Open-Loop Piezo Controllers
Chapter 6: Bandwidth Performance
Page 14
Rev C, July 7, 2015
Chapter 6 Bandwidth Performance
The MDT693B and MDT694B are precision, low-noise, low-drift, high-voltage
amplifiers designed specifically for driving piezo actuators. There are two
limitations that need to be considered. Since piezoelectric crystals can be modeled
as capacitors, it is easy to predict their dynamic performance using a basic set of
equations as described below.
6.1. Low-Pass
Filter
Since the piezo can be modeled most accurately as a capacitor, the piezo will
create a low-pass filter with the output impedance of the driver. For both the
MDT693B and MDT694B, the output impedance is specified as 150
, 1 nF.
1
2
Where,
For example, consider connecting a piezo with a 1.6
F capacitance to the
MDT693B. Using the output impedances given in the specification section (see
page
Error! Bookmark not defined.
):
1
2 150
1nF
1.6μF
663
6.2.
Output Current Limit
The output current limit also needs to be considered when determining system
performance. Since the piezo is electrically equivalent to a capacitor (C
PIEZO
), the
charge rate (
∆
V/
∆
t) needs to be considered. The max current for the MDT693B
and MDT694B is 60 mA (I
OUT
).
∆
∆
The output current requirements depend upon how quickly the piezo needs to be
charged, not the voltage applied to it. Below are some common waveforms and
their calculations.
