MMF M68 Series User manual

Operator’s Manual

Metra Mess- und Frequenztechnik Radebeul

Meissner Str. 58 - D-01445 Radebeul

Phone +49-351-836 2191 Fax +49-351 836 2940

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

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

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.

2. Function and Operation

2.1. Introduction

Fig. 2 shows the block diagram of Model M68 with its

most important functional groups.

Q/10

ICP

supply

J1 (ICPon/off)

Low pass

Frequency

High

pass

0.1Hz

3Hz

V2 1. Int. 2. Int.V1

Gain a

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

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.

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.

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.

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

Iconst

Piezo

ceramics

CCRI

Iconst

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.

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

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

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