AN1988 Freescale Semiconductor / Motorola, AN1988 Datasheet

AN1988

Manufacturer Part Number

AN1988

Description

AN1988

Manufacturer

Freescale Semiconductor / Motorola

Datasheet

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Freescale Semiconductor

Application Note

±1.5g Dual Axis Micromachined

Accelerometer Power Supply Rejection

Ratio (PSRR) Suggestions

by: Peter Schultz

accelerometer family with sensitivity parallel to the device’s

mounting plane. The device utilizes variable capacitance

sensing elements and a two-channel interface IC all in a

16-pin QFN package.

power supply rejection ratio (PSRR), how it affects device

performance, and suggestions for improvement.

Rejection Ratio

the device rejects the noise on the power supply line. The

MMA6200Q series is capable of being used in several

different applications. In some cases it may be possible for

supply line noise to adversely affect the output signal. This

phenomenon can create large output signals when the supply

line noise frequencies are roughly equal to the device’s

oscillator frequency and/or harmonics. The oscillator drives

the g-cell sampling as well as the internal low pass filter. When

the difference between the frequency of the noise on the

supply and the oscillator frequency is less than the low pass

filter cutoff frequency the aliased signal passes through the

filter. This aliased signal is then amplified internally by the

device creating an even more adverse effect. This noise at the

output can be as much as ten times the amplitude of the input

noise at the oscillator frequency or its harmonics.

Rejection Ratio

oscillator frequency, large amounts of noise may be observed

at the output of the device. This does not become an issue

until the noise frequency is a little larger than the oscillator

frequency; a little larger here meaning still within the internal

low pass filter’s bandwidth. In application a simple low pass

The MMA6200Q series is a two axis (X and Y)

This section provides a general description of the device

The power supply rejection ratio is a measure of how well

If the power supply contains noise approaching the

Product Engineer

INTRODUCTION TO POWER SUPPLY

IMPROVING THE POWER SUPPLY

COMPONENT DESCRIPTION

filter at the input helped to resolve this issue by attenuating

much of the offending noise before it enters the device.

device, made up of a resistor in series with a capacitor in shunt

with V

capacitor there is a large reduction in the PSRR, roughly 10X.

This is now well below unity gain; meaning at the output of the

device there is noticeably less noise than was supplied to the

device, at the oscillator frequency and/or its harmonics. The

first harmonic is at the oscillator frequency of roughly 15 KHz.

The following illustration shows the effect on the PSRR when

placing different values of capacitance at the input and

retaining the 50 ohm resistor. There is six harmonics

represented in the illustration the first of which is at about

15 KHz. These data points at the different harmonics

represent the ratio of the output noise to the input noise.

Depending on the application needs one can use this

illustration to help determine the capacitive needs for the filter

(see

SUPPLY

POWER

A simple two element low pass filter at the input of the

With this combination of a 50 ohm resistor and a 2 µF

Figure

Figure 1. RC Input Loading for PSRR at V

, works rather well (see

2). The plot is given in Log scale.

50 OHM

Figure

2 µF

1).

Rev 1, 10/2004

MMA6200Q

AN1988

out

Related parts for AN1988

AN1988 Summary of contents

Page 1

... KHz. These data points at the different harmonics represent the ratio of the output noise to the input noise. Depending on the application needs one can use this illustration to help determine the capacitive needs for the filter (see Figure 2). The plot is given in Log scale. AN1988 Rev 1, 10/2004 1). MMA6200Q X out V ...

Page 2

... As a result there was greater than a factor of eight improvement with the third and fifth harmonics. This RC and RLC circuit produced the following PSRR plot (see AN1988, Rev 1 Figure 4). This is more MMA6200Q µ ...

Page 3

... It is still well below unity for this configuration. Figure 7. PSRR with 150 Ohm and 0.5 µF Input Capacitance Sensor Products Freescale Semiconductor Figure 5. PSRR with RC and RLC Input Filter POWER SUPPLY Figure 6. Alternate RC Input Loading for PSRR at V AN1988, Rev 1 MMA6200Q X out V dd 150 OHM 0.5 µF Y ...

Page 4

... The preceding gives an introduction to power supply rejection ratio as well as some information to better understand this potential problem. Also included are some example filters, the effects on PSRR, as well as suggested choices. AN1988, Rev 1 Sensor Products Freescale Semiconductor ...

Page 5

... Sensor Products Freescale Semiconductor NOTES AN1988, Rev 1 5 ...

Page 6

... NOTES AN1988, Rev 1 Sensor Products Freescale Semiconductor ...

Page 7

... Sensor Products Freescale Semiconductor NOTES AN1988, Rev 1 7 ...

Page 8

... Freescale Semiconductor Literature Distribution Center P.O. Box 5405 Denver, Colorado 80217 1-800-441-2447 or 303-675-2140 Fax: 303-675-2150 LDCForFreescaleSemiconductor@hibbertgroup.com AN1988 Rev. 1 10/2004 Information in this document is provided solely to enable system and software implementers to use Freescale Semiconductor products. There are no express or implied copyright licenses granted hereunder to design or fabricate any integrated circuits or integrated circuits based on the information in this document ...

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