MMF M68 Series User manual

Operator’s Manual
Metra Mess- und Frequenztechnik Radebeul
Meissner Str. 58 - D-01445 Radebeul
Phone [phone] Fax [phone]
Email: [email protected] Internet: www.MMF.de
Charge
Amplifiers
M68
Series

Contents
1. Application 2
2. Function and Operation 3
2.1. Introduction 3
2.2. Power Supply 4
2.3. Inputs 6
2.4. Amplifier 9
2.5. Filters 10
2.6. Integrators 12
2.7. Rack Mounting Cases for Model M68R1 14
3. Technical Data 16
Appendix: Limited Warranty
Declaration of Conformity
“ICP” is a trade mark of PCB Piezotronics Inc.
Jan. 04 #162

1
Output socket (BNC)
Input socket (BNC)
Input selector switch
Screws for battery compartment
ICP constant current LED
>5 % level indicator
Overload indicator
Battery indicator
Low pass switch
High pass / integrator switch
Gain switch
Fig. 1: Front view of Model M68D1 with control elements

2
1. Application
The Signal Conditioners of M68 series are intended for
connection of piezoelectric acceleration, force or pres-
sure transducers. The input is suitable for sensors with
charge output as well as for ICP®compatible transduc-
ers or microphones.
By means of the M68 the sensor signal can be best
possibly adapted to the existing measuring equipment or
PC-based data acquisition systems. The Signal Condi-
tioners provide the following functions:
• Adaptation of the sensor signal and sensor supply
• Amplification
• High- and low-pass filtering (for example anti-
aliasing filter)
• Integration of the sensor signal, for instance, to
measure velocity or displacement.
Models M68D1 and M68D3 are housed in a rugged
aluminum case. Both models can be used in laboratory
as well as under field conditions. Model M68D1 may
also be operated from batteries. Model M68R1 has been
developed for multichannel measuring systems. It fits
into 19”-rack systems.

3
2. Function and Operation
2.1. Introduction
Fig. 2 shows the block diagram of Model M68 with its
most important functional groups.
U
Q/10
Q
U
Q/10
Q
Q1
Q2
ICP
supply
J1 (ICPon/off)
Low pass
Frequency
High
pass
0.1Hz
3Hz
V2 1. Int. 2. Int.V1
Gain a
v
d
Gain
Input
Overload
Output
ICP
+
Compa-
rator >5%
V=1
Compa-
rator
Fig. 2: Block diagram
Description
of the
Signal Path
Depending on the position of the input switch, the input
signal passes the impedance converter Q1 or Q2. If ICP
operation is selected, the signal is directly connected to
the amplifier. At ICP operation a constant current is fed
into the input socket to supply the sensor electronics.
The constant current source can be switched off by the
internal jumper J1, in case an AC voltage shall be con-
nected to the input.
The input circuit is followed by the first amplifier stage,
low pass and high pass filters. The low pass filter has 6
selectable limiting frequencies. The high pass filter has
a limiting frequency of 3 Hz, which can be bypassed by
the switch “HIGH PASS / INTEGRATOR”. In this case
the full bandwidth down to 0.1 Hz comes into effect.
The filters are followed by the second amplifier stage.
The divided gain before and after filtering provides
sufficient dynamic range, even for signal components
outside the filter range. At the same time a high signal-
to-noise ratio is achieved.
Before reaching the output driver the signal may pass
one or two integrating stages. The output is DC cou-
pled.
A control LED for the output modulation indicates an
output signal higher than 5 % of full-scale modulation.
An overload LED shows if the output signal exceeds

4
90 % of full-scale modulation. It also indicates overload
before the filter stages.
Models M68D1, M68D3, and M68R1 have identical
electronic circuits.
2.2. Power Supply
External
Supply The Signal Conditioners M68 are powered by an exter-
nal DC voltage
• Models M68D1 and M68D3 come with a mains
plug adapter for 115/230 VAC. The power supply
socket according to DIN 45323 is located at the
rear of the instruments. Any other voltage of 5 V to
15 V DC and about 300 mA (for M68D1) or 1 A
(for M68D3) may be connected to this socket. The
positive supply terminal is connected to center pin
(tip). The POWER ON/OFF switch is located at the
rear.
• The M68R1 also has its power supply connector at
the rear. It is a 4-pole frame connector type WAGO
232. The pin designation is shown in Fig. 3.
+ 5 .. 15 V
0 V
Signal ground
0 V
Fig. 3: Power supply socket of Model M68R1
The supply voltage is connected to the terminals “+
5 .. 15 V“ and “0 V”. A special plug with screwed
contacts for the rear socket is delivered together
with the instrument. This way the power supply can
be wired manually. In case you use the offered 19”-
mounting racks with internal power supply unit,
this connection is realized by the backplane (see
chapter 2.7). Model M68R1 has no power on/off
switch.

5
Battery
Operation
(M68D1)
Model M68D1 has a battery compartment for four
“AA” size batteries (type LR 6). It is opened by un-
screwing four plastic knobs and removing the cover.
The right polarity is shown on the battery holder. To
ensure long battery life it is recommended to use alka-
line batteries. Accumulators may be used as well. You
can operate the instrument on NiMH or NiCd. How-
ever, by reason of the lower voltage of accumulators,
the battery control will not work.
)
))
)Please take discharged batteries out of the instrument to
avoid damage by leakage. Also, remove the batteries if
the unit is not in use for a longer period.
Voltage
Control All models of M68 series have an LED “BAT O.K.”
indicating sufficient supply voltage. It lights up green,
as long as the voltage is above 5 V. It works for battery
operation as well as for external power supply.
Fault
Protection The instruments are protected against false polarization
and short-time excess voltage up to 60 V.
Grounding
Conception The inputs and outputs of the signal conditioners are
single ended, i.e. asymmetrical. In case an additional
signal ground connection is required, ground is avail-
able via a separate connector at the rear of the instru-
ments. For the Models M68D1 and M68D3 this con-
nector is a 4 mm banana jack. The signal ground of
Model M68R1 can be found at the 4-pole frame con-
nector. The case of the instruments is internally con-
nected to ground.
The power supply is separated from signal ground. In
some cases it may be of advantage to connect the minus
pole of the power supply to signal ground, to avoid
ground loops. For this purpose you can plug in the
4 mm jumper (delivered with the instruments) at the
rear of Models M68D1 and M68D3. At Model M68R1
the terminals of the power supply socket can be con-
nected by a wire.

6
2.3. Inputs
The Signal Conditioners M68 are designed for both
sensors with charge output and with integrated imped-
ance converters to ICP®standard as well. You can
switch from one to the other type of transducer by
means of the slide switch next to the input socket. Both
types use the same BNC input socket.
Charge
Mode Capacitive signal sources, usually piezoelectric sensors
with charge output, are connected to the charge input
(Q). The input is fed to an amplifier with capacitive
feedback. All M68 instruments have two input stages
for charge. In the position “Q/10” of the switch the gain
is divided by 10.
The advantage of charge measurement is, that cable
capacitance and insulation resistance have almost no
influence to the measuring result. For sensors with
charge output it is strongly recommended to use special
low-noise cables. Ordinary cable will cause a consider-
able measuring error at mechanical stress, as a result of
the so-called triboelectrical effect. Cables with low
insulation resistance, for example caused by humid
connectors, reduce the accuracy of measurement at
lower frequencies. A desirable insulation resistance is
higher than 10 GΩ. Cables longer than 10 m are not
recommended at the charge input.
ICP


Mode The abbreviation ICP means “Integrated Circuit Pie-
zoelectric”. It has been established between many other
names as industrial standard for piezoelectric trans-
ducers. The integrated sensor circuit transforms the
charge signal of the piezo-ceramics, with its very high
impedance and high EMI sensitivity, into a voltage
signal with low impedance. The converted signal can be
easier transmitted. The cable length at this input may be
more than one hundred meters. Ordinary low cost co-
axial cable can be used.
A peculiarity of ICP®is, that power supply and meas-
uring signal use the same line. So, an ICP®transducer
needs, like a transducer with charge output, only one
single-ended line.
Fig. 4 shows the circuit diagram. To separate the low
impedance sensor signal from the power supply, the
integrated circuit is supplied with constant current.

7
This constant current must be fed into the measuring
line and simultaneously separated from the following
amplifier stages. The yellow LED “ICP ON” indicates
the flow of constant current.
Integrated amplifier
U
Iconst
s
QU
Piezo
ceramics
CCRI
CC
Iconst
RI
Coupling capacitor
Constant supply current
Input resistance
UsSupply voltage of
constant current source
coaxial cable,
> 100 m
ICP Transducer Signal Conditioner
Fig. 4: ICP principle
By supplying the sensor with constant current a positive
DC voltage arises over its terminals. This static bias
voltage depends on manufacturer and specimen and
amounts to about 5 through 14 V. The sensor signal is
superposed on this bias voltage. The output voltage of
the transducer never changes to negative values. Its
minimum value is the saturation voltage of the inte-
grated impedance converter (0.5 V to 1 V). The supply
voltage of the constant current source determines the
maximum value of the output voltage. For the M68 this
voltage amounts up to 24 V and guarantees an optimum
dynamic range for all available sensors. Fig. 5 shows
these relations.

8
Max. output voltage =
supply voltage of
constant current source
Min. output voltage =
saturation voltage
(see data sheet)
Sensor bias voltage
(see data sheet)
negative overload
0V
positive overload
24 V
0.5..1 V
5 .. 14 V
Fig. 5: Dynamic range of ICPcompatible transducers
Switching Off
the ICP Supply In some cases it may be necessary to switch off the
constant current supply, in order to use the input for
normal AC sources. For this purpose, please change the
position of the jumper, which you will find at Models
M68D1 and M68R1 behind the front panel. Remove the
cover of Model M68D1 by unscrewing the four plastic
screw heads. To remove the cover of Model M68R1,
four screws at the side and two at the back are un-
screwed. Jumper J1 is located left at the front side of
the printed circuit board. Plug it into the position “OFF”
to switch off the constant current source.
After removing the front cover of Model M68D3 you
will see only the jumper of channel 1. To reach the
jumpers of channels 2 and 3, please remove front and
rear panel.
Avoiding
Ground
Loops
Earthing or ground loops are often the reason for meas-
uring errors in multichannel measuring systems. In most
cases you will find a superimposed 50 Hz or 100 Hz
voltage on the measuring signal. One reason for this
effect may be, that the transducers are connected to
ground not only via their cable at the signal conditioner,
but also in addition at the measuring point through their
case. Vibration transducers are often mounted at

9
grounded machine parts. Within earthing systems tran-
sient currents may appear. These transient currents
cause a potential drop across the earthing or grounding
wires. Via the signal input of the amplifier they may
result in a considerable measuring error.
To avoid this, insulated attachment of the transducers is
recommended.
Metra offers several industrial vibration transducers
with insulated mounting base and different insulating
flanges for non-insulated sensors.
A star-shaped grounding network is the ideal solution to
avoid ground loops. Star-shaped means that all
grounding wires of the sensors and the amplifier outputs
are tied to ground at the signal conditioners, without
any transverse connections. In many cases this is more
difficult to realize for the outputs than for the inputs,
because the following measuring equipment may have
single-ended, inputs. If you have the choice to use dif-
ferential inputs, which can be found on many data ac-
quisition boards, you should preferably use them.
2.4. Amplifier
The instruments of M68 Series have the following
measuring ranges:
Charge mode: 0.1 / 1 / 10 / 100 / 1000 mV/pC
ICP mode: 1 / 10 / 100 / 1000 times
The gain selection switch ”GAIN” has four positions. In
position “Q/10” of the input selection slide switch the
measuring range of all charge ranges is divided by 10.
This may be advantageous for measurement with high
sensitivity transducers or for shock measurement.
)
))
)After connecting a sensor and occasionally after
changing the measuring range, the amplifier needs a
certain settling time because of a short term overload.
Therefore it may take about 30 s, until the output volt-
age is free of DC components.

10
Output The amplifier output is buffered and DC-coupled.
Therefore, possible offset currents fed into the amplifier
output by the following equipment (for instance a PC
data acquisition board), do not cause a DC offset.
Minimum
Modulation and
Overload
Control
LEDs indicate minimum modulation and overload
condition.
The LED “>5%” lights up at an output voltage higher
than 0.7 V. The LED “OVL” lights up if the output
voltage exceeds 9 V.
The optimum gain range is selected, if the LED “>5%”
lights up and the LED “OVL” remains dark. If both
LEDs remain dark, the gain should be increased. If both
LEDs light up the gain should be reduced.
The overload detector monitors both the amplifier
output and the filter input (see Fig. 2). By that means
overload condition will also be indicated when high
signal components beyond the filter pass band occur.
)
))
)An overload detector at the integrator input is not pro-
vided. In some cases high level components at higher
frequencies may overload the amplifier stage before the
integrator while at the integrator output no overload
condition can be detected. To avoid this, make sure to
check the signal level in the switch position „ACC“
(integrator off) before switching on the integrators.
When the LED indicates overload you can use the low
pass filter to attenuate high frequencies.
2.5. Filters
Low Pass To eliminate disturbing noise or to comply with the
Shannon theorem: “Signal frequency should be less than
half of the sampling frequency”, it can be advantageous,
to use a low pass filter. For higher accuracy in the time
domain it is recommended to set the low pass at 110 the
sampling frequency.
The instruments of M68 series have 6 internal low pass
filters. The scale at the positions of the filter switch
“LOW PASS” is shows the 3 dB limiting frequencies in
kHz. The following table shows the 3 dB and the 10 %
limiting frequencies of the low pass filters:

11
3 dB Frequency
100 Hz
300 Hz
1 kHz
10 kHz
20 kHz
50 kHz
10 % Frequency
70 Hz
200 Hz
700 Hz
7 kHz
14 kHz
35 kHz
The slope of the low pass filters is 40 dB per decade.
180°
-180°
-90°
90°
0
0,1 1 10 100
kHz
0 dB
-20 dB
-40 dB
0,1 1 10 100
kHz
1 kHz
1 kHz
10 kHz
10 kHz 20 kHz
50 kHz
20 kHz
50 kHz
180°
-180°
-90°
90°
0
0 dB
-20 dB
-40 dB
10 100 1000 10000
Hz 10 100 1000 10000
Hz
100 Hz
300 Hz
100 Hz
300 Hz
Fig. 6: Typical frequency and phase response of the low pass filters
High Pass The M68 signal conditioners have a high pass filter with
a lower limiting frequency of 3 Hz (-3 dB). By means of
this filter low frequency noise can be removed. Low
frequency noise may occur, for example, by the influ-
ence of temperature transients to piezoelectric compres-
sion type accelerometers. The slope of the 3 Hz high
pass filter is 40 dB / frequency decade (Fig. 7). The
3 Hz high-pass filter is switched on by turning the
“INTEGRATOR HIGH PASS” switch into position
“ACC 3 Hz”. If the high pass filter is switched off
(switch position “0.1 Hz ACC”), the lower limiting
frequency of the amplifier is 0.1 Hz.
)
))
)When using the integrators, the 3 Hz high pass filter is
switched on in any case.

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0 dB
-20 dB
-40 dB
0,1 1 10 100
Hz
180°
-180°
-90°
90°
0
0,1 1 10 100
Hz
Fig. 7: Frequency and phase response of the high pass filter
As the filters are located between the amplifier stages
(see Fig. 2) the instrument does not become overloaded,
even if the measured signal has higher spectral compo-
nents outside the filter range.
2.6. Integrators
The M68 can integrate the measuring signal one or two
times. Integration can be useful for vibration measure-
ment by means of accelerometers on rotating machin-
ery.
Single integration of vibration acceleration results in
velocity, double integration in displacement.
The „INTEGRATOR HIGH PASS“ switch activates the
integrators. In position “ACC“ (acceleration) the meas-
uring signal passes without integration. At the position
“VEL” (velocity) the signal is integrated once, in posi-
tion “DISP” (displacement) twice.
With switched on integrators the 3 Hz high pass filter is
always activated.

13
Relation
Between
Output Voltage
and Vibration
Quantity
The following calculations show how the output of the
M68 (uout) corresponds to the three vibration quantities.
The selected gain range of the M68 is G and the accel-
erometer sensitivity (see transducer data sheet) is Bua.
Vibration acceleration a (without integration):
au
GB
out
ua
=•
(a in m/s²; uout in mV; G in mV/mV; Bua in mV/ms-2)
Vibration velocity v (single integration):
vu
GB s
out
ua
=••10
(v in mm/s; uout in mV; G in mV/mV; Bua in mV/ms-2)
Vibration displacement d (double integration):
du
GB s
out
ua
=••100 2
(d in µm; uout in mV; G in mV/mV; Bua in mV/ms-2)
The equations above apply for ICPcompatible acceler-
ometers. For accelerometers with charge output, G is re-
placed by the selected charge amplifier range in pC/g and Bua
is replaced by the transducer’s charge sensitivity Bqa.
Example Vibration velocity is measured using an accelerometer with a
sensitivity of Bqa= 5 pC/ms-2. The M68 is operated in the
range G=100 mV/pC. Its output voltage is 300 mVrms. What
is the corresponding vibration velocity?
Solution: vmV
mV pC pC ms smms
rms
=••=
--
300
100 5 10 6
21
// /
Normalization
of the Output
Signal
Often a direct connection between the output voltage of
the M68 and the measured physical quantity (for exam-
ple “1 mV corresponds to 1 mm/s”) is desired.
This can be achieved by adjusting the connected meas-
uring equipment or by typing in a correction factor in a
PC based data acquisition system. In the example given
above this correction factor would be 0.02.
Dynamic
Range Over
Frequency
At higher frequencies the output voltage will have only
small amplitudes after integration. The dynamic range
and the signal-to-noise ratio therefore become lower in
the kHz-range (Fig. 8).

14
100
10
1
%
dB
Hz
Double integration
Single integration
Modulation limits
Max.
modulation
0.1 1 10 100 1000
20
0
-20
-40
Fig. 8: Frequency response of the integrators
180°
-180°
0
90°
-90°
0.1 1 10 100 1000
Fig. 9: Phase response of the integrators
Overload at
Integrator
Input
In some cases signal components with high frequency
and magnitude may overload the amplifier although no
overload can be detected at the M68 output. The over-
load LED remains dark. This can occur due to the at-
tenuation of higher frequencies by the integrator
(compare Fig. 8). To avoid possible overload, make
sure to check the signal level in the switch position
„ACC“ (integrator off) before using the integrator. If an
overload condition should be indicated, reduce high
frequency components by an appropriate low pass
frequency.

15
2.7. Rack Mounting Cases for Model M68R1
For the 19” unit M68R1 the following rack mounting
cases are available:
Model
M68A6
M68A12
M68B6
M68B12
Channels
6
12
6
12
Built-in power supply
no
no
yes
yes
Fig. 10: Rack case M68A6 for 6 channels
The rack mounting cases without internal power sup-
plies Model M68A6 and M68A12 have an open rear
side (see Fig. 11). The rear terminals of the modules
must be wired manually to an external DC power sup-
ply.
Fig. 11: Rear view of case Model M68A6

16
The rack cases with internal power supply Models
M68B6 and M68B12 supply the plugged-in modules
via a backplane. They can be operated with both 115
VAC and 230 VAC without changing any settings.
Replacing
the fuse The fuse holder of the rack cases M68B6 and M68B12
with mains power supply is located inside the mains
socket at the rear. It can be pulled out using a screw
driver. The fuse facing to the back of the drawer is a
spare fuse. The rear one is the mains fuse.
)
))
)Important: Unplug the device from the mains voltage
before replacing the fuse.
Make sure that the fuse to be replaced has the rating
T 800 mA.
3. Technical Data
Measuring inputs Charge and ICPcompatible, RI > 5 MΩ
BNC socket, single-ended
ICPsensor supply 3.8 .. 5.6 mA constant current,
compliance voltage 24 V,
switched off by internal jumper,
LED indicator
Gain 0.1 / 1 / 10 / 100 / 1000 mV/pC (charge)
1 / 10 / 100 / 1000 (ICP)
Accuracy ±1 % typical
±2 % maximum
Low pass filter (-3 dB) 0.1 / 0.3 / 1 / 10 / 20 / 50 kHz, two poles, 40 dB/decade
High pass filter (-3 dB) 3 Hz, two poles, 40 dB/decade, can be switched off
Frequency range
of integrators Single integration: 3 .. 1000 Hz
Double integration: 3 .. 100 Hz
Output ±10 VPEAK, DC coupled, DC offset < 10 mV,
ROUT = 50 Ω, BNC socket, single-ended
Cross-talk attenuation > 60 dB (M68D3 at 1 kHz / V=1000)
Output noise < 10 mVRMS (50 kHz bandwidth),
< 6 mVRMS (20 kHz bandwidth)
LED indicators Minimum modulation: > 0.7 VPEAK
Overload: > 90 % of full-scale output
Battery: supply voltage > 5 V

17
External supply 5 .. 15 VDC
< 300 mA (M68D1, M68R1)
< 1 A (M68D3)
connector to DIN 45323 (M68D1 / M68D3)
4 pin frame connector (M68R1)
Battery supply
(only M68D1) 4 x “AA” size (LR6)
> 10 h lifetime with alkaline cells
Mains supply
(only M68B6 / M68B12) Wide range input 85 .. 264 VAC
Socket for IEC 320 mains cord
Grounding required
Power consumption: < 40 W
Fuse: 800 mA (slow) in mains socket
Mains plug adapter
(only M68D1 / M68D3) Wide range input 100 .. 240 VAC, 50 / 60 Hz
with two pole Euro plug
Output: 12 VDC / 0.5 A (M68D1) / 1 A (M68D3)
Warm-up time 15 minutes
Operating temperature -10 .. 50 °C, 95 % rel. humidity without condensation
Dimensions
(width x height x depth) 105 x 40 x 150 mm³ (M68D1)
105 x 90 x 140 mm³ (M68D3)
7 width units x 3 height units x 190 mm (M68R1)
Limited Warranty
Metra warrants for a period of
24 months
that its products will be free from defects in material or workmanship
and shall conform to the specifications current at the time of shipment.
The warranty period starts with the date of invoice.
The customer must provide the dated bill of sale as evidence.
The warranty period ends after 24 months.
Repairs do not extend the warranty period.
This limited warranty covers only defects which arise as a result
of normal use according to the instruction manual.
Metra’s responsibility under this warranty does not apply to any
improper or inadequate maintenance or modification
and operation outside the product’s specifications.
Shipment to Metra will be paid by the customer.
The repaired or replaced product will be sent back at Metra’s expense.

18
Declaration of Conformity
Products: Charge Amplifiers
Models: M68D1, M68D3, M68R1
It is hereby certified that
the above mentioned products
comply with the demands
pursuant to the following standards:
• EN 50081-1
• EN 50082-1
• EN 61000-3
• EN 60950
Responsible for this declaration is the producer
Metra Mess- und Frequenztechnik
Meißner Str. 58
D-01445 Radebeul
Declared by
Manfred Weber
Radebeul, 29th of May, 2001