General Overview & Principles Operation - TTI 1906 Service Manual

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.
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