In many piezoelectric testing occasions, it is necessary to convert the charge signal into a voltage or current signal. At this time, a special kind of amplification-a charge amplifier is needed.
The principle of the charge amplifier: using the concept of the amplifier input virtual ground, the charge signal is directly integrated into the integrating capacitor in the circuit, so that the output of the amplifier is the output of the sum of all charges. At the same time the charge conversion is completed, the charge on the input terminal will be depleted.
In reality, the charge amplifier is not ideal, because any electronic amplifier itself has a bias voltage bias current, and the integral capacitor itself has leakage resistance, which leads to errors in the charge conversion, and after the charge conversion is completed, the integral capacitor is also Because of the above factors, it will fluctuate up and down with time. At present, the domestic field of solving this problem is almost blank, so that foreign high-end amplifiers to solve this problem sell for tens of thousands of yuan.
Charge amplifier classification:
1. DC charge amplifier: used to measure the amount of charge that changes slowly. But its biggest disadvantage is that non-ideal amplifier leakage current, bias current, etc. will cause the measurement result to be slowly and continuously injected at the same time. As time goes by, the error becomes larger until the amplifier is fully saturated. The ideal amplifier can solve this problem, but the price is tens of thousands of yuan.
The compromise method is: in the ordinary DC charge amplifier, add an integral discharge circuit, before each measurement, discharge the capacitor to 0 output.
2. AC charge amplifier: often used to measure the amount of charge that changes back and forth. It is also the most widely used and most used type at present. The principle is to add a discharge resistance of several hundred megabytes or even G to both ends of the integral capacitor of the DC charge amplifier. Compared with the amplifier resistance, the leakage current and bias current of the amplifier have little effect, so it is close to the ideal value. However, it can also be seen from its structural principle that it is not suitable for testing signals with too low frequency, because when the charge frequency is low, the charge integral is the discharge of the resistance to the integral. Therefore, it is generally used to measure frequencies greater than 50 Hz. If you want to test a lower frequency, you can increase the value of the discharge resistance and choose a charge sensor with a larger amount of charge.
The relationship between input and output voltage:
Take the precision AC charge amplifier of Shenzhen Vkinging Co., Ltd. as an example. ICA101, ICA102, ICA103, and ICA104 represent that the internal integral capacitors are 100PF, 1000PF, 10000PF, and 100000PF respectively.
ICA101: Input charge of 100pC, output peak value is 1V, maximum input is 500pC when supplying +-6V
ICA102: Input charge of 1000pC, output peak value is 1V, maximum input is 5000pC when supplying +-6V
ICA103: Input charge of 10000pC, output peak value is 1V, maximum input is 50000pC when supplying +-6V
ICA104: Input charge of 100000pC, output peak value is 1V, maximum input is 500000pC when supplying +-6V
Application points of AC charge amplifier:
1. Piezoelectric ceramics, acceleration sensors, etc., need to be charge type. (current output type is currently available on the market, and current output type is not high in accuracy)
2. For regular tests such as sine, the effective value of the measuring point is the peak-to-peak value.
3. If it is a single pulse test, the effective value is the highest peak value; the next pulse test cannot be performed until it falls back to 0V. Because there is an integral discharge resistance inside the AC amplifier, the output will slowly fall back from the peak value. If you start a new test before returning to the origin, it will be inaccurate.
