Charge amplifier for sensors outputting electrical charge

A charge amplifier for sensors outputting electrical charge, particularly for piezoceramic pressure sensors, includes a voltage integrator having an output, an integration capacitor and a resistor connected parallel to the integration capacitor. A current-to-voltage converter is connected upstream of the voltage integrator and has an input. A negative feedback branch has a series circuit of a farther voltage integrator and a voltage-to-current converter. The negative feedback branch is connected between the output of the voltage integrator and the input of the current-to-voltage converter.

The invention relates to a charge amplifier for sensors that output 
electrical charge, particularly for piezoceramic pressure sensors, having 
an integrator with an integration capacitor to which a resistor is 
connected in parallel. 
Measuring the course of combustion chamber pressure in internal combustion 
engines affords the opportunity, among others, of cylinder-selective 
recognition of defective combustion processes, such as misfiring and 
engine knocking. Piezoceramic sensors which are used for such a purpose 
are mounted between the spark plug and the cylinder head, or in an 
indicator bore of the cylinder head that is directly exposed to the 
combustion chamber pressure. Such sensors emit a charge signal that is 
proportional to the course of the pressure in the cylinder. 
The charge signal furnished by the sensor is amplified by a charge 
amplifier and converted into a voltage signal. In the ideal case, a charge 
amplifier is a current integrator. 
Piezoceramic and other sensors that output an electrical charge have 
properties that impair the proportionality between the measurement 
variable (the combustion chamber pressure) and the sensor signal. The 
sensor-caused drift in the charge signal over the temperature, which 
results from a sudden engine load change, for instance, severely limits 
the useful signal range within the supply voltage range. Moreover, the 
current frequency response (spurious frequency response) of the charge 
amplifier leads to a high direct component in the output signal 
(hysteresis of the temperature-dictated signal drift). 
In practice, known charge amplifiers (as shown in FIG. 1 which is discussed 
below) are driven by a resistor connected parallel to the integration 
capacitor. A resultant first-order high-pass filter in the charge 
frequency response prevents the charge amplifier from entering saturation 
in response to constantly integrated leakage currents within a short time. 
The current frequency response (spurious frequency response) corresponds to 
a first-order low-pass filter, which does not damp temperature errors at 
all. 
Moreover, in many cases, such known charge amplifiers tend to oscillate. 
It is accordingly an object of the invention to provide a charge amplifier, 
which overcomes the hereinafore-mentioned disadvantages of the 
heretofore-known devices of this general type, in which both the 
temperature drift and the hysteresis are substantially less and which does 
not tend to oscillate. 
With the foregoing and other objects in view there is provided, in 
accordance with the invention, a charge amplifier for sensors outputting 
electrical charge, particularly for piezoceramic pressure sensors, 
comprising a voltage integrator having an output, an integration capacitor 
and a resistor connected parallel to the integration capacitor; a 
current-to-voltage converter being connected upstream of the voltage 
integrator and having an input; and a negative feedback branch including a 
series circuit of a further voltage integrator and a voltage-to-current 
converter, the negative feedback branch being connected between the output 
of the voltage integrator and the input of the current-to-voltage 
converter. 
In accordance with another feature of the invention, the resultant circuit 
structure (as shown in FIG. 2 which is discussed below) is split into a 
stable current-to-voltage converter, that is advantageously constructed as 
a current amplifier, and a voltage integrator in which the resistor, that 
is connected to an output voltage divider and connected parallel to the 
integration capacitor, can have a relatively low resistance, which is 
uniformly demanded (integration capability, leakage currents) for use in 
motor vehicles. 
More particularly, in accordance with a further feature of the invention, 
there is provided a voltage divider being connected between the output of 
the voltage integrator and reference potential of a voltage supply, the 
voltage divider having a tap, the voltage integrator having an input, and 
the resistor connected parallel to the integration capacitor being 
connected between the tap of the voltage divider and the input of the 
voltage integrator. 
Moreover, the circuit affords major advantages in signal processing. The 
integrator in the negative feedback branch leads to a charge frequency 
response that acts as a second-order high-pass filter. Temperature 
gradient errors of the sensors are suppressed twice as well as in the 
known charge amplifier, as are other temperature-caused effects. The drift 
in the output voltage upon a load change is then only half as pronounced. 
The current frequency response exhibits bandpass behavior, so that errors 
in hysteresis are suppressed completely. As a result, the sensor no longer 
exhibits practically any apparent hysteresis effect in the output signal 
drifting when a load change occurs. 
Moreover, in this circuit, it is possible to use substantially smaller 
capacitors for both integrators. Low capacitances make lesser demands of 
the output stage of the charge amplifier, because the amplifier need not 
furnish a high charge current. Low-capacitance ceramic capacitors with 
very low tolerances (1%) are available, which means that the charge 
amplifier is usable at the higher temperatures arising in motor vehicles, 
unlike foil capacitors, which are available only with substantially larger 
tolerances for use in higher temperature ranges. Yet low tolerances are 
important, since the capacitance directly determines the magnitude of the 
output voltage. 
The necessity for high capacitances in conventional charge amplifiers 
accordingly makes them virtually impossible to use in motor vehicles at 
high temperatures. 
In accordance with an added feature of the invention, the 
voltage-to-current converter is a resistor. 
In accordance with a concomitant feature of the invention, the 
voltage-to-current converter is a controlled current source. 
Other features which are considered as characteristic for the invention are 
set forth in the appended claims. 
Although the invention is illustrated and described herein as embodied in a 
charge amplifier, it is nevertheless not intended to be limited to the 
details shown, since various modifications and structural changes may be 
made therein without departing from the spirit of the invention and within 
the scope and range of equivalents of the claims.

Referring now to the figures of the drawing in detail and first, 
particularly, to FIG. 1 thereof, there is seen a circuit of a known, 
discrete-type charge amplifier, including an operational amplifier OP that 
is wired as an integrator 3, having an integration capacitor C which has a 
resistor R connected parallel to it and performs the function of a current 
integrator with charge amplification. Its advantages and disadvantages 
have already been discussed above. The charge amplifier of FIG. 1 converts 
a charge signal Qe, obtained from a piezoceramic sensor 1, into an output 
signal Ua that is as proportional as possible. 
FIG. 2 shows a basic circuit diagram of a charge amplifier according to the 
invention. This circuit diagram illustrates individual functions and is 
therefore advantageous, because exemplary embodiments in different 
techniques, such as an integrated version, in which the embodiment of the 
various function blocks may differ considerably from the exemplary 
embodiment, made by a discrete technique shown in FIG. 3. 
An output signal of the piezoceramic sensor 1, that is the charge Qe or a 
sensor current Is, is converted in a current-to-voltage converter 2 into a 
voltage signal, which is converted in a following voltage integrator 3 
into the output signal Ua. This output signal Ua is negatively fed back 
from the output of the voltage integrator 3 to the input of the 
current-to-voltage converter, through an integrator 4 and a 
voltage-to-current converter 5. The mode of operation and advantages of 
this basic circuit have likewise already been explained in detail above. 
Finally, FIG. 3 shows an exemplary embodiment of a charge amplifier 
according to the invention, which corresponds to the basic circuit diagram 
of FIG. 2 but is of a discrete type, which is constructed with operational 
amplifiers. 
In this embodiment, the current-to-voltage converter 2 is constructed as a 
current amplifier, which includes an operational amplifier OP1 wired with 
a resistor R1, that converts the input signal Qe or Is of the piezoceramic 
sensor 1 into a voltage signal. This signal is integrated to make the 
output signal Ua through the use of the following voltage integrator 3, 
which includes an operational amplifier OP2 wired with an input resistor 
R2 and a negatively fed back integration capacitor C1 with a resistor R5 
that is connected in parallel with the integration capacitor C1 through a 
tap of an output voltage divider R3, R4. This output signal Ua is then 
further processed in a non-illustrated manner, for instance to produce 
pressure and/or knocking signals. 
In the negative feedback branch, the output signal Ua is integrated to make 
a voltage signal Ug, through a further voltage integrator 4 which includes 
an operational amplifier OP3 that is wired with an input resistor R6 and 
an integration capacitor C2, in this case without any parallel resistor. 
Through the use of a voltage-to-current converter 5 formed by a resistor 
R7, this voltage signal Ug is then converted into a current signal Ig that 
is superimposed on the output signal Is of the sensor 1. 
Advantageously, the voltage-to-current converter 5 (which is equivalent to 
the resistor R7) of FIGS. 2 or 3 may be constructed as a controlled 
bipolar current source between the supply terminals and the input of the 
current-to-voltage converter 2. This source is controlled by the output 
signal Ug of the negative feedback integrator 4. 
This is especially advantageous if the circuit is to be an integrated 
circuit, since the entire circuit except for the two capacitors C1 and C2 
is integratable. In that case, the negative feedback integrator 4, 
including the resistor R6, which like the resistor R7 is of relatively 
high impedance when made in discrete form, can be constructed with current 
source circuits, so that these two resistors need not be made in 
high-impedance integrated form.