REV B ECN 11862 07/24/2015 3
Using equation 2 and making the appropriate
substitutions and solving for the output voltage of
the amplifier in terms of input charge, amplifier
loop gain, and input and feedback capacitance we
have:
-qin -qin 1
Vout = ---------------- = ----------- x ------------------ Eq 3
CA / A + Cf(A + 1) Cf(1 + 1 / A) 1 + CA / Cf(A + 1)
where A is the open loop gain of the op-
amp.
Now, letting gain A approach infinity, we have:
-qin
V
out = ------- Eq 4
Cf
This result (Eq 4) shows clearly that the
transfer function (gain) of a charge amplifier is a
function only of the value of the feedback
capacitor Cf. Notice that input capacitance CAhas
no effect on the sensitivity of the charge amplifier.
This means that cable capacitance, for example,
has no effect on the sensitivity, a significant find
when switching cable lengths and types.
INSTALLATION
(Refer to Outline/Installation drawing 127-
3255C) To install Model 3255C, is necessary to
prepare (or find) a flat mounting area of
approximately 0.5 inch diameter. Ideally, the
mounting surface should be flat to .001 in. TIR.
The flat mounting surface ensures intimate contact
between accelerometer base and mounting
surface for best high frequency transmissibility,
thus accuracy.
At the center of the mounting area, drill
and tap a 10-32 mounting port in accordance with
instructions on drawing 127-3255C. Clean the
area to remove all traces of machining chips,
burrs, etc.
Next, thread the Model. 6200 mounting
stud into the base of the 3255C. The stud should
enter easily and thread in up to the raised collar of
the stud by hand. This collar prevents the stud
from bottoming inside the tapped hole in the
3255C where it could possibly cause stresses in
the base structure which could, in turn, cause
anomalous behavior of the accelerometer at
higher frequencies.
After seating the stud, spread a light
coating of silicone grease, or other lubricant, on
either of the mating surfaces and thread the
accelerometer/stud combination into the tapped
hole by hand, until the accelerometer base seats
against the mounting surface. Check to see that
the mating surfaces are meeting properly, i.e., that
they are meeting flush and that there is not an
angle formed between the two surfaces indicating
that they are not co-planar. If this condition is
observed, torquing the accelerometer down will
strain the base causing possible poor frequency
response and even erroneous reference
sensitivity. Inspect the perpendicularity of the
tapped hole.
If the hand tight meeting between the two
surfaces is satisfactory, torque the 3255C to the
mating surface with 10 to 12 lb-inches of torque,
preferably measuring the torque with a torque
wrench torquing on the hex surface only.
Proper torque will ensure the best high
frequency performance from the instrument as
well as repeatability of sensitivity when mounting
and remounting.
Connect the cable (typically low-noise
Models 6013AXX or 6019AXX) to the
accelerometer snugging up the threaded lock ring
tightly by hand.
NOTE: Do not use pliers or vise grips on
the knurled lock ring. This could damage
the connector of the 3255C and/or the
cable connector.
To avoid stressing the cables which could
lead to early failure, especially under larger
excursions of the test object, it is good practice to
tie the cable down to a fixed surface near the
mounting area at a point approximately one inch
from the accelerometer.
If there is excessive motion between the
accelerometer and the nearest tie point, allow a
strain loop of cable to let relative motion occur
without stressing the cable.
Connect the other end of the cable to the
charge amplifier. Consult the operating manual for
the particular charge amplifier employed for
instructions on its use.
HIGH FREQUENCY RESPONSE
All piezoelectric accelerometers are
basically rigid spring mass systems, i.e., second
order systems with essentially zero damping. As a
result, these instruments will exhibit a rising
characteristic as the resonant frequency is
approached.
