BRUEL & KJAER 2626 User guide

Conditioning Amplifier
Type 2626
Sr u.l & Ki<»r
c.,.,.,... .. . Sensitivi ty pC /g
Conditioning Amplifier
Type 2626
This charge amplifier contains a
three
digit sensitivity adjustment
network offering wide conditioning
possibilities to different transducers
(1 -1100 pC/g). Output of the
amplifier can be adjusted stepwise
between 1
mV /g and 10 V/g
depending on the
transducer's
sensitivity and is available either
directly or through an output
transformer. The lower and upper
limiting frequencies can be adjusted
stepwise.

CONDITIONING AMPLIFIER TYPE 2626
September 1971

CONTENTS
1. INTRODUCTION . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5
1.1. General . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5
1.2. Charge
Amp Iifier . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6
1.3. Principle
of the Charge Amplifier . . . . . . . . . . . . . . . . 7
2. CONTROLS . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9
2.1.
Front Panel
2.2. Rear Panel
................................. 9
12
3. OPERATION ........................................ 13
3.1. Input ..................................... 13
3.2. Output . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 14
3.3. Power Requirements ......... . ................ 14
3.4. Overload
Reset . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 14
3.5. Use in Servo-Loops of Exciter Control Systems . . . . . 14
3.6. Low Frequency Measurements .................. 15
3.7. Hum & Noise Reduction
....................... 15
3.8. Evaluation
of Noise Referred to Input ............ 15
4. DESCRIPTION
....................................... 19
4.1. Input ..................................... 19
4.2. Conditioning Section
......................... 20
4.3. Filters . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 20
4.4. Output . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 20
4.5. Signal Level Indicators · ........................ 21
4.6. Power Supply . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 21
4. 7. Output Characteristics . . . . . . . . . . . . . . . . . . . . . . . . 21
4.8. Noise . . ................................... 23
4.9. Phase Characteristics . . . . . . . . . . . . . . . . . . . . . . . . . . 23

5. USE WITH OTHER INSTRUMENTS ...................... 25
5.1. Sensitivity Calibration of Accelerometers .......... 25
5.2. Wide Band Random Test
...................... 27
5.3. Mechanical Impedance Measurements
............. 28
5.4. Correlation Measurements ..................... 28
6. ACCESSORIES ....................................... 29
7. SPECIFICATIONS .................................... 31

1. INTRODUCTION
1.1. GENERAL
When measuring vibration by means of an accelerometer, it is necessary
to incorporate a preamplifier between it and the measuring amplifier. The
preamplifier is introduced in the measuring circuit for two reasons:
1.
to transform th~ high output impedance of the accelerometer to a
lower value and
2.
to amplify the relatively weak output signal from the accelerometer.
The signal from the piezoelectric accelerometer appears as a voltage
across a capacitive impedance. As the capacitive output impedance of the
accelerometer is very high, the associated amplifier must be of a special
design having a high input impedance. This is necessary in order to avoid
loading of the accelerometer and thereby obtaining decreased sensitivity and
limitation at the low end of the frequency range.
The combination of a charge amplifier and a piezoelectric accelerometer
gives a sensitivity which is independent of cable length within very wide
limits. This feature makes a charge amplifier especially attractive in
vibration systems where different cables are used. The Conditioning Ampli-
fier Type 2626 may be used with cable lengths up to several thousand
meters.
Another advantage the 2626 Amplifier offers is the wide conditioning
possibilities to different transducers and measuring requirements. It features
a 3 digit sensitivity
adjustment network which enables the amplifier sensitiv-
ity to be adjusted to the particular transducer used. The network is
calibrated in pC/g. Output of the amplifier can be adjusted stepwise
between 1 rnV/g and
10 V/g, depending on the transducer sensitivity. A
maximum sensitivity
of 1 V /pC is available. The output signal is available
either directly coupled or through a transformer (switchable) for floating
output.
Stepwise adjustable HP and LP filters are incorporated, by which the
frequency limits can be adjusted independent of sensitivity. The filter scales
5

give the 5% as well as 3 dB frequency limits. The lowest frequency limit for
the amplifier is 0.3 Hz which makes it quite suitable for measurement of
impulses.
Two neon indicators are connected to the output, one for overload and
the other for underload which lights when the signal level is at least 1 V
peak. On
account of quick recovery time after overload, the amplifier is also
suitable for use
in a servo-loop in exciter systems.
1.2. CHARGE AMPLIFIER
The Conditioning Amplifier Type 2626 has been designed for use with
accelerometers, where it
is required that the measuring system be
independent
of the cable length connecting the accelerometer to the
preamplifier.
The active elements of a piezoelectric accelerometer are small discs of
polarized ceramic which when deformed mechanically, develop a charge
across opposite faces
of the discs. The equivalent circuit of this system can
be represented
either as a capacitance Ct across which a charge Sq a: appears
(Fig.1.1a),
or as a capacitance Ct in series with a voltage generator Sv a:
(Fig.1.1 b) where
Sq charge sensitivity (pC/g)
Sv voltage sensitivity (mV/g)
a: acceleration (g)
a. b.
Q=S•O<
Fig. 1. 1. a) Charge equivalent circuit b) Voltage equivalentcircuit
From the voltage equivalent circuit it can be seen that the cable
capacitance Cc and Ct act as a voltage divider, reducing the signal appearing
at
the other end of the cable. The cable capacitance is proportional to its
length, which means that for very long cables, the voltage signal appearing at
6

the end, which is fed into the preamplifier is very small. Also, each time the
cable length is changed, a new voltage sensitivity factor must be calculated,
since the voltage divider has new component values.
If, however, ·an amplifier which gives an
output voltage proportional to
input charge is used, it can be seen from Fig.1.1 a) that the cable capacitance
would have no effect on the sensitivity.
This,
then, is the difference between a charge amplifier and a voltage
amplifier. A voltage amplifier
is sensitive to variations in input voltage and
so
is sensitive to cable capacitance. A charge amplifier is sensitive to
variations in input charge, and since the capacitance across which the charge
appears does
not alter the charge, the charge amplifier output is
independent of the cable tapacitance.
1.3. PRINCIPLE OF THE CHARGE AMPLIFIER
Basically, a charge amplifier consists of a high gain operational amplifier
with a feedback capacitor, Ct, as in the equivalent circuit of Fig.1.2. It can
be shown
that the voltage output of this circuit is given by
OA
Fig.1.2. Equivalent circuit of charge amplifier
7

Since A (the open loop gain of the amplifier), is very large, then usually
so the output becomes
-Q
Hence if C1 remains constant, the output voltage is directly proportional
to the charge appearing across the accelerometer and the cable capacitance
Cc normally has a negligible effect. Only when Cc becomes comparable to
AC
1 will it
affect the gain.
8

2. CONTROLS
2.1. FRONT PANEL
Conditi on i ng Amp lifi er
8r U.I & klalr Type 2626
,~~0~
Fig.2. 1. View of front panel
The front panel of the preamplifier is as shown in Fig.2.1, and the
functioning of the various control knobs is as follows:
VOLT/gOUT:
Depending on the accelerometer sensivity,
the output sensitivity may be selected
between 1 mV/g -
10 V/g, as shown in the
ranges below.
9

SENSITIVITY pC/g:
FREQUENCY LIMIT:
Lower
FREQUENCY
LIMIT:
Upper
10
Maximum sensitivity
of 1 V/pC is available.
Minimum sensitivity of 0.1 mV /pC is avail-
able.
Ace. Sensitivity Output Sensitivity
1- 11 pC/g 0.001-0.01-0.1-1.0 V/g
10- 110 pC/g 0.01-0.1-1.0-10.0 V/9
100-1100 pC/g 0.1-1.0-10.0 V/g
A neon lamp will be lit indicating the
decimal point for the accelerometer sensi-
tivity, depending on the range chosen.
The three knobs
permit conditioning of
accelerometer charge sensitivity within the
range 1.00-1099 pC/g. The decimal point
is indicated by one of the three neon lamps
placed above the sensitivity knobs.
For impulse measurements the signal
can
be attenuated (if required) by selecting
higher accelerometer sensitivity positions
than the actual sensitivity
of the accelero-
meter used.
On both DIRECT and TRANSFORMER
outputs, the choice of lower limiting fre-
quencies (3 dB down)
are 0.3, 3, 10, and
30 Hz. These figures correspond to lower
limiting frequencies 5% down at 1, 10, 30,
and 100 Hz. The cut-off slope is 6 dB/
octave.
The upper
limiting frequencies (3 dB
down)
can be selected as 1, 3, 10, 30kHz
and linear, corresponding to 0.3 1, 3, 10,
and 30kHz maximum 5% down. Cut-off
slope 12 dB/octave.

RESET:
OVERLOAD:
-20 dB:
INPUT:
OUTPUT:
The reset button restores the preamplifier
to normal working conditions after over-
load. This
is especially useful when the pre-
amplifier
is used for low f ~ Efquency shock
measurements.
It should be noted that for normal impulse
overload, the recovery
time is negligible.
The neon indicator lights up when the
signal is greater than 10 Volt peak.
The neon
indicator li .ghts up only when the
signal is above 1 V peak.
Miniature coaxial socket
for connection of
accelerometer cable. Plug type JP 0012.
Miniature coaxial socket for connection of
cable to analyzing equipment.
Although in the direct
output mode the
connection
is not DC blocked, the DC off-
set is negligible. Typical 1-2 mV. Max.
± 10 mV.
In the transformer output mode, the out-
put is floating.
11

2.2. REAR PANEL
Fig.2.2. View of rear panel
Fig.2.2 illustrates the sockets on the rear panel.
POWER:
INPUT:
OUTPUT:
12
This switch connects power to the instru-
ment. Any line voltage between 90 and
265 Volts AC can be used without adjust-
ment. Maximum consumption is approx.
7 Watts.
An alternative input connected in parallel
with the input socket on the front panel,
plug
JP 0012.
BNC output socket, connected in parallel
to front panel output. For many purposes
it could be more convenient to use than
the miniature coaxial output.

3. OPERATION
The main purpose of the 2626 Conditioning Amplifier is to convert a
high impedance signal from an accelerometer
to a low impedance signal
scaled
to suitable sensitivity for use with subsequent instrumentation. Exact
information
about the vibration at the point of measurement can then be
obtained provided
the following conditions are fulfilled:
1. The accelerometer sensitivity must be known over the applicable fre-
quency range. Such information is generally supplied, otherwise the
accelerometer must be calibrated.
2. The accelerometer must be fixed to the vibrating specimen in such a
way
that the frequency response curve is not altered. This is partic-
ularly important where frequencies higher than about 2000Hz are
concerned.
3. The mass
of the accelerometer must not significantly affect the
vibration at the measuring point.
4. The conditioning attenuators of
the Preamplifier must be set to the
correct accelerometer sensitivity. The output sensitivity should be
chosen
to be as high as possible (without overloading), to get best S/N
performance.
3.1. INPUT
The input is single-ended with microplug connections in parallel at the
front and rear of the instrument. Whenever the front panel input is used, the
rear input should be protected by the small extension nut, and vice versa, in
order
to eliminate hum pick-up.
For low frequency measurements it is important to keep the cable con-
nections clean,
both at the accelerometer and at the preamplifier end, in
order to preserve the high input impedance of the amplifier.
13

Mininoise coaxial accelerometer cable is available in any length up to
200m (600ft) from Bruel & Kj<Er. There are two types, Type AC 0010
(PVC insulated) for ordinary temperatures, and Type AC 0005 (Teflon) for
a wide temperature range (-90 to+ 250°C). The appropriate microplugs are
also available,
with tools and instructions for mounting in the kit UA 0129.
See Accessories.
3.2. OUTPUT
If the output is taken via a miniature cable, an adapter may be necessary
to feed the output signal to the next instrument. An alternative BNC output
socket is available for connection to the input of the analyzing and
indicating
instrumentation. In the DIRECT output mode, the signal could
be off-set
by max.± 10 mV. When the preamplifier is used in a servo-loop in
exciter systems,
the TRANSFORMER output mode may advantageously be
used since
the output is floating. Problems arising from ground loops can
thus be avoided. Care should, however, be taken with this output below
40Hz and above 10kHz. Below 40Hz the available output amplitude is
limited by saturation of the transformer iron at a rate of 6 dB/octave.
Above
10kHz large capacitive loads may introduce peaking effects.
3.3. POWER REQUIREMENTS
The preamplifier can be operated from any line voltage between 90 and
265 Volts AC, without adjustments required for the instrument. Maximum
power
consumption is approximately 7 Watts.
3.4. OVERLOAD RESET
The use of this button restores immediately normal operating conditions
after overload when the preamplifier is used in low frequency modes for
impulse (shock) measurements.
3.5. USE IN SERVO LOOPS OF EXCITER CONTROL SYSTEMS
On account of quick recovery time the preamplifier can be used in servo
loops in exciter systems. It should
be noted, however, that it is dangerous to
unplug loads or alter switch positions during a vibration test, as sudden
changes
in levels could occur at the exciter table resulting in damage to the
specimen or the table itself.
14

3.6. LOW FREQUENCY MEASUREMENTS
Special care should be taken when low frequency measurements are
carried out. Piezoelectric ceramic accelerometers on account of pyroelectric
effects will yield a low frequency output when subjected to temperature
variations. Since the frequency response
of the Preamplifier can go down to
0.3 Hz, these low frequency variations can be seen on the output of the
amplifier. To eliminate this effect, the position of the selector giving the
highest possible
low frequency cut-off should be used as a general rule. If
wide temperature fluctuations are encountered, Quartz Ace. Type 8304
should be used.
3.7. HUM & NOISE REDUCTION
For correct operation of the instrumentation, the effects of hum and
noise fields should be cut down to a minimum. The following remarks may
help as a useful guide in reducing the effects.
Avoid earth loops through which serious hum problems could arise. Make
the screen of the output cable the only connection between the preamplifier
and the following instruments. If the main transformer in a power amplifier
has considerable capacitance between its windings, it is best to ground the
amplifier, making this the one and only earth point in the system. Since
most accelerometers have their casing as one connection, they must be
isolated from the test object if it is grounded by a metal structure. If the
transformer
output of the Preamplifier Type 2626 is used, this is not neces-
sary. For minimizing wind-induced noise, an efficient draught shield can be
of great help, even in an apparently still room. If necessary an accelerometer
of higher charge sensitivity may be used.
3.8. EVALUATION OF NOISE REFERRED TO INPUT
Noise in a charge amplifier is dependent upon active input devices and
input source capacitance C5 (consisting of transducer capacitance and cable
capacitance). For evaluation of the noise the feedback capacitance Cf of the
amplifier and input noise must be known. As the Conditioning Amplifier
Type 2626 has a Field Effect Transistor input stage, only the voltage noise
en has to be considered for an approx. calcuI at ion of wide band noise,
Fig.3.1.
15

Fig.3.1. Input stage of amplifier
The charge amplifier output noise can be calculated from
- (Cs + Cf)
- en cf
If the noise is referred to input source the expression should be divided
by the signal gain
~- cs
es -c;-
. (Cs + Ct) h . h . f d .
1.e. ens = enc w ere ens IS t e no1se re erre to mput.
s
The noise charge qns referred to input is therefore
The value
of en for the Preamplifier Type 2626 is max. 5 pV RMS in a
100 kHz frequency band. The feedback capacitance Cf which is dependent
on the position
of the output sensitivity switch is given in Table 3.1.
RANGE
V/g
1-
10 pC/g .001 .01 .1 1
10-
100 pC/g
.01 .1 1 10
100- 1000 pC/g .1 1 10
Ct (nF) 10 1 0.1
0.1
Table 3. 1. Feedback capacitance Cf as a function of control settings
I
16

To illustrate the evaluation of the noise referred to input the following
example
is included in which a piezoelectric accelerometer Type 4338 is
used.
Ace. Capacitance
Ace. Sensitivity
Ace. Cable Capacitance
Amplifier Output Sensitivity
Preamplifier Range
Noise charge referred
to input
1000 pF
100 pC/g
100 pF (1.2 meter cable length)
1 V/g
10-100 pC/g
= qns
=en (Cs + Cf)
=5 X 10-6 (1000 + 100 + 100) X 10-12
= 6 X 1 o-1 S C = 6 X 1 o-
3 pC.
Converting
input noise charge to acceleration for a sensitivity of
100 pC/g gives 6 x 10-
5 g
or 60 pg referred to input. With 1 V/g sens. this
is equal to 60 pV RMS at the output. From the above calculations it is
obvious that a combination of a high sensitivity accelerometer and use of
high gain in the amplifier will yield the best S/N ratio. With large cable
lengths, however,
the noise is to a great extent determined by the cable.
Furthermore the cable itself will produce considerable noise when exposed
to mechanical vibration (Tribo-electric effect). Therefore for low level
measurements low noise cables should be used and must be rigidly
supported between the accelerometer and the preamplifier.
17

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