Accelerometers

H34C Accelerometer

Overview - H34C Accelerometer

The model H34C accelerometer is a ultra small and thin 3-axis accelerometer module, produced by MEMS (Micro Electro Mechanical System) technology. It is composed of a precise sensor's chip, and CMOS-IC chip with the op-amplifiers and has the several excellent functions written below. For each products, the performance variations among products, and moreover those drifts over temperature are compensated before shipment. Therefore the H34C accelerometer could be used without calibration for most applications. And also the H34C accelerometer has the high reliability due to the ceramic package and the air-tight seal.

Key Features H34C

• Detect three axes (triaxial) simultaneously
• Very small and thin package: 3.4 x 3.7 x .92mm
• Single supply voltage of +2.2 to +3.6V
• Very low power consumption and standby
• Operation current 0.36mA at 3V
• Stand by current 1 µ A max.
• "Freefall Detection": While outputs of each axis (X,Y,Z) are nearly zero g, the terminal to output flag signal is equipped as a standard specification.
• Capable to detect "Static(Tilt) and "Dynamic"
• acceleration and measurement range is +/-3g
• High shock durability (>5000g)
• Temperature sensor and a function to adjust the performance drifts due to different temperature integrated.
• High reliability accelerometer sensor due to the ceramic package, air tight seal and factory calibration.
• ROHS Compliant
• Low cost and high volume manufacturing (Tape and Reel) ready.
• Precise factory calibration - 100 % of the accelerometer is factory calibrated. As a result, customer side is free from re-calibration. No additional cost for re-calibration is required.

Direct Benefits of Triaxial Accelerometer

Oultine - H34C Accelerometer

H34C is a 3-axis accelerometer, composed of CMOS IC chip and a sensor chip mounted into one package. The CMOS IC chip has the new function which is to compensate the variation of sensitivity and zero g voltage, and those drift over temperature, for each products. The analog voltages proportional to the acceleration of X, Y, and Z axes are outputted simultaneously. The output voltage at the acceleration of 1g is about 333mV(operating voltage 3V).

Voltage of operation is the single power supply of +2.2V to +3.6V. The measurement range of the accelerometer H34C is ±3g. H34C has a temperature sensor and a function to adjust the performance drifts due to different temperature. The output of a temperature sensor in the CMOS IC can be taken out at the external terminal of a package, which can be used as a simple temperature sensor.

Furthermore, it has the another function which is to detect the free fall of apparatus. While outputs of each axis (X,Y,Z) are nearly zero g, the terminal to output a flag signal is equipped as a standard specification.

About the temperature sensor and the free fall detection mentioned above, another application notes are prepared.

From pin numbers of 5, 4, and 3, the analog voltages of AOX, AOY, and AOZ, proportional to the acceleration of X, Y, and Z axes, are put out. When the acceleration applied to the
sensor is 0g, those outputs are about Vcc/2(V). The voltage(Tout) of a temperature sensor is outputted from the pin number of 1. The voltage at the temperature of 25 degrees C is about Vcc/2(V), and at 65 degrees C, it is about Vcc/2+0.4(V). As for Vref, which is the reference voltage, its voltage is about Vcc/2 (V) outputted from pin number of 13.

In the adjustment process at the factory before shipments, all outputs are adjusted on the basis of the output voltage Vref. Therefore, on using H34C, it is recommended to use the difference between the output voltages(AOX, AOY, AOZ, and Tout) and the reference voltage(Vref) .

From the terminal 18, while detecting outputs of all axes(X,Y,Z) are almost 0(g) at the same time, Vcc voltage is outputted as a flag.

A STBYB terminal performs standby control. Please apply 0(V) at the time of standby, and Vcc(V) at the time of operation.

Microcomputer Interfaces

The Vref pin of H34C accelerometer is a terminal from which standard voltage（Vcc/2) is outputted. Output voltages(AOX, AOY, AOZ and Tout ) from the accelerometer and from the temperature sensor are calibrated on the basis of this Vref voltage. Therefore, in the normal usage, it is recommended that Vref voltage is taken into a microcomputer through an A/D converter with output voltages of AOX, AOY, AOZ and Tout, and that exact digital values are calculated by the microcomputer, taking the differences between outputs voltages and Vref voltage. Or, on using an A/D converter of a differential input type, the calibrated differential voltages can be directly obtained on the basis of Vref voltage.

The output voltages of H34C accelerometer are all designed to be ratiometric to the supply voltage Vcc. Therefore, to get high accuracy, the following point is important. Regarding the reference power supply for an A/D converter (which determines the A/D conversion span), should be used the same power supply as Vcc, or the resister-divided voltage of Vcc, in order that the span of the A/D conversion is set to be ratiometric to Vcc and that the final outputs are not affected by the change of Vcc voltage.

About load restrictions of an output terminal (input impedance of an A/D converter)

All output signals, i.e., detected acceleration voltages (AOX, AOY, AOZ) and detected temperature voltage (Tout), and the reference voltage (Vref), are outputted to the respective terminals through output resistances of 32k ohms. Therefore, when the A/D converter, connected to these output terminals, works like the DC load, some errors may occur in measured value.

Although there are many kind of A/D converters, for most of types, the input voltages are sample-held at the sampling capacitor (Csamp). Since the sampled electric charge is cleared each time, on seeing from the signal source, the A/D converter seems to absorb the electric charge for every sampling. Then, in cace that a sampling frequency is fsamp, equivalent input resistance (Rin) of the A/D converter is expressed with the following formula.

Rin = 1/(fsampxCsamp)

In order to suppress the measurement error by the load effect within 0.3%, Rin is desired to be more than 10M ohms. Usually, Csamp is several pF to several 10pF. In the calculation result, in case that Csamp is 10pF and that fsamp is 1kHz, Rin becomes 100M ohms. This is considered to be sufficiently high resistance such as to be able to disregard the measurement error occurred by the output resistance.

Since the equivalent input resistance becomes low in inverse proportion to a sampling frequency, it is desirable to set a sampling frequency to the minimum. From this viewpoint, the A/D converter of successive approximation register (SAR) type is suitable for the usage in the low sampling frequency. Because this type of A/D converter converts one data by one sampling.

On the other hand, in a sigma-delta A/D converter, a high order over-sampling is performed to conversion of one data, and equivalent input resistance (Rin) is low. Therefore, this type is not recommended. When using this type, it is necessary to choose a product equipped with an input buffer amplifier.

In addition, generally, source resistance affects also settling time* for every one sampling. However, by the usage of H34C, since the external capacitors (Cx, Cy, Cz, and Ct) functions as the buffers to high frequency, there is no concern that the settling time gets worse by source resistance.

* Time necessary to complete charging a sampling capacitor.

Setup of external capacitor(Cx,Cy,Cz,Ct)

The Block diagram of the H34C accelerometer is shown in Fig. 2. (see Accelerometer H34C for diagrams and figures) Detected acceleration signals (AOX, AOY, AOZ) and detected temperature voltage (Tout) are outputted through the buffer amplifiers and output resistances (design value 32k ohms). Low pass filters are generated by the output resistances and external capacitors (Cx, Cy, Cz, and Ct), and bandwidth is restricted. The bandwidth (BW) is roughly expressed by the following equation.

BW=5/C (Hz)

where, C stands for Cx, Xy, Cz and Ct in unit of µF

However, the resistances of output resistors have about ±30% of errors. Therefore, the deviation about BW, depending on errors, also occurs. In a setup of frequency bandwidth (BW), it is important to take into consideration synthetically a frequency response, a noise performance (S/N), turn-on time, etc. In a frequency band of 200Hz or less, they have the following relation to the frequency bandwidth (BW).

Frequency response -- Proportional to BW.

Noise performance (S/N) -- Inverse proportion to the square root of BW.

Turn-on time (Ton) -- Inverse proportion mostly to BW
Ton = 750/BW (ms) 99% turn on

Especially, the point, of which should be careful, is that a noise performance (S/N) and a frequency response or turn-on time have the relation of a trade-off. Usually, it is best to design BW to the necessary minimum value, in consideration of a frequency response and turn-on time.

When Cx, Cy, and Cz are set to 0.047uF(s), bandwidth is about 100Hz, S/N is 60dB on full scale, turn-on time is about 8ms.

In addition, Ct and Cref should be recommended to be 0.01uF.


Static Acceleration and Output Voltage

1) When Zero G detection not necessary, Pin No.18 must not be connected.

2) When the H34C accelerometer unit is dropped to the hard floor, there is a possibility that large impact over specification is generated and that the characteristic changes. Care should be exercised in handling.

3) Although the H34C accelerometer features proprietary ESD protection circuitry, permanent damage may occur on devices subjected to high energy electrostatic discharges. Therefore, proper ESD precautions are recommended to avoid performance degradation or loss of functionality.

Technical Sheets (PDF)

No.	Accelerometer Sheets	File
1	"Catalog of H34C Accelerometer"	Accelerometer H34C (PDF, 306k)

 

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