TTI 1906 Service Manual page 9

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.
i.e.
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.
8
CTRL7 CTRL6
0 0
0 1
1 0
1 1
IC2 gate
0
1
2
3