Measurement Equipment

Transducers with charge output

Transducers with charge output have some special properties which require particular attention in order to obtain precise measuring results:

• Always use special low noise cables.
• The cable length must not exceed 10 meters.
• The cable should not be moved during measurement.
• All connector nuts must be tightened.
  

Preferably charge amplifiers should be used. It is also possible to use AC voltage amplifiers with high impedance input. Both principles are described below.

1. Charge amplifiers

Accelerometers with charge output generate an output signal in the range of some picocoulombs (1 pC = 1000 fC) with a very high impedance. To process this signal by standard AC measuring equipment it needs to be transformed into a low impedance voltage signal.
Preferably, charge amplifiers are used for this purpose. The input stage of a charge amplifier features a capacitive feedback circuit which balances the effect of the applied charge input signal. The feedback signal is then a measure of input charge. The following picture shows a typical charge input stage:

The input charge q_in is applied to the summing point (inverting input) of the amplifier. It is distributed to the cable capacitance C_c, the amplifier input capacitance C_inp and the feedback capacitor C_f. 

The node equation of the input is therefore:
q_in = q_c + q_inp + q_f

 
Using the electrostatic equation:
q = u · C
and substituting q_c, q_inp and q_f:
q_in = u_inp · (C_c + C_inp) + u_f · C_f

 
Since the voltage difference between the inverting and the non-inverting input of a differential amplifier becomes zero under normal operating conditions, we can assume that the input voltage of the charge amplifier u_inp will be equal to GND potential. With u_inp = 0 we may simplify the equation:
q_in = u_f · C_f
and solving for the output voltage u_out:
u_out = u_f = q_in / C_f

 
The result shows clearly that the output voltage of a charge amplifier only depends on the charge input and the feedback capacitance. Input and cable capacitances have no influence on the output signal. This is a significant fact when measuring with different cable lengths and types.
Referring to the upper picture, the feedback resistor R_f has the function to provide DC stability to the circuit and to define the lower frequency limit of the amplifier.

 
The circuit in the upper picture only represents the input stage of a charge amplifier. Other stages like voltage amplifiers, buffers filters and integrators are not shown.
Typical charge amplifiers are, for example, the M68 series signal conditioners.

2. High-impedance voltage amplifiers

Instead of charge amplifiers, high impedance voltage amplifiers can be used with charge mode transducers. In this case, however, the capacitances of sensor, cable, and amplifier input must be considered (see the following picture).

The voltage sensitivity of an accelerometer with known charge sensitivity B_qa and inner capacitance C_i is calculated to:
B_ua = B_qa / C_i
The values for B_qa and C_i can be found in the sensor data sheet.

 
Taking into account the capacitance of the sensor cable C_c and the input capacitance C_inp of the voltage amplifier, the resulting voltage sensitivity B´_ua will become lower than B_ua:
B´_ua = C_i / (C_i + C_c + C_inp)
A typical 1.5 m low noise cable model 009 has a capacitance of approximately 135 pF.
 
The lower frequency limit f_l will also be influenced by C_c, C_inp and R_inp:

The lower frequency limit increases with decreasing input resistance.

Example: A charge mode accelerometer Model KS50 with inner capacitance C_i = 1.4 nF is connected to a typical scope input with R_inp = 10 MOhms and C_inp = 20 pF. The sensor cable has a capacitance of 135 pF.

Result: The lower frequency limit will be at about 10 Hz.

IEPE compatible accelerometers

A special feature of IEPE compatible transducers is that power supply and measuring signal are transmitted via the same cable. That is why an IEPE compatible transducer, like a transducer with charge output, requires only one single-ended shielded cable.
The following picture shows the principle circuit diagram.

The integrated sensor electronics is powered with constant current in the range between 2 and 20 mA.

A typical value is 4 mA. Some battery powered instruments even work at 1 mA.

The constant current I_const is fed into the signal cable of the sensor.

The supply current and the length of the cable may influence the upper frequency limit. 

The de-coupling capacitor C_c keeps DC components away from the signal conditioning circuit. The combination of C_c and R_inp acts as a high pass filter. Its time constant should be sufficiently high to let all relevant low frequency components of the sensor signal pass.

Important:

• Under no circumstances a voltage source without constant current regulation should be applied to an IEPE compatible transducer.
• False polarization of the sensor cable may immediately destroy the built-in electronics.
  

The following picture shows that IEPE compatible transducers provide an intrinsic self-test feature.