TTI 1906 Manuale utente

1906

Multimeter

Service Manual

Part Number 48511-1110 Iss.2

Table of Contents

Specification 2

Safety 5

General Overview & Principles Operation 6

Detailed Circuit Operation 7

Calibration 19

Parts List 24

Circuit Diagrams 33

Specification

Accuracy

Accuracies apply for 1 year 18°C to 26°C. Temperature coefficient outside these limits is <0.1x quoted

range accuracy per °C. Warm-up time to rated accuracy is 1 hour.

DC VOLTS

Range Accuracy Input Impedance Max Input

200Mv 0.017% + 3D 10MΩor 1kV DC or

2V 0.012% + 3D >1GΩAC Peak,

20V 0.019% + 3D 10MΩany range

200V 0.019% + 3D

1000V 0.019% + 3D

RESISTANCE (OHMS)

Range Accuracy Max. Measuring Current Max Input

200 0.025% + 4D 1.5mA

2k 0.019% + 3D 1.5mA 300V DC or RMS

20k 0.019% + 3D 0.4mA continuous,

200k 0.019% + 3D 40uA any Ohms input,

2M 0.022% + 3D 4uA any range

20M 0.07% + 3D 400nA

Max open circuit voltage 3.5V.

Audible continuity check available on all ranges.

TRMS AC VOLTS

Range Accuracy

45Hz - 5kHz 5kHz - 10kHz 10kHz - 20kHz 20kHz - 50kHz

200mV 0.2% + 150D 0.2% + 250D 3% + 500D -

2V 0.2% + 150D 0.2% + 250D 0.5% + 300D 3% + 1500D

20V 0.2% + 150D 0.2% + 250D 0.5% + 300D 2% + 1000D

200V 0.2% + 150D 0.2% + 250D 0.5% + 500D -

750V 0.2% + 150D - - -

Additional error at crest factor=3 typically 0.7%

Input impedance 1MOhm//<100pF any range.

Max input 750V rms, 1kV peak, any range.

AMPS

Range Accuracy Voltage Burden Max Input

200uA 0.08% + 12D 1A,300V

2mA 0.08% + 12D 300mV max fuse protected

20mA 0.08% + 12D

200mA 0.08% + 12D

10A up to 2000mA 0.06% + 10D 650mV max 10A, 300V

above 2000mA 0.12% + 10 fuse protected

TRMS AC AMPS

Range Accuracy 45Hz - 1kHz Voltage Burden Max Input

200uA 0.37% + 100D 1A,300V

2mA 0.37% + 100D 300mV max fuse protected

20mA 0.37% + 100D

200mA 0.37% + 100D

10A up to 2000mA 0.34% + 100D 10A, 300V

above 2000mA 0.4 % + 100D 650mV max fuse protected

Additional error at crest factor=3 typically 0.7%.

DISPLAY

Display Type: 13mm LED, 8 digit.

Scale Length: Selectable 5½ digit or 4½ digit.

Reading Rate: 3 readings/sec (5½ digit).

10 readings/sec (4½ digit).

Overange Indication: Displays -OL- if input too great for range.

Overflow Indication: Displays OFLO if calculated result too large for display.

Annunciators: LED annunciators for range, function and program modes.

Input Characteristics

Input Current: <100pA.

DC NMR: >60dB at 50/60Hz.

1k Unbalanced CMR: >120dB at DC/50Hz/60Hz, DC ranges;

>60dB at DC/50Hz/60Hz, AC ranges;

Hi Z: Switchable high impedance (>1GOhm) on 200mV and 2V DC ranges.

COMPUTING FUNCTIONS

Null: Operates over full range; values can be stored for every function.

Digital Filter: 10 options, including optimised default values for each range and function.

T/HOLD

(Touch & Hold): Reading is held when stable.

% DEV: Displays % deviation from entered reference value.

dB: Displays measurement in dB relative to 1V, 1mA, 1kOhm or user entered

value, or in dBm.

Ax+B: Linear scaling of results, with offset.

Limits: Reading displayed with 'H','L',or'P' (pass) with respect to user-defined high

and low limits.

Min/Max: Minimum and maximum reading stored.

Data Logger: Manual or automatic storage of 100 measurements.

INTERFACES

RS232: Complies fully with the ARC (Addressable RS232 Chain) interface

standard. Address selectable from the front panel. Baud rates 9600, 1200

and 300

GPIB (Optional): Fully complies with IEEE-488.2

POWER REQUIREMENTS

AC Input: 110/120 Volts AC nominal 50/60Hz or 220/240 Volts AC nominal 50/60Hz

by rear panel adjustment. The instrument will operate safely and meet

specification within normal AC supply variations, viz. 99-132 Volts AC and

198-264 Volts AC respectively.

Power Consumption: 25VA max.

Safety

This multimeter DMM is a Safety Class I instrument by IEC classification; it has been designed to meet

the requirements of both IEC348 and IEC1010-1.

This multimeter has been designed for use in a Pollution Degree 1 environment in the temperature

range 5°C to 40°C, 20% - 80% RH (non-condensing). It has been designed for Installation Category

II measurement use to 600VDC/rms and Installation Category I measurement use to 1000V DC/rms.

General

Environmental

Operating Range: +5°C to +4°C, 20% to 80% RH

Environmental

Storage Range: –40°C to +60°C

Size: 260(W) x 88(H) x 235(D)mm, excluding handle and feet

Weight: 2.2kg.

Safety

When operating the instrument with the covers off the users attention is drawn to the fact that the

measurement circuit analogue common is capable of being driven up to 1kV above earth ground.

All measurement circuits are therefore potentially hazardous, including the CPU section of the

main board and the display/keyboard mounted behind the front panel. Instrument safety in normal

use with the cover on is dependent on the safety earth connections to the front and rear panels

and to the remote interface section of the main board.

It is therefore essential that these connections are only disturbed when absolutely necessary and

are then replaced correctly. In this case the safety tests at the end of this manual should be

carried out to ensure that the meter still meets the specified safety standard.

Mains Fuse

The mains input is protected by a fuse located in a drawer beneath the mains input socket. This

fuse should only be replaced with one of the type specified on the rear panel label. Note that the

fuse drawer orientation selects the mains voltage required to power the instrument.

Current Range Protection Fuses

The mA‚ ranges, measuring currents up to 200mA, are protected by a 1A, 20mm HRC fuse

mounted on the rear panel.

The 10A‚ range is protected by a 10A, 20mm HRC fuse mounted on the main board. To change

this fuse first remove the top cover of the instrument. The 10A fuse is designated FS2 and is

located near the input sockets.

Fig. 1 – Block Diagram

General Overview & Principles Operation

(refer to the block diagram fig.1)

The input signal conditioning includes the input attenuator, current shunts, and analogue switches

required for range switching. All analogue switches are either CMOS gates or MOSFETs. By

software calibrating every meter range high precision components are avoided, the only

requirement is stability.

The reference for voltage and current measurements consists of a reference diode with dividers

connected to analogue ground. Resistance measurements use a separate Ohms reference

voltage and are made using a ratiometric method so that the reference actually becomes the

voltage across the reference resistor.

The input multiplexer consists of analogue gates which feed the A/D converter with the signals

necessary to perform a complete meter reading . Each complete meter reading requires up to

four 'sub-readings', each with a different signal fed to the A/D converter for the duration of one

20ms or 100ms conversion period. The complete reading is calculated from the sub-readings

using the following generalised formula:-

reading = (input signal hi - input signal lo)/(ref hi - ref lo)

DC voltage readings require two sub-readings, one where the input attenuator's output is fed to

the A/D converter and one where the bottom of the attenuator is fed to the converter. As in all

voltage and current measurements reference hi and reference lo sub-readings are performed at

timed intervals once the meter function has been established.

The AC voltage and all current functions require only one sub-reading per displayed reading, the

reference lo sub-reading also being used as the signal lo sub-reading. This has the advantage of

speeding up the reading rate. For AC voltage and current measurements the signal hi sub-

reading is taken from the AC/DC converter output. For DC current measurements the signal HI

sub-reading is taken directly from the input current shunt. Resistance measurements require four

sub-readings.The details of the signal paths used for the sub-readings of each meter function are

given in the sections describing detailed circuit operation.

The buffer amplifier, IC40, which drives the A/D converter is switched between gains of x1 and

x10 according to the meter function and range.

The A/D converter is of the charge balance type controlled directly by the CPU. The counters for

the A/D converter reside within the CPU. The dividing line between A/D and CPU sections is

therefore more blurred than the diagram suggests.

The basic remote interface is an opto-isolated RS232, which is incorporated in each instrument.

The optional GPIB interface works via the RS232.

The display consists of multiplexed 7-segment LEDs. The front panel LEDs are also driven via the

main display multiplexing and the keyboard is scanned by the CPU using the display multiplexing

signals.

Detailed Circuit Operation

The following sections describe the detailed operation of the circuit blocks outlined in the general

overview. Refer to the main circuit diagrams where information is not presented on simplified

schematics.

Measurement Control Signals and Signal Drivers

The CPU is responsible for setting up all the measurement signal paths implemented using

CMOS analogue gates and MOSFETS. In addition the CPU controls the A/D conversion process

in real time.

The CPU controls all instrument functions using signals at two different levels. All control signal

names start with 'CTRL' followed by the signal number and if the signal swings nominally

between +5V and -10V it is suffixed with 'A'. 5V logic signals have no suffix. All +5/-10V level

control lines are translated from their logic level equivalents by the amplifiers in IC21, IC22, IC23

and IC24.

Note that the signals CTRL1A, CTRL2A and CTRL10A are used to switch relays RL1,RL2 and

RL3. All three relays are bistable latching types which require pulses of positive polarity to switch

one way and negative polarity to switch the other way. This is achieved by ac coupling the control

lines via C35, C36 and C37. No dc current flows through the relay coils so little power is

dissipated in them.

The supply to IC22 is raised a Vbe by D22. This is to guarantee that CTRL4A swings positive

enough to turn on MOSFET F3 which switches in the 200mA current shunt.

Fig.6 presents a complete list of control signals and their states for each meter function and

range.

Input Signal Conditioning‚ - Guard Tracks

All input sense paths which would be susceptible to board leakage are protected by a guard track

driven by IC20-D. The guard track is driven with a voltage which approximates to the mean buffer

input. The guard track can be identified on the PCB by the absence of solder resist over it.

Input Signal Conditioning‚ - DC Volts

(refer to the main circuit diagram sheet 1)

Each complete measurement requires two sub-readings representing the incoming signal HI,

either direct or attenuated, and the incoming signal LO connected to AG (Analogue Ground). The

displayed measurement is calculated from these two sub-readings and the regularly updated

reference sub-readings.

On the 200mV and 2V DC ranges the input voltage is passed to the buffer unattenuated via R93,

R1, RL1, R44 and IC4-D. RL1 is switched to pin 4 by a low going CTRL1A signal. Input

protection on this sense line is provided by Q5 and Q6 which limit the voltage to approximately ±

3.6V. The input multiplexer switches between IC4-D and IC4-B, the analogue common being

sensed, as for all DC Volts ranges, via IC4-B, IC7-B, IC6-C and F5. F7 is turned off by a low

CTRL12f or all DC Voltage measurements.

The input impedance on the 200mV and 2V ranges is determined by R78 and RP1-B. When LOZ

is selected these resistors are connected in parallel by IC8-B and IC1-A and the combination is

connected to the HI input terminal via RP1-A, RL3 and R11, giving an input impedance of 11 MΩ.

All other gates which could contribute to this impedance are turned off. When HIZ is selected IC8-

B and IC1-A are turned off so that the input impedance effectively becomes infinite.

On 20V DC ranges and above RL1 is switched to pin 5 to isolate this sense line from incoming

high voltages and the input multiplexing takes place between IC4-A and IC4-B. IC4-A senses the

output from the input attenuator via R46, R45 and IC5-A. IC5-B and IC5-D are turned off for all

DC voltage measurements. The two element DC input attenuator is formed by RP1-A in series

with RP1-C, RP1-D or RP1-E in the 20V, 200V and 1kV ranges respectively. The measurement

current is fed to the bottom element of the attenuator via IC1-A in the 20V range and via IC1-D

plus the appropriate gate in IC2 in the 200V and 1kV ranges. The voltage across the bottom of

the attenuator is sensed via IC1-C or IC1-B and IC2.

The gate selection in IC2 is controlled by CTRL6 and CTRL7, which select the gates according to

a binary code with CTRL6 the LSB.

CTRL7 CTRL6 IC2 gate

0 0 0

i.e. 0 1 1

1 0 2

1 1 3

The bottom of the attenuator is connected to AG by F5. The LO voltage is sensed at the drain of

F5 via IC6-C, IC7-B and IC4-B.

IC11-B and associated components form an active filter which is switched in on critical ranges via

IC5-E. The ranges where the filter is switched in are indicated in fig.6 where CTRL11A is

shown to be high.

Input Signal Conditioning - DC Current

(refer to main circuit diagram sheet 1)

DC current readings require only one sub-reading per complete reading. This is the sense

reading from the hi end of the current shunt. The lo end of the current shunts is connected to and

sensed via AG. This sub-reading is made when the reference readings are made at regular

intervals of a few seconds.

The DC current shunt sense voltage is connected to the A/D converter buffer input via IC4-D,

RL1, IC8-A and IC6-B, pin 2 of IC6 selecting the sense voltage up to 200mA and pin 1 selecting

the 10A sense.

The 200uA to 200mA current shunts are contained in RP2 and R74 is the 10A shunt. The current

is switched into the appropriate shunt in RP2 by MOSFETs F6, F1, F2 or F3. The 10A shunt is fed

from a separate input socket and so is not switched. The MOSFETs are turned on by high going

signals on their gates. The 10A input is fuse protected by FS2 and lower current ranges are

protected by FS1. V4 and V5 prevent high voltage transients from occurring if the current is fed

from an inductive source when the fuse blows; these could otherwise prevent the arc from

extinguishing within the fuse body which could therefore result in instrument damage. D1 to D5

prevent overvoltage from damaging the current switches and shunts and result in FS1 blowing

if sufficient current is drawn.

Input Signal Conditioning - AC/DC Converter

(refer to main circuit diagram sheet 1)

The AC/DC conversion is performed by true RMS converter IC15. The signal to be measured is

input to pin 2 and negative DC appears at pin 6. The input signal is AC coupled via C19 with

bootstrapped DC bias supplied by IC11-C and R32 for good LF performance. The source of the

signal is selected by

IC6-A, pin 13 of IC6 being the AC Volts input and pin 12 being the AC current input.

C12 is the converter's averaging capacitor which averages a logarithmic voltage at pin 5. R106 is

switched across the averaging capacitor by IC4-C for inputs below 1/200th of full scale to allow

the reading to reach zero for zero input.

The converter's output is switched into the A/D buffer via IC5-D and IC4-A with post filtering of the

converter's output performed by IC11-B and associated components.

Fig. 2 - AC Volts I/P - Simplified Schematic

Input Signal Conditioning‚ - AC Volts Attenuation‚

(refer to simplified schematic fig. 2)

The AC voltage readings require one sub-reading. The LO terminal is sensed via AG and is

measured during the regular reference updates. IC4-A is therefore switched on permanently

except during the reference sub-readings. The AC/DC converter works in the range 0-200mV

RMS input signal, so all AC inputs are first converted to this level. For AC Volts measurements the

multimeter uses an active attenuator centred around the amplifier IC51, configured as an

inverting amplifier with the virtual earth node, pin 3, biased via IC7-A at one Vbe above analogue

common. The signal is AC coupled into the amplifier's inverting input by C76 with DC stabilisation

provided by feedback resistors R115 and R116. D21 and D6 prevent large transients on the input

causing latch up.

The AC attenuator presents a constant input impedance on all ranges of nominally 990kOhms,

consisting of R75, R76 and R77. In the 200mV range the input resistors are connected to the

amplifier inverting input by IC8-B, IC8-D and IC5-B. IC1-D and IC1-B are turned off. R120 is the

200mV feedback resistor around IC51. The output from IC51 is divided by about ten at the

junction of R119 and R121 and the feedback is taken from this point. This gives a nominal overall

gain of one whilst maintaining a relatively low feedback resistance for better HF performance. It is

essential for HF performance also that F7 is turned off in the 200mV range as this isolates

spurious capacitive feedback through RP1 and associated gates. R117 provides current limiting

for IC6-A's input protection diodes if the output of IC51 goes more positive than +5V.

Indice

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