Frequently Asked Questions about Measurement - Part 23: "Charge Output Accelerometers and Charge Converters"

This measurement column addresses frequently asked questions received by our customer support center and provides answers to those questions.

Our company offers two types of accelerometers: preamplifier-integrated accelerometers and charge-output type accelerometers. Because charge-output type accelerometers do not have a built-in preamplifier, they are suitable for measuring shock vibrations such as those encountered in collision and drop tests, as well as vibration measurements in high-temperature environments. When analyzing the charge signal output of a charge-output type accelerometer using analysis equipment such as an FFT analyzer or data station, the signal is converted to a voltage signal using a charge amplifier or charge converter before being input to the analysis equipment.

Our CH-6130 and CH-6140 charge converters are easy to use as they can be attached to the BNC connector of the analysis device. However, because the gain is fixed, depending on the combination, they may only be able to measure accelerations smaller than the maximum usable acceleration of the charge output type accelerometer alone. Therefore, when selecting an accelerometer and charge converter, it is necessary to check the magnitude of the acceleration actually occurring and select an appropriate product.

Charge converters CH-6130, CH-6140

Table 1 shows the main specifications of the CH-6130 and CH-6140 charge converters. The only difference between the CH-6130 and CH-6140 is the gain and price. For measuring large accelerations, the CH-6130 with its lower gain is suitable. For measuring small accelerations, the CH-6140 with its higher gain is suitable, but because the output is capped by a maximum output voltage of ±5 V, it cannot measure very large accelerations.

Table 1 Main specifications of charge amplifiers CH-6130 and CH-6140

item	CH-6130	CH-6140
gain	1.0 mV/pC	10 mV/pC
Frequency range	5 Hz~15 kHz (±0.5 dB) 2 Hz~45 kHz (±3 dB)	5 Hz~15 kHz (±0.5 dB) 2 Hz~45 kHz (±3 dB)
Maximum output voltage	10 Vp-p or more ±5 V or more	10 Vp-p or more ±5 V or more
Input-calculated noise	0.05 pC (rms) or less	0.05 pC (rms) or less
CCLD power supply	Voltage: 18~36 V Constant current: 2.0~20 mA	Voltage: 18~36 V Constant current: 2.0~20 mA

Figure 1 shows the appearance of the CH-6130. The appearance of the CH-6140 is the same as the CH-6130 except for the model name.

Figure 1 Charge converter CH-6130

A combination of a charge-type accelerometer and a charge converter.

When using a charge-type accelerometer in combination with a charge converter, the voltage sensitivity is the product of the accelerometer's charge sensitivity (pC/(m/s²)) and the charge converter's gain (mV/pC). The NP-2106 has a charge sensitivity of approximately 0.035 pC/(m/s²) and the CH-6140 has a gain of 10 mV/pC, so the voltage sensitivity is the product of these two values, 0.35 mV/(m/s²). This indicates that for an acceleration of 1 m/s², an output of 0.35 mV is produced.

The maximum usable acceleration for a charge-type accelerometer is the maximum acceleration at which the accelerometer operates correctly.
Due to limitations in the maximum output voltage of the charge converter, the maximum usable acceleration when combined may be less than the maximum usable acceleration of the accelerometer alone. When the NP-2106 and CH-6140 are combined, the voltage sensitivity is 0.35 mV/(m/s²), and the maximum output voltage of the CH-6140 is ±5 V, so the maximum usable acceleration when combined is 5 V ÷ (0.35 mV/(m/s²)) = 14,285 m/s². The maximum usable acceleration of the NP-2106 alone is 100,000 m/s², but when combined with the CH-6140, it can only measure up to 14,285 m/s². To measure accelerations greater than this, it is necessary to combine it with the CH-6130 or a charge amplifier, etc.

The input-referred noise of the CH-6130 and CH-6140 charge converters is less than 0.05 pC. Even with zero acceleration, a noise signal equivalent to less than 0.05 pC will be output. The charge sensitivity of the NP-2106 is approximately 0.035 pC/(m/s²), so when converted to acceleration, it becomes 0.05 pC ÷ (0.035 pC/(m/s²)) = 1.43 m/s². Depending on the measurement method and the required accuracy of the measurement, a value 10 or 30 times the input-referred noise value (m/s²) (14.3 to 42.9 m/s²) is a rough guideline for the lower limit of measurable acceleration. The NP-2106 is an acceleration detector for collision and drop tests that can measure up to 100,000 m/s², so it is not suitable for measurements below several hundred m/s². For measuring small accelerations, please use an accelerometer with high sensitivity.

When our charge output type accelerometer and charge converters CH-6130 and CH-6140 are combined...
The voltage sensitivity, maximum operating acceleration, and input-referred noise are shown in Tables 2 and 3.
When combined, the maximum operating acceleration of all acceleration detectors is lower when combined than when used individually. This also applies to the CH-6130; for the NP-2710, NP-2910, NP-2810, and NP-2120, the maximum operating acceleration is lower when combined. If you need to measure acceleration greater than the maximum operating acceleration when combined, please use a charge amplifier such as our CH-1200A.

Table 2 Characteristics when combined with charge converter CH-6130

Charge output type accelerometer				CH-6130 (1.0 mV/pC) and Characteristics when combined
Model name	Features	Charge sensitivity (typ.)	maximum usage acceleration	Voltage sensitivity (typ.)	maximum usage acceleration	Input conversion noise
NP-2106	Ultra-compact and lightweight	0.035 pC/(m/s2)	100,000 m/s2	0.035 mV/(m/s2)	100,000 m/s2	1.43 m/s2
NP-2110	Small and lightweight	0.16 pC/(m/s2)	10,000 m/s2	0.16 mV/(m/s2)	10,000 m/s2	0.31 m/s2
NP-2710	Small size/high temperature	0.306 pC/(m/s2)	22,600 m/s2	0.306 mV/(m/s2)	16,339 m/s2	0.16 m/s2
NP-2910	Small and general-purpose	0.3 pC/(m/s2)	50,000 m/s2	0.3 mV/(m/s2)	16,667 m/s2	0.17 m/s2
NP-2810	small size	1.2 pC/(m/s2)	20,000 m/s2	1.2 mV/(m/s2)	4,166 m/s2	0.042 m/s2
NP-2120	General purpose	5 pC/(m/s2)	8000 m/s2	5 mV/(m/s2)	1000 m/s2	0.010 m/s2
NP-2560	Lightweight 3-axis	0.04 pC/(m/s2)	25,000 m/s2	0.04 mV/(m/s2)	25,000 m/s2	1.25 m/s2

Table 3 Characteristics when combined with charge converter CH-6140

Charge output type accelerometer				CH-6140 (10 mV/pC) and Characteristics when combined
Model name	Features	Charge sensitivity (typ.)	maximum usage acceleration	Voltage sensitivity (typ.)	maximum usage acceleration	Input conversion noise
NP-2106	Ultra-compact and lightweight	0.035 pC/(m/s2)	100,000 m/s2	0.035 mV/(m/s2)	14285 m/s2	1.43 m/s2
NP-2110	Small and lightweight	0.16 pC/(m/s2)	10,000 m/s2	1.6 mV/(m/s2)	3125 m/s2	0.31 m/s2
NP-2710	Small size/high temperature	0.306 pC/(m/s2)	22,600 m/s2	3.06 mV/(m/s2)	1633 m/s2	0.16 m/s2
NP-2910	Small and general-purpose	0.3 pC/(m/s2)	50,000 m/s2	3 mV/(m/s2)	1666 m/s2	0.17 m/s2
NP-2810	small size	1.2 pC/(m/s2)	20,000 m/s2	12 mV/(m/s2)	416 m/s2	0.042 m/s2
NP-2120	General purpose	5 pC/(m/s2)	8000 m/s2	50 mV/(m/s2	100 m/s2	0.010 m/s2
NP-2560	Lightweight 3-axis	0.04 pC/(m/s2)	25,000 m/s2	0.04 mV/(m/s2)	12500 m/s2	1.25 m/s2

summary

Each accelerometer has a specified maximum operating acceleration, and it is not possible to measure accelerations exceeding that value. When using a charge output type accelerometer in combination with a charge converter, the maximum measurable acceleration may be less than the maximum operating acceleration of the accelerometer alone due to the limited output voltage of the charge converter.

This time, we presented the maximum usable acceleration value when a charge output type accelerometer and a charge converter are combined. We also presented the charge sensitivity and input-referred noise values when the two are combined.

Accelerometers and charge converters must be selected to match the magnitude of acceleration generated by the object being measured. In some cases, a charge amplifier may be required instead of a charge converter.

When measuring an object for the first time, the magnitude of the acceleration generated is unknown. Furthermore, collision and drop tests can sometimes generate acceleration far greater than anticipated. To allow you to verify the actual acceleration generated in such cases, we offer demo unit rentals and product demonstrations. Please contact your nearest sales office for details.

(Excerpt from the email newsletter issued on February 20, 2019)

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