CETDQ801B Charge Amplifier for Weigh In Motion System

The CETDQ801B charge amplifier is a multi-channel charge amplifier that outputs a voltage proportional to the input charge. It supports 8 or 12 channels (optional) and is designed to work with piezoelectric sensors to measure mechanical quantities such as acceleration, pressure, and force. It is widely used in sectors including water conservancy, power, mining, transportation, construction, seismology, aerospace, and defense. The instrument features the following characteristics:

1)      Optimized circuit design with high-precision, low-noise, low-drift imported components ensures stable and reliable product quality.

2)      Eliminates signal attenuation caused by the equivalent capacitance of input cables, allowing the use of long cables without affecting measurement accuracy.

3)      Outputs ±5Vp at 50mA.

4)      Supports input from 8 or 12 sensors, with a DB15 output interface, and operates at DC 12V.

2. Working Principle

The CETDQ801B charge amplifier consists of a charge conversion stage, adjustment stage, low-pass filter, high-pass filter, final power amplifier overload stage, and power supply. The working principle is illustrated in the block diagram below:

1)      Charge Conversion Stage: Centered around operational amplifier A1.

The CETDQ801B charge amplifier can be connected to piezoelectric acceleration sensors, force sensors, and pressure sensors. These sensors convert mechanical quantities into proportional weak charges (Q) with extremely high output impedance (Ra). The charge conversion stage transforms the charge into a proportional voltage (1pC/1mV) and converts high output impedance to low output impedance.

o    Ca: Sensor's inherent capacitance, typically several thousand pF. The lower frequency limit is determined by 1/2πRaCa.

o    Cc: Capacitance of the low-noise output cable.

o    Ci: Input capacitance of operational amplifier A1, typically 3pF.

The charge conversion stage uses a high-input-impedance, low-noise, low-drift, broadband precision operational amplifier from the USA. The feedback capacitor (Cf1) has four settings: 101pF, 102pF, 103pF, and 104pF. According to Miller's theorem, the effective capacitance at the input is C = (1 + K)Cf1, where K is the open-loop gain of A1, typically 120dB. When Cf1 is set to 100pF (minimum), C is approximately 108pF. For a 1000-meter low-noise cable, Cc is 95000pF. Assuming Ca is 5000pF, the total parallel capacitance of Ca, Cc, and Ci is approximately 105pF, which is 1/1000 of C. Thus, even with a 1000-meter cable, the impact on Cf1's accuracy is only 0.1%. The output voltage of the charge conversion stage is Q / Cf1, so the output is 10mV/pC, 1mV/pC, 0.1mV/pC, and 0.01mV/pC for the respective feedback capacitor settings.

2)      Adjustment Stage: Comprises operational amplifier A2 and a sensitivity adjustment potentiometer (W). This stage ensures normalized voltage output when using piezoelectric sensors with different sensitivities.

3)      Low-Pass Filter: A second-order Butterworth active filter centered around A3, designed to effectively eliminate high-frequency interference.

4)      High-Pass Filter: A first-order passive high-pass filter composed of C4 and R4, designed to suppress low-frequency interference.

5)      Final Power Amplifier: Centered around A4, providing gain and output short-circuit protection with high precision.

3. Technical Specifications

1)      Input Characteristics: Maximum input charge ±106pC.

2)      Sensitivity: 0.1-1000mV/pC (with source capacitance of 1nF at -40 to +60dB).

3)      Sensor Sensitivity Adjustment: 4 ranges with charge sensitivity from 1-109.9pC/Unit.

4)      Accuracy:

o    1mV/Unit, 10mV/Unit, 100mV/Unit, 1000mV/Unit.

o    When input cable equivalent capacitance is less than 10nF, 68nF, 22nF, 6.8nF, or 2.2nF, the error is less than ±1% under 1kHz reference conditions and less than ±2% under rated working conditions.

5)      Filter and Frequency Response:

o    High-Pass Filter: Lower cutoff frequencies of 0.3, 1, 3, 10, 30, and 100Hz. Tolerance: 0.3Hz, -3dB to +1.5dB; 1, 3, 10, 30, 100Hz, -3dB ±1dB. Attenuation slope: -6dB/octave.

o    Low-Pass Filter: Upper cutoff frequencies of 1, 3, 10, 30, and 100kHz. Tolerance: -3dB ±1dB. Attenuation slope: -12dB/octave.

6)      Output Characteristics:

o    Maximum output amplitude: ±5Vp.

o    Maximum output current: ±100mA.

o    Minimum load resistance: 100Ω.

o    Harmonic distortion: Less than 1% at frequencies below 30kHz with capacitive loads <47nF and full-scale output.

7)      Noise: Less than 5µV (referred to input at maximum gain).

8)      Warm-Up Time: Approximately 30 minutes.

9)      Output Interface: DB15.

10)  Input Channels: 8 or 12 channels.

11)  Input Power: DC 12V.