Description

[Application Note 40518]

Piezoelectric transducers produce small electrical charges in response to pressure. Their sensitivity, ruggedness, and stability at high temperatures make them an essential building block of many accelerometers and pressure sensors. Applications include turbine vibration measurement (accelerometers) and combustion dynamics monitoring (pressure sensors).

Typically, the high-impedance charge output produced by a piezoelectric transducer must be conditioned and amplified before transmission across system cabling to the data acquisition and measurement instrumentation. This conversion is accomplished with a device called a charge amplifier.

Problem

Over time, a charge amplifier’s signal response can drift. Damage to the charge amplifier or associated cabling is also common. Periodic calibration, consequently, is required to ensure accurate measurement of the physical phenomenon exciting a piezoelectric transducer. Many charge amplifiers offer adjustable bandwidth and gain settings that should also be confirmed.

Unfortunately, testing charge amplifier gain and frequency response – as well as system cabling – can be a challenging task. That’s because most signal generators are not equipped with a charge output, and those that are may not have the precision and frequency range necessary to fully test the charge amplifier. Additionally, these types of sensors are typically installed in remote, hostile environments where portable testing equipment is a requirement.

Solution

The 1510A Signal Generator from MTI Instruments can simulate most types of piezoelectric transducers for the test and calibration of charge amplifier electronics and associated cabling. Test results using two commercially available products – the Endevco (Meggitt) Model 2777A and the MTI Instruments 55CA – demonstrate its ability to verify frequency response of a typical charge amplifier.

Endevco (Meggitt) Model 277A

MTI’s 1510A Signal Generator can be used to test the gain and frequency response of a typical charge amplifier. Tests on two
charge amplifiers, the Endevco (Meggitt) Model 2777A and the MTI Instruments 55CA, show both charge amplifiers functioning
as expected.

In both instances, the 1510A was used to generate 100 picocoulombs (pC) RMS input signals at various frequencies. An AC voltmeter recorded the RMS voltage outputs. Results show both charge amplifiers functioning as expected.

MTI Instruments 55CA

MTI Instruments 55CA

Specifically, the 2777A datasheet specifies a high-pass -3dB frequency of 8.59 Hz and a low-pass -3dB frequency of 17.5 kHz. The expected gain is 10 mV/pC in the passband. Results matched the expected voltage output: near 70% at 8.59 Hz and 17.5 kHz, and near 100% in the passband.

Frequency (Hz)Output (V rms)
50.315
80.662
8.6
0.714
100.809
200.981
600.99
1000.997
2000.997
20000.997
100000.951
175000.689
20013000.575
Graph Output vs. Frequency

The 55CA label specifies a high-pass -3dB frequency of 10 Hz and a low-pass -3 dB frequency of 5 kHz. The expected gain is 10 mV/pC in the passband. Again, voltage output met expectations: near 70% at 10 Hz and 5 kHz, and close to 100% in the passband.

Frequency (Hz)Output (V rms)
50.062
80.386
100.713
200.994
600.997
1000.997
2000.997
20000.932
50000.717
70000.492
Graph Output vs. Frequency

Benefits

A charge accuracy exceeding 0.2% of output makes the 1510A an excellent tool for calibrating all types of charge amplifiers. Portability and ruggedness, moreover, make it convenient for testing cables and equipment installed in remote and hostile environments. Related features include

  • Single-ended and differential charge output options.
  • Charge amplitude configurable from 0 to 10,000 pC.
  • Frequency configurations from 0 to 100,000 Hz.
  • Frequency sweep function

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