MLX90821

Relative Pressure Sensor IC

Page 18 of 40

REV 004 – 4 MAR 2021

3901090821

Analog Front End

The analog front end of the MLX90821 consists of a chopping stage and 3 amplification stages as can be seen in

Figure 9. There are also several input diagnostics integrated into this front end to be able to detect a broken InP

or InN connection or an input which is out of range. This diagnostic information is transferred to the

microcontroller to handle further action for example flagging a diagnostic message.

OPA

OPA

G = 4.5 to 10.5

3 bits

G = 1.6, 3.2 or 6.4

InP

InN

G = 1.25 or 3.5

CSOF: 1/3 to 2/3

of VDDA

Stage 1:

Instrumentation amplifier

Stage 2:

Differential amplifier

Stage 3:

Integrator

Chopping

1us/phase

Input

Diag

nostics

Figure 9: Analog front end block diagram

The first stage is an instrumentation amplifier of which the gain can be programmed using 3 bits to cover a gain

range between 4.5 and 10.6.

Transfer equation:

OUTP1 – OUTN1 = Gst1*(InP – InN) in phase 1

OUTP1 – OUTN1 = Gst1*(InN – InP) in phase 2

The second stage is a fully differential amplifier. The gain of the amplifier can be calibrated using 1 bit.

Transfer equation:

OUTP2 – OUTN2 = -Gst2*(OUTP1 – OUTN1) – Gst2*(CSOF1 – CSOF2) in phase 1

OUTP2 – OUTN2 = -Gst2*(OUTN1 – OUTP1) – Gst2*(CSOF2 – CSOF1) in phase 2

The CSOF1 and CSOF2 signals are generated by the coarse offset DAC with the following transfer functions:

( )

127

]

0

:

6

[

*

2

*

3

1

3

2

*

1

2

1

7

CO

VDDA

VDDA

CSOF

CO











 −

−

+

=

( )

127

]

0

:

6

[

*

2

*

3

1

3

2

*

1

2

2

7

CO

VDDA

VDDA

CSOF

CO











 −

−

−

=

CO[6:0] fixes the DAC output. CO7 is used for the polarity.

The third stage is an integrator which is controlled using 2 bits to set a gain between 1.6 and 6.4

Transfer equation at the outputs of the amplifier:

OUTP3 – OUTN3 = -N*(C1/C2)*(OUTP2 – OUTN2)

OUTP3_common_mode and OUTN3_common_mode = VCM = VDDA/2

In this equation N represents the number of integration cycles which is a fixed value of N = 40.

C2 is a fixed feedback capacitor of approximately 5pF. C1 can have 3 different values: 0.2pF, 0.4pF or 0.8pF.

Transfer equation after the ADC: