Biomedical Engineering Reference
In-Depth Information
FIGURE 5-13
Schematic showing
(a) piezoelectric
accelerometer
connected to (b) a
charge amplifier.
In short, the amplifier acts as a charge integrator that balances the sensor's electrical charge
with a charge of equal magnitude and opposite polarity. This ultimately produces a voltage
across the range capacitor. In effect, the purpose of the charge amplifier is to convert the
high impedance charge input,
q
, into a usable output voltage,
V
out
.
Two of the more important considerations in the practical use of charge amplifiers
are time constant and drift. The time constant,
τ
c
, is defined as the discharge time of
an alternating current (AC)-coupled circuit. In a period of time equivalent to one time
constant, a step input will decay to 37% of its original value. The time constant of a charge
amplifier is determined by the product of the range capacitor,
C
r
, and the time constant
resistor,
R
t
:
τ
c
=
R
t
C
r
(5.21)
Drift is defined as an undesirable change in output signal over time and is independent of
the input signal. In a charge amplifier it can be caused by low insulation resistance at the
input,
R
i
, or by leakage current of the input MOSFET or JFET in the op amp.
Drift and time constant simultaneously affect a charge amplifier's output. One or the
other will be dominant. Either the charge amplifier output will drift toward saturation
(power supply rail) at the drift rate, or it will decay toward zero at the time constant rate.
5.3.3 Practical Considerations
It must be remembered that op amps do not really have infinite gain or infinite input
impedances, among other problems.
In addition, real op amps exhibit an imbalance caused by a small mismatch between
the input transistors, which results in unequal bias currents flowing through the input
terminals and offsets the output voltage. This offset can be balanced out by introducing a
compensating bias voltage.
As mentioned earlier, one of the main uses of differential amplifiers in biomechatronic
applications is to eliminate common-mode voltages. In reality, this rejection is not perfect
