AN1668

Sensors

Freescale Semiconductor

3

AMPLIFIER SELECTION AND AMPLIFIER

INDUCED ERRORS

A differential amplifier is needed to convert the differential

output of the MPX2010 sensor to a high level ground-

referenced (single-ended). The classic three-op amp

instrumentation amplifier can be used. However, it requires

additional components (3 op-amps and possibly a split power

supply). An instrumentation topology shown in Figure 1

requires only a single supply and only 2 op-amps and 1%

resistors.

Figure 1. MPX2010 Amplifier Circuit

The circuit uses a voltage divider R+S1 and R+S2 to

provide the reference (level shift), U1A and U1B are non-

inverting amplifiers arranged in a differential configuration with

gain resistors R1, R2, R3, and R4. Note that U1B is the main

gain stage and it has the most gain. It is recommended to

place a 0.015 µF capacitor in it's feedback loop (in parallel with

R4) to reduce noise. The amplifier output can be

characterized with the equation below:

Equation 4 is the differential gain of the amplifier and

equation 5 is the resulting offset voltage of the amplifier.

The above equations assume that the amplifier is close to

bias currents). Since an ideal op-amp is hard to come by, the

customer should select an op-amp based on cost and

performance. Below are some points to keep in mind when

selecting an op-amp and designing the amplifier circuit.

Note that the ratio R2*R4/R1*R3 controls the system offset

as well as the common mode error of the amplifier.

Mismatches in these resistors will result in an offset and

common mode error which appear as offset. It is therefore

recommended to use 1% metal film resistors to reduce these

comparison with R1 in order to reduce common mode error.

Amplifier input offset and input bias currents can induce

errors. For example, an input offset (Vio) of the amplifier can

become significant when the closed-loop gain of the amplifier

is increased. Furthermore, there is also a temperature

coefficient of the input voltage offset which contribute an

additional error across temperature. If the input bias current of

the amplifier is not taken into account in the design, it can also

become a source of error. A technique to reduce this error is

to match the impedance the source impedance of what the op-

amp input pins sees.

It is important to note that high performance op-amps are

more expensive. An MC33272 op-amp has a low input offset

and low input bias current which is suitable for the two-op amp

amplifier design. We can see that there is a tradeoff between

accuracy and cost when designing a solution with the

MPX2010.

When designing a system based on the MPX2010, it is

important to take into account errors due to parametric

variation of the sensor (i.e., offset calibration, span calibration,

TcS, TcO), power supply and the inherent errors of the

amplification circuit. The offset and span errors greatly

determines the resolution of the system (which adds to the

system error). Even though the system offset error can be

2

3

6

X1

3

12

4

R+S1

R+S2

S +

S –

Pressure Sensor

5

U1B

R1

R2

U1A

R4

R3

1

7

+

–

–

+

(4)

Gain

R4

R3

-------- 1

+

=

(5)

Voffset

VREF

R2 R1

⋅

R1 R3

⋅

---------------------

⎝⎠

⎛⎞ VSCM R2 R4

⋅

R1 R3

⋅

---------------------

⎝⎠

⎛⎞

1

–

–

=

(6)

Vout = (S+ - S-) Gain + Voffset

(7)

where (S+ - S-) = Sensor differential output + Sensor offset