MPXM2102A datasheet(2/10 Pages) FREESCALE

2 / 10 page

AN1979

Sensors

Freescale Semiconductor

Amplifier Selection and Amplifier Induced Errors

The main goal of the signal conditioning circuit is to convert

the MPX2102A differential output to a single-ended, ground-

referenced output. The differential output is extremely small

for the MCU to process so a conditioning circuit also needs to

provide amplification.

This reference design has a barometric pressure range of

64 kPa to 105 kPa. The output of the sensor is ratiometric to

the supply voltage and the supply voltage is 5.0 V, the FSS,

Sensitivity, and Offset are 5.0 V/10 V, or half, of the specified

values at a 10 V supply. Using these calculated sensitivity and

offset ranges, the lowest and highest possible values were

calculated.

VOUT = (Applied Pressure * Sensitivity) ± Offset

VOUT at 64 kPa = 64 kPa * 0.2 mV/kPa — 1 mV = 11.32 mV

VOUT at 105 kPa = 105 kPa * 0.2 mV/kPa + 1 mV = 21.0 mV

These values were found to be 11.32 mV to 22.79 mV

differential output from the sensor.

Two-Stage Design

This two-stage design level shifts the differential output

voltage of the sensor by subtracting an offset voltage from

each of the sensor outputs, then uses a differential

amplification as shown in Figure 2.

After the first stage of amplification, the output of op-amp A is:

VA = (1+R8/R6) x V4 – (R8/R6) x VS(1)

= (1+10/4.42k) x V4 – (10/4.42k) x 5.0 V

= (1+10/4.42k) x V4 – 11.3 mV

and the output of op-amp B is:

VB = (1+R7 / R5) x V2 – (R7 / R5) x VS(2)

= (1+10/4.42 k) x V2 – (10/4.42 k) x 0 V

= (1+10/4.42 k) x V2 – 0

The second stage of amplification connects these two

outputs to a common differential amplifier (op-amp C) also

shown in Figure 3. With some algebraic manipulation, the

output voltage (VOUT) of the entire amplification circuit is

VC = (R12/R11) x [(1+R8/R6) x (V2 - V4) – (R8/R6) x VS](3)

= (412K/1 k) x [(1+10/4.42 K) x (V2 - V4) – (10/4.42 K) * 5 V]

= (412) x [(1.002) x (V2 - V4) – 11.3 mV]

= 412 x (V2 - V4) –11.3 mV

Table 1. MPXM2102A Operating Characteristics

Characteristic

Symbol

Min

Typ

Max

Unit

Pressure Range

POP

—

100

kPa

Supply Voltage

—10

Vdc

Supply Current

—6.0—

mAdc

Full Scale Span

VFSS

38.5

41.5

Offset

Voff

-1.0

-2.0

—

1.0

2.0

Sensitivity

MPX2102D Series

MPX2102A Series

∆V/∆P

—0.4—

MV/kPa

Linearity

MPX2102D Series

MPX2102A Series

—

-0.6

-1.0

—

0.4

1.0

%VFSS

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