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 cannot be compensated

by the MAX1458 without 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 (IRO)

The input-referred offset 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.)

First, calculate the ideal IRO correction voltage using

the following formula, and select the nearest setting

from Table 1:

where IROideal is the exact voltage required to perfect-

ly null the sensor, O(T1) is the sensor offset voltage in

excitation voltage at +25°C. In this example, 30mV

must be subtracted from the amplifier front end to null

the sensor perfectly. From Table 1, select an IRO value

of 3 to set the IRO DAC to 27mV, which is nearest the

ideal value. To subtract this value, set the IRO sign bit

to 0. The residual output-referred offset error will be

corrected later with the Offset DAC.

Determining OFFTC COEF Initial Value

Generally, OFFTC COEF can initially be set to 0, since

the offset TC error will be compensated in a later step.

However, sensors with large offset TC errors may

require an initial coarse offset TC adjustment to prevent

the PGA from saturating during the compensation pro-

cedure as temperature is increased. An initial coarse

offset TC adjustment is required for sensors with an off-

set TC greater than about 10% of the FSO. If an initial

IROideal

- O T1 x V

T1

- 0.012V/V x 2.5V

- 30mV

BDRIVE

=

( )

( )

[

]

=

(

)

=

A

OUTFSO

SensorFSO

4V

0.0375V

106V/V

=

=

R

R

x 500ppm/ C

70k

x 500ppm/ C

70k

FTC

ISRC

≅

°

≅

°

°

°

=

Ω

TCR - TCS

2600ppm/ C - -2100ppm/ C

|

|

|

|

Ω

1%-Accurate, Digitally Trimmed

Sensor Signal Conditioner

______________________________________________________________________________________

13

Programmable-gain amplifier gain

IRO

Internal (approximately 75k

Ω) or user-

supplied resistor that programs the nomi-

nal sensor excitation current.

Input-referred offset correction DAC

value

DESCRIPTION

IRO Sign

PARAMETER

Input-referred offset sign bit

IRS

Internal resistor selection bit

OFF COEF

Offset correction DAC coefficient

OFF Sign

Offset sign bit

OFFTC COEF

Offset TC compensation DAC coefficient

OFFTC Sign

Offset TC sign bit

FSO COEF

FSO trim DAC coefficient

FSOTC COEF

FSO TC compensation DAC coefficient

Internal (approximately 75k

Ω) or user-

supplied resistor that compensates

FSO TC errors.

Table 7. Compensation Components and

Values