Otherwise, set IRS high and connect external resistors

as shown in Figure 13:

where Rb(T) is the sensor input impedance at tempera-

ture T1 (+25°C in this example).

When using an external resistor, use the equation

resistor between ISRC and FSOTC. Since the 12-bit

FSOTC DAC provides considerable dynamic range, the

value within ±50% of the calculated value is accept-

able:

This approximation works best for bulk, micromachined,

ventional sensor behavior that can be compensated by

the MAX1459 with additional external circuitry.

Selecting the PGA Gain Setting

To select the PGA gain setting, first calculate

SensorFSO, the sensor full-span output voltage at T1:

∆P

= +1.5mV/V per PSI x 2.5V x 10 PSI

= 0.0375V

sensor excitation voltage (initially 2.5V), and

∆P is the

maximum pressure differential.

Then calculate the ideal gain using the following formula,

and select the nearest gain setting from Table 2:

where OUTFSO is the desired calibrated transducer

full-span output voltage, and SensorFSO is the sensor

full-span output voltage at T1.

In this example, a PGA value of 2 (gain of +95V/V) is

the best selection.

Determining Input-Referred OFFSET

The input-referred offset (IRO) register is used to null

any front-end sensor offset errors prior to amplification

by the PGA. This reduces the possibility of saturating

the PGA and maximizes the useful dynamic range of

the PGA (particularly at the higher gain values).

A

OUTFSO

SensorFSO

4V

0.0375V

+106V/V

≅=

R

R

500ppm/ C

60k

500ppm/ C

60k

FTC

ISRC

≅

×°

≅

×°

°°

≅Ω

TCR - TCS

2600ppm/ C - -2100ppm/ C

||

||

Ω

R

Rb(T

kk

≈×

≈

)

12

12

5

60

ΩΩ

2-Wire, 4–20mA

Smart Signal Conditioner

16

______________________________________________________________________________________

Internal (approximately 100k

Ω) or user-

supplied resistor that compensates

FSOTC errors

FSOTC compensation DAC

FSOTC COEF

FSO trim DAC

FSO COEF

Offset TC sign bit

OFFTC Sign

Offset TC compensation DAC coefficient

OFFTC COEF

Offset sign bit

OFF Sign

Offset correction DAC coefficient

OFF COEF

Internal resistor selection bit

IRS

Input-referred offset sign bit

PARAMETER

IRO Sign

DESCRIPTION

Input-referred offset correction DAC

value

Internal (approximately 100k

Ω) or user-

supplied resistor that programs the nomi-

nal sensor excitation current

IRO

Programmable-gain amplifier gain

Table 9. Compensation Components and

Values

Maximum Input Pressure

10 psi

Minimum Input Pressure

Sensitivity Linearity Error

as % FSO, BSLF

(best straight-line fit)

0 psi

S(p)

0.1% FSO,

BSLF

Offset Tempco

OTC

Offset

-1000ppm/°C

of FSO

O(T)

+12mV/V at

+25°C

Sensitivity Tempco

TCS

Sensitivity

-2100ppm/°C

S(T)

+1.5mV/V per

PSI at +25°C

PARAMETER

Bridge Impedance

Tempco

SENSOR

DESCRIPTION

TCR

Bridge Impedance

2600ppm/°C

Rb(T)

TYPICAL

VALUES

5k

Ω at +25°C

Table 8. Sensor Information for Typical PRT