges.

Customization

Maxim can customize the MAX1450 for unique require-
ments including improved power specifications. With a
dedicated cell library consisting of more than 90 sen-
sor-specific functional blocks, Maxim can quickly pro-
vide customized MAX1450 solutions. Contact the
factory for additional information.

Applications

Piezoresistive Pressure and Acceleration

Transducers and Transmitters

Manifold Absolute Pressure (MAP) Sensors
Automotive Systems
Hydraulic Systems
Industrial Pressure Sensors

Features

1% Sensor Signal Conditioning

Corrects Sensor Errors Using Coefficients Stored
in External Trimmable Resistors, Potentiometers,
or DACs

Compensates Offset, Offset TC, FSO, FSO TC,
and FSO Linearity

Rail-to-Rail

Analog Output

Programmable Current Source for Sensor
Excitation

Fast Signal-Path Settling Time (< 1ms)

Accepts Sensor Outputs from 10mV/V to 30mV/V

Fully Analog Signal Path

MAX1450

Low-Cost, 1%-Accurate Signal Conditioner

for Piezoresistive Sensors

________________________________________________________________

Maxim Integrated Products

PGA

A = 1

OUT

ISRC

BDRIVE

INP

INM

SOTC
SOFF

OFFTC
OFFSET

BBUF

FSOTRIM

CURRENT

SOURCE

MAX1450

INM

BDRIVE

ISRC

SOTC

I.C.

INP

TOP VIEW

I.C.

OUT

OFFTC

SOFF

I.C.

FSOTRIM

BBUF

OFFSET

MAX1450

SSOP

Pin Configuration

19-1365; Rev 0; 5/98

PART

MAX1450CAP

MAX1450C/D

MAX1450EAP

-40°C to +85°C

0°C to +70°C

TEMP. RANGE

PIN-PACKAGE

20 SSOP

Dice*

20 SSOP

Dice are tested at T

= +25°C, DC parameters only.

Functional Diagram

Ordering Information

Rail-to-Rail is a registered trademark of Nippon Motorola, Ltd.

MAX1450

Low-Cost, 1%-Accurate Signal Conditioner
for Piezoresistive Sensors

_______________________________________________________________________________________

ABSOLUTE MAXIMUM RATINGS

ELECTRICAL CHARACTERISTICS

= +5V, V

= 0, T

= T

MIN

to T

MAX

, unless otherwise noted. Typical values are at T

= +25°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.

Supply Voltage, V

to V

......................................-0.3V to +6V

All Other Pins ...................................(V

- 0.3V) to (V

+ 0.3V)

Short-Circuit Duration, OUT, BBUF, BDRIVE .............Continuous
Continuous Power Dissipation (T

= +70°C)

SSOP (derate 8.00mW/°C above +70°C) ....................640mW

Operating Temperature Range

MAX1450CAP .....................................................0°C to +70°C
MAX1450EAP ..................................................-40°C to +85°C

Storage Temperature Range .............................-65°C to +165°C
Lead Temperature (soldering, 10sec) .............................+300°C

= +25°C (Note 1)

DC to 10Hz, gain = 39,
sensor impedance = 5k

OUT

= (V

+ 0.25V) to (V

- 0.25V)

= +25°C

(Note 5)

(Note 4)

load to V

or V

DD,

= +25°C

From V

to V

63% of final value

(Notes 2, 3)

Eight selectable gains (Table 3)

CONDITIONS

µV

RMS

500

Output Noise

-1.0

1.0

(sink)

(source)

Output Current Range

SS +

DD -

0.25

Output Voltage Swing

V/V

Minimum Differential Signal
Gain

V/V

39 to 221

Differential Signal Range Gain

2.8

3.5

Supply Current

mV/V

10 to 30

Input-Referred Adjustable
Full-Span Output Range

±100

Input-Referred Adjustable Offset
Range

CMRR

Common-Mode Rejection Ratio

Output Step-Response Time

1.0

Input Impedance

µV/°C

±0.5

Input-Referred Offset
Temperature Coefficient

0.01

Amplifier Gain Nonlinearity

UNITS

MIN

TYP

MAX

SYMBOL

PARAMETER

No load, T

= T

MIN

to T

MAX

SS +

DD -

0.05

4.5

5.0

5.5

Supply Voltage

At any gain

ppm/°C

±50

Differential Signal Path
Temperature Coefficient

V/V

1.15

Offset TC Gain

V/V

1.15

Offset Gain

OUT

OFFSET

OUT

OFFTC

GENERAL CHARACTERISTICS

ANALOG OUTPUT (PGA)

ANALOG INPUT (PGA)

SUMMING JUNCTION (Figure 1)

MAX1450

Low-Cost, 1%-Accurate Signal Conditioner

for Piezoresistive Sensors

_______________________________________________________________________________________

ELECTRICAL CHARACTERISTICS (continued)

= +5V, V

= 0, T

= T

MIN

to T

MAX

, unless otherwise noted. Typical values are at T

= +25°C.)

Note 1:

Contact factory for high-volume applications requiring less than 1.5mA.

Note 2:

All electronics temperature errors are compensated together with the sensor errors.

Note 3:

The sensor and the MAX1450 must always be at the same temperature during calibration and use.

Note 4:

This is the maximum allowable sensor offset at minimum gain (39V/V).

Note 5:

This is the sensor's sensitivity normalized to its drive voltage, assuming a desired full-span output (FSO) of 4V and a bridge
voltage of 2.5V. Operating at lower bridge excitation voltages can accommodate higher sensitivities.

BDRIVE

= 2.5V

No load

BDRIVE

- V

BBUF

) at V

BDRIVE

= 2.5V, no load

CONDITIONS

µA

-100

100

Current Drive

SS +

DD -

1.3

Voltage Swing

-20

OFS

Offset Voltage

UNITS

MIN

TYP

MAX

SYMBOL

PARAMETER

BDRIVE/

ISRC

(Figure 2)

SS +

DD -

1.3

BDRIVE

Bridge Voltage Swing

0.1

0.5

2.0

BDRIVE

Bridge Current Range

µA/µA

Current-Source Gain

SS +

DD-

1.3

ISRC

Current-Source Input Voltage
Range

BUFFER (BBUF)

CURRENT SOURCE

______________ Detailed Description

Analog Signal Path

The MAX1450's signal path is fully differential and com-
bines the following three stages: a 3-bit PGA with
selectable gains of 39, 65, 91, 117, 143, 169, 195, and
221; a summing junction; and a differential to single-
ended output buffer (Figure 1).

Programmable-Gain Amplifier

The analog signal is first fed into a programmable-gain
instrumentation amplifier with a CMRR of 90dB and a
common-mode input range from V

to V

. Pins A0,

A1, and A2 set the PGA gain anywhere from 39V/V to
221V/V (in steps of 26).

MAX1450

Low-Cost, 1%-Accurate Signal Conditioner
for Piezoresistive Sensors

_______________________________________________________________________________________

NAME

FUNCTION

INP

Positive Sensor Input. Input impedance is typically 1M

. Rail-to-rail input range.

PIN

2, 3,

12, 16

I.C.

Internally connected. Leave unconnected.

SOTC

Offset TC Sign Bit Input. A logic low inverts V

OFFTC

with respect to V

SS.

This pin is internally pulled to V

via a 1M

(typical) resistor. Connect to V

to add V

OFFTC

to the PGA output, or leave unconnected (or

connect to V

) to subtract V

OFFTC

from the PGA output.

OFFTC

Offset TC Adjust. Analog input summed with PGA output and V

OFFSET

. Input impedance is typically 1M

Rail-to-rail input range.

PGA Gain-Set LSB Input. Internally pulled to V

via a 1M

(typical) resistor. Connect to V

for a logic

high or V

for a logic low.

PGA Gain-Set Input. Internally pulled to V

via a 1M

(typical) resistor. Connect to V

for a logic high or

for a logic low.

SOFF

Offset Sign Bit Input. A logic low inverts V

OFFSET

with respect to V

. This pin is internally pulled to V

via

a 1M

(typical) resistor. Connect to V

to add V

OFFSET

to the PGA output, or leave unconnected (or con-

nect to V

) to subtract V

OFFSET

from the PGA output.

OUT

PGA Output Voltage. Connect a 0.1µF capacitor from OUT to V

PGA Gain-Set MSB Input. Internally pulled to V

via a 11k

(typical) resistor. Connect to V

for a logic

high or V

for a logic low.

FSOTRIM

Bridge Drive Current-Set Input. The voltage on this pin sets the nominal I

ISRC

. See the

Bridge Drive section.

BBUF

Buffered Bridge-Voltage Output (the voltage at BDRIVE). Use with correction resistor R

STC

to correct for FSO

tempco.

OFFSET

Offset Adjust Input. Analog input summed with PGA output and V

OFFTC

. Input impedance is typically

. Rail-to-rail input range.

Pin Description

Positive Supply Voltage Input. Connect a 0.1µF capacitor from V

to V

INM

Negative Sensor Input. Input impedance is typically 1M

. Rail-to-rail input range.

Negative Power-Supply Input.

BDRIVE

Sensor Excitation Current Output. This pin drives a nominal 0.5mA through the bridge.

ISRC

Current-Source Reference. Connect a 50k

(typical) resistor from ISRC to V

OFFTC SOTC

OFFSET SOFF

INP

INM

A1 A0

PGA

A = 1

OUT

Figure 1. Signal-Path Functional Diagram

Summing Junction

The second stage in the analog signal path consists of
a summing junction for offset, offset temperature com-
pensation, and the PGA output. The offset voltage
(V

OFFSET

) and offset temperature-compensation volt-

age (V

OFFTC

) add or subtract from the PGA output

depending on their respective sign bits, offset sign
(SOFF), and offset TC sign (SOTC). V

OFFSET

and

OFFTC

can range in magnitude from V

to V

Output Buffer

The final stage in the analog signal path consists
of a unity-gain buffer. This buffer is capable of swinging
to within 250mV of V

and V

while sourcing/sinking

up to 1.0mA, or within 50mV of the power supplies with
no load.

Bridge Drive

Figure 2 shows the functional diagram of the on-chip
current source. The voltage at FSOTRIM, in conjunction
with R

ISRC

, sets the nominal current, I

ISRC

which sets

the FSO (refer to Figure 3 for sensor terminology.) I

ISRC

is additionally modulated by components from the
external resistor R

STC

and the optional resistor R

LIN

STC

is used to feed back a portion of the buffered

bridge-excitation voltage (V

BBUF

), which compensates

FSO TC errors by modulating the bridge-excitation cur-
rent over temperature. To correct FSO linearity errors,
feed back a portion of the output voltage to the current-
source reference node via the optional R

LIN

resistor.

Applications Information

Compensation Procedure

The following compensation procedure assumes a pres-
sure transducer with a +5V supply and an output voltage
that is ratiometric to the supply voltage (see

Ratiometric

Output Configuration section). The desired offset voltage
(V

OUT

at P

MIN

) is 0.5V, and the desired FSO voltage

OUT

MAX

) - V

OUT

MIN

)) is 4V; thus the FS output volt-

age (V

OUT

at P

MAX

) will be 4.5V. The procedure requires

a minimum of two test pressures (e.g., zero and full scale)
and two temperatures. A typical compensation procedure
is as follows:

1) Perform Coefficient Initialization

2) Perform FSO Calibration

3) Perform FSO TC Compensation

4) Perform OFFSET TC Compensation

5) Perform OFFSET Calibration

6) Perform Linearity Calibration (Optional)

Coefficient Initialization

Select the resistor values and the PGA gain to prevent
gross overload of the PGA and bridge current source.
These values depend on sensor behavior and require
some sensor characterization data. This data may be
available from the sensor manufacturer. If not, it can be
generated by performing a two-temperature, two-pres-

MAX1450

Low-Cost, 1%-Accurate Signal Conditioner

for Piezoresistive Sensors

_______________________________________________________________________________________

BDRIVE

13 (I

ISRC

)

BDRIVE

ISRC

BBUF

INP

INM

SENSOR

ISRC

(EXTERNAL)

BDRIVE

FSOTRIM

BBUF

OUT

A = 1

STC

(EXTERNAL)

LIN

(OPTIONAL)

(EXTERNAL)

MAX1450

Figure 2. Bridge Drive Circuit

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