BRUEL & KJAER 2624 User guide

Charge Amplifier
Type 2624
The charge ampI ifier enables operation with
accelerometers without attention to the length
of cable. The Type 2624 is fully transistorised
and of modular design. Frequency range ex-
tends down to 0. 003 Hz.
BRUEL& ~~AC.R
NA:.RUM, DENMARK. Teleph.: 80 05 00 . Cable: BRUKJA . Telex: 5316. COPENHAGEN
BB 2624. 0300067

Charge Amplifier
Type 2624
October 1968

Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3
1.1. General . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3
1.2. Charge Amplifier . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3
1.3. Principle of the Charge Amplifier . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4
1.4. Low Frequency Response . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5
2. Description . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6
2.1. Circuit . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6
2.2. Front Panel . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. . . . . . . . . . . . . 7
2.3. Rear Panel . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8
2.4. Module System . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8
2.5. Frequency Response . .... .................................. . . 10
2.6. Input Characteristics ...... . ......... . ......................... 10
2.7. Output Characteristics .. ...................................... 11
2.8. Sensitivity . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11
2.9. Harmonic Distortion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11
2.10. Noise ....................................................... 12
3. Operation .. .. .. .. ........................... ........ ... ~ .......... 14
3.1. Power Requirements
......................................... 14
3.2. Calibration Check ............................................ 14
3.3. Output Connection . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 14
3.4. Accelerometers .............................................. 14
3.5. Overload Reset .............. ... .................. ····- . .. · . . . . . . 14
3.6. Use in Feedback loop of Control System ...................... 14
3.7. Low Frequency Measurements ................................ 15
4. Accessories . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 16
5.
Specifications . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 18

1. Introduction
1.1. General
The combination of a charge amplifier and a piezoelectric accelerometer gives
a sensitivity
which is independent upon cable length within very wide limits.
This makes a
charge amplifier especially attractive in vibration systems where
different cables are used. The Charge Amplifier Type 2624 may be used with
cable lengths up to several thousand metres. A charge amplifier also gives
possibility of very low frequency measurements and in the case of Type 2624,
the
limit is 0.003 Hz in the least sensitive mode, this makes the preamplifier
ideal for measurements of shocks and long duration transients. The pre-
amplifier is provided with an overload indicator which lights up when the
dynamic range is exceeded and when the amplifier is blocked due to satura-
tion. A reset button is used
for recovery of normal working conditions after
overload.
The output may be scaled to 0.1, 1 or 10 mV/pC giving different low frequency
limits. See specifications.
Input and output sockets are microsockets 10-32 NF
and the
preamplifier is completely protected against overload and short
circuits
N.B. The Type 2624 Charge Amplifier is not recommended for use in the
feedback loop of a control system, due to the blocking action after overload.
1.2. Charge Amplifier
The Type 2624 Charge Amplifier has been designed for use with piezoelectric
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 the B & K piezoelectric accelerometers 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
Q=SqOC
c,
Fig. 1.1. a) Charge equivalent circuit.
Fig. 1.1 b) Voltage equivalent circuit.
3

be represented either as a capacitance C, across which a charge Sqa appears
(Fig. 1.1a), or as a capacitance C, in series with a voltage generator Sva
(Fig. 1.1 b) where
Sq = charge sensitivity (pC/ g)
Sv = voltage sensitivity (mV/ g)
a = acceleration (g)
From
the voltage equivalent circuit it can be seen that the cable capacitance
Cc, and C, 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 the end, and
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 ampli-
fier. 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 capacitance.
1.3. Principle of the Charge Amplifier
Basically, a charge
amplifier consists of a high gain operational amplifier
with a feedback capacitor , C ~, as in the equivalent circuit of Fig. 1. 2.
It can be shown that the voltage output of this circuit is given by
Vo = QA
C, + Cc+ Ct (1 -A)
Vo
Fig. 1.2. Equivalent circuit of charge amplifier .
4

•
Since A is very large, i.e. the open loop gain of the amplifier, then usually
i Ct + Cc I ~ I cf (1 -A) I
so the output becomes Q
Vo=-
Cf
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 AC1
will it affect the gain.
1.4. Low Frequency Response
The 2624 is designed primarily as an AC instrument, but it does possess a
frequency response down to approximately DC. However, low frequency
measurements down to the limits of the amplifier's frequency response (0.003
Hz) require special attention. The problems involved are discussed in Sec-
tion 3.7.
5

2. Description
2.1. Circuit
A block diagram of the circuit is given in Fig. 2.1.
Input Input
Circuit
Feedback
L-----------,.--ca_,P. citor
c,
ClassB
Output 1-----...,......---G
Overload
Indicator
Fig. 2.1. Block diagram of preamplifier .
The input circuit has a field effect transistor, which gives the Type 2624 a
high input
impedance of over 10 GQ.
The operational
amplifier is followed by a class 8 output stage which delivers
the relatively high capacitive current required by the feedback capacitors at
high frequencies. A
feedback resistor, R ,, is necessary for stabilization of the
DC level of the amplifier , and this gives a lower limiting frequency of
2 n
~ ~ R1
The normal feedback resistor is 5 GQ giving the lower limiting frequencies
shown at the upper switch positions. These frequencies are so low that it takes
a long
time for restoration of normal working conditions after an accidental
overload, so the 2624 is provided with a reset button which recharges the
capacitors restoring working conditions immediately.
If the very low frequency response is not required, the other three positions
of the sensitivity control are used, giving lower limiting frequencies 100 times
higher (R t = 50 MQ), as marked on the instrument.
The
output impedance is low , Jess than 1 Q. The output is not decoupled in-
ternally, so is
at about 12 .5 V positive. The overload indicator lights up for
both positive and negative overload.
6

•
2.2. Front panel
The front panel of the 2624 is shown in Fig. 2.2. The controls are as follows:
SENSITIVITY I LOWER LIMITING FREQUENCY.
This knob has 10 positions; the four unmarked positions earth the input.
There are three charge sensitivities available: 0.1, 1, and 10 mV/pC. The
upper three positions of the knob give lower limiting frequencies 100 times
lower than the corresponding lower positions of the knob. All the relevant
figures are marked on the front panel, and it can be seen that the frequency
response can extend down to 0.003 Hz.
Charge Amplifie r
Type2624
S&nsitivity rnV/pC
Lower Limiting Frequency
0.1 0.03Hz 10
0.003 Hz • 0.3 Hz
10
30Hz •
1
3Hz
0.1
0.3Hz
Bruel & Kje2r
Copenhogen
Overload Reset
-
\: ~
Fig. 2.2. Front panel. Fig. 2.3. Rear panel.

OVERLOAD
This lights up when the amplifier is negatively or positively overloaded.
RESET
Pressing this button recharges the capacitors after overload.
Whilst pressed, a
25 kQ resistor is connected directly across the input.
INPUT
Miniature coaxial socket for connection of accelerometer cable.
Plug Type JP 0012.
OUTPUT
Miniature coaxial socket for connection of cable to analysing equipment. This
connection is not DC blocked, and so is at about 12.5 V positive. This DC
voltage must be either balanced out or blocked by a capacitor, although the
input
to most other equipments is normally blocked by a capacitor .
2.3. Rear Panel
The rear panel of the instrument is shown in Fig. 2.3.
Power
28 V
The upper miniature coaxial socket takes the lead from the power supply,
with the inner conductor positive.
OUTPUT
An alternative output, connected in parallel with the output socket on the
front panel.
The three screws hold the top and side panels in position.
When the screws are removed, the panels can slide
off backwards.
2.4. Module System
The preamplifier is part of the B & K module system of instrumentation as
shown in Fig. 2.4.
This system makes rack mounting of instrument combina-
tions very convenient.
A
one tier system contains the required combination of modules mounted in
the metal case KK 0010. This again fits into the wooden case KA 0020, making
a
portable instrument system. Standard 19" rack mounting is simple, using the
rack mounting lugs which are screwed onto the case KK 0010.
A three tier system is made up of three metal cases KK 0010 containing the
required
modular instruments, all mounted in the metal case KK 0011 . This
assembly again fits into the wooden case KA 0020.
The modules are mounted in the metal cases KK 0010 and KK 0011 as follows :
8
1. Loosen the finger screw in the front plate of the module and slide out
the bottom plate.
2. Unscrew the feet from the bottom plate.
•

•
Fig. 2.4. Module system.
9

3. Using the same screws, fix the bottom plate inside the case KK 0010.
4. Slide the module on to the bottom plate and fix it with the finger screw.
Fig. 2.5. Mounting of module.
2.5. Frequency Response
The frequency characteristics of the 2624 are shown in Fig. 2.6, for all settings
of the sensitivity selector switch.
Capacitive input load causes a high frequency boost with a maximum peak at
frequencies
higher than 20 kHz. Under full capacitive load conditions the
increase in sensitivity at
20 kHz is less than 10 %.
6
dB
A
-2
-·
-6
I
-8
I
-10
0.001
--
a.
/
/
I
I
I
5 0.
01 '
a: 0.1mV/pC, 0.003Hz (-3dB)
b: 1mfpC . 0. 03 Hz (-3dB)
c: 10 rrtJ/pC , 0. 3
Hz (-3dl!l
,.....r-
/
b,
c. d.
/
/ /
I
I
5 0.1 '
d: 0.1rrtJ/pC. 0 .3
Hz (-3dl!l
e: 1mfpC, 3
Hz (-3dB)
f: 10rrl\(pC . 30
Hz (-3dB) c.f./
/
/
....... ......
r--..
e,.
f.
a. ...
/ /
/ /
I
I
5
10 ' 5 1000 ' 5 '
Hz 10k 5 ·100k
Fig. 2.6. Frequency characteristics of preamplifier for the various settings of
selector switch.
10
•

•
2.6. Input Characteristics
Maximum Capacitive Load: 1 ,u F, 0.1 ,u F, 10 nF, respectively for the three
sensitivities
(0.1, 1.0, 10 mV /pC) will cause less than 1 °/o reduction in sen-
sitivity.
Input DC Shunt Resistance:
~reater than 10 GQ.
With RESET button pressed,
25 kQ ± 10%.
2.7. Output Characteristics
Maximum voltage:
20 V peak to peak, at approx. 12.5 V DC.
Maximum current, ± 10 mA.
Output Impedance: Less than 1 Q with less than 1/4 full input load 3-5 Q
at full input load.
The
output is short-circuit protected.
The maximum current available limits the high frequency response when loaded
with long cables. A set of curves for 0.5% distortion and different cable
capacity are shown in Fig. 2. 7.
10
Output
Voltage
RMS
2
tO
OJ
0.01
""'
~
10
10'
"
""'
~
~
0
~
~
~
~
~
~
10'
"
~
~
0
10'pF
~
~
~
~
~
~
10'~
~
~
~
0
~
~
~
~
"""
~
. ~
~
~
~
~
10' 2 5 10'
Frequency (Hz)
Limit for 0.5
•t. disto rtion ,
no input capacitance,
5 •J. distortion, maximum
input capacitance.
Fig. 2.7. Limit of high frequency response due to capacitive loading .
2.8. Sensitivity
The three sensitivities
of 0.1, 1, 10 mV/pC are accurate to within 1% at 100Hz.
With an accelerometer charge sensitivity of 10 pC/g, the sensitivities of the
combination of
amplifier and accelerometer are 1, 10 and 100 mV/g, re-
spectively.
For
the effect of input capacitance on sensitivity, see Sec. 2.6, Input Charac-
teristics.
2.9.
Harmonic Distortion
Less than 0.5% at full output and no capacitive load, and less than 5% at
full output and full capacitive load, at frequencies below 20 kHz.
11

2.10. Noise
The noise level in the 2624 in the frequency range 20-40,000 Hz Is shown in
Figs. 2.8,
9, 10, as a 1/3 octave spectrum analysis. For each spectrum at the
three sensitivities, two lengths of cable are represented, connected between
accelerometer and amplifier.
Total noise voltages in the frequency range 20 Hz - 45 kHz with weighting net-
works,
A, B, C, or linear, appear at the right hand end of the scale.
-
OODDDDOODDODOOODODODDDDDODODDDODODODDOOOOODODOODO
M.o-lootO!otoel:~ ••
ai"mUi,-p~ ·
0.1 mv full scgle
1 ----- "" 0.01mV
:~
...., ___ _
'-'- -- - ~ • 0.001mV
~La~
w•. s,., __ _
_ .., _ _ _
~ .... --..,. .i__ 0 010
OP1123 10
67m cable
(6000pF) ·
10000
1. 2m cable ::J::::l=f::::
(105 pF) =F='f=F.
JDODO c/1.0000,\. cu..
IIU / :mi)A I CU..
' "
0. 1!1
Corresponding
g values for
' S. = 10 pC!g
0. 01!1
• " 0. 001g
Fig. 2.8. Spectrum of inherent noise in 0.1 mV/pC mode.
-
OOODOOOODODODDOOODDDOOODDDDOOOODOODOOOOOOODODOOOO
I•G:!!!J•• .... !10 • O. l:V"'- 10 0.01g
....._,.Oiotocc_l62.L ••
1 ---- - "" 0.01mV
..... __ _
:___DGB_____
...., __ __
'-'--- - • •0 .001mV
~""' ' sOda
w •. s,., __ _
_ .., __ _
..........,.,_ __ ..,. ,.1__ 0 0
10
OP1123 10
67m cable
(6000 pFl
10000
• Corr•&pondi ng
!I values for
• Ss,s 10 pC/g
• 0.001(}
1. 2m cabl~ ~i::::f: , ..
0.000111
(105 pF) =f:=l:::=
Fig. 2.9. Spectrum of inherent noise in 1 mV/pC mode.
12
•

.
oocoaooooooooooooooooooooooooooaooooooooooooooooo
"'~
1
., +so a
1
mV•q., ·o.01o
_.,.,_~ ....
ian~@~·
1 mV full scgle
( ----- "" 0.1mV
...... __ _
:~
'-'- ---~ • 0. 01mV
~~
w •. s,. __ _
_ .., __ _
~"-s-lo"' "_l__ 0 0 10
OP1123 10
67mcable
(6000 pF)
10000
• Corresponding
g
values for
• S.= 10 pC/g
0.001g
1.2m cableE ' " 0.0001 g
(105pF) =H=
20CIXI c/o 6CIOQOA I C ~
( I U/21'11)A I CU..
'- 121..1
Fig. 2.10. Spectrum of inherent noise in 10 mV!pC mode.
13

3. Operation
3.1. Power Requirements
The 2624 must be operated from an external 28 V supply, (28 V ± 10 %}.
Current drain is typically 17 rnA; 40 rnA with overload indicator lit; 150 rnA
max.
without output terminals shorted.
Power is supplied via the miniature coaxial socket on the rear panel, using
plug Type JP 0012.
3.2. Calibration Check
If an oscillator is connected to the input via a 1000 pF series capacitor, the
amplifier will work as a voltage amplifier with sensitivity in V /V equal to the
charge sensitivity in mV/pC.
3.3. Output Connection
Since the output must be taken via a miniature cable, an adaptor may be
necessary to feed
the output signal into the next instrument. See Accessories
section. It must be remembered that the output is directly connected , and is
thus at approximately + 12 .5 V. It may be necessary to block or balance out
this DC voltage. See Section 3.7.
3.4. Accelerometers
The B & K accelerometers Types 4332 - 33 - 34 - 35 - 36 - 40 are all con-
nected via the miniature cable and plugs Type JP 0012. An individual calibra-
tion chart is supplied with each accelerometer, giving the charge sens.itivity
Sq. (Sq = S.C,o ,, where s. is voltage sensitivity, C ,o, is total capacitance of
accelerometer, both including cable}. At an amplifier sensitivity of S (mV/pC)
the overall sensitivity to acceleration at the output of the amplifier is SSq mV/g
where Sq is in pC /g.
Many accelerometers of conventional design produce low frequency volt ages
when subjected to temperature variations. These will be measured by the 2624
when it is used in
one of the modes with an extended low frequency response.
See Section 3.7.
3.5. Overload Reset
The use of this button was explained in Section 2. It should be pressed after
an overload to restore operating conditions , when used in the low frequency
positions.
14

3.6. Use in Feedback Loop of Control System
The Type 2624 is not recommended for use in the feedback loop of an accelera-
tion control system, because of the blocking action in the event of an over-
load. After overload, on the lower frequency ranges, the amplifier is blocked
for a long time, or until the RESET button is pressed. This would mean that
no
feedback was applied during this time, which could be dangerous for the
vibration system.
3.7. Low Frequency Measurements
Several problems are likely to be encountered when very low frequency
measurements are attempted with the 2624.
The
first of these is not a problem with the amplifier but of the accelerometer.
Accelerometers of normal design will give a low frequency output when
subjected to temperature variations which cause variations in strain within
the accelerometer. These variations in strain, if transmitted to the piezoelectric
elements, will cause an output at the output terminals. Since the 2624 can have
a
frequency response down to 0.003 Hz, these low frequency variations will be
seen at the output
of the amplifier. They can be eliminated by using one of the
positions of the selector knob which gives a higher low frequency cut-off. As
a general rule, the position of the selector giving the highest possible low
frequency cut-off should be used.
The second problem is that
of the DC voltage on the output of the amplifier.
The
amplifier must be DC coupled if the low frequency response is to be pre-
served. Many measuring instruments, however, do have a capacitor input; the
B & K instruments
for example, give a lower cut-off frequency of 2 Hz. If a
capacitor is used to decouple the output of the 2624, the lower limiting fre-
quency will be 2 n ~ R ;n where C is the capacitance used and R ;n the input
resistance of the measuring instrument. ·
If the DC voltage (about 12.5 V positive) on
the output is balanced out,
problems of drift in the balancing voltage and in the amplifier power supply
may arise. These are not to be confused with drift due to temperature vari-
ations of the accelerometer. Nevertheless, whatever the origin of the low
frequency signal, the high frequency components on the output of the accelero-
meter will be correctly measured by the amplifier as long as it is not over-
loaded.
15

4. Accessories
The following accessories are available for accelerometers and preamplifiers .
16
UA 0125
Set of studs containing 10 isolated studs YS 0420, 10 steel
studs YQ 2960, 10 nuts YM 0414, 10 mica washers YO 0534,
UA 0142 1 set
of clamping magnet containing 5 permanent magnets
UA 0070 with isolated mounting.
UA 0129 Set
of 20 miniature plugs JP 0012 with tools and instruction
for mounting of the plugs on cable.
UA 0130 Set of 25 miniature plugs JP 0012.
AO 0037 1.2 m (4 ft.) of mininoise cable for operation to 100°C (212° F)
fitted with miniature plugs. Individually calibrated.
AO 0038 1.2 m (4 ft.) of mininoise cable for operation to 260°C (500° F)
fitted with miniature plugs. Individually calibrated.
AC 0010 Mininoise cable up to 600ft. in one length.
90 pF/m or 30 pF/ft., for operation to 100 °C (212° F).
AC 0005 Mininoise cable up to 600 ft. in one length.
90 pF/m or 30 pF/ft., for operation to 260 °C (500°F).
Fig.
4.1 . Photograph of UA 0129 a nd instruct ions tor mounting of the plug
JP 0012.

-outc>r surfa ce- of cen tr e- in su lation is graphit ec oa tea .
Cle an ~ xpose d_ i nsu la tio n t op j: P._ vent ·tealfjzge
17

5. Specifications
Sensitivity: 0.1 - 1 - 10 mV/pC ± 1 %at 100Hz.
Input Load: Maximum 1 p,F, 0.1 p,F, 10 nF respectively for the
3 sensitivities, to cause less than 1 %
reduction in
sensitivity.
Input Shunt Resistance: 10 GQ.
Frequency Range:
Output Voltage and
Current:
Output Impedance:
Harmonic Distortion:
18
25 kQ ± 10% while resetting.
Low frequency half power points (- 3 dB):
0.003, 0.03 and 0.3 Hz.
Alternatively 0.3, 3 and 30 Hz.
Tolerance ±
20%.
With negligible capacitive load at input, the frequency
ranges in the six positions are:
Sensitivity
-3 dB I Frequency Range
mV/pC point, Hz Hz
0.1 0.003 0.03-30,000 ± 1 %
1 0.03 0.3-30,000 ± 1 %
10
0.3 5-10,000 ± 5%
0.1 0.3 3-30,000 ± 1 %
1 3 30-30,000 ± 1 %
10
30
150-10,000 ± 5%
Capacitive input load causes a high frequency boost
with a maximum at above 20 kHz, which is less than
a
10% increase with full capacitive input load.
Max.
20 V peak to peak, 12.5 V DC.
± 10 rnA, short circuit protected.
< 1 Q at less than 1/4 full load.
3-5 Q at full input load.
< 0.5 %, full output, no capacitive load.
< 5%, full output, full capacitive load.
Both at frequencies <
20 kHz.