METRON QA-90, User & Service Manual

Page: 63 / 112

6-3
module comprising D1, D2 and C2150, C2151 and out to the measur-
ing object via a 102 kOhm series resistance. The series resistances
protect the voltage generators, the equipment under test, and users
against the high test voltages.
Schematic Diagrams E 160.20.2400.U3 (Applied Part Matrix) and
E 160.20.2500.U3 (Measurement Matrix) comprise relays that are set
up in accordance with the different measurements to be performed.
Diagram 5 includes relays that belong to measurements on the patient
leads. Diagram 6 includes relays for other measurement items, relays
for connecting an attenuator and for a filter (patient equivalent).
Schematic Diagram E160.20.2600.U3 (Prog. Amplifier and Lowpass)
is the measuring unit itself with a balanced measurement amplifier
B&B PGA202 CU2600) with variable gain controlled by µP via opt-
ical isolators. The measurement amplifier module is supplied by a
separate power supply, based on a DC-DC converter that pr
15 Volt. The SGND earth system for this power is isolated from GND
and the mains. OPA602 and R2616 are used to offset adjustment of
the measurement amplifier. Normally this is unnecessary, thus there is
a jumper between pins 1 and 2 on S2600 to avoid using the DC offset
function. U2610 is a filter that is operated by the S2420 relay during
"insulating resistance" measurements. Signals from the measurement
amplifier continue to an RMS to DC converter AD536AK, which in
turn generates a DC signal on a circuit for voltage to frequency con-
version (U2641) LM231WM. The output signal (A/D frequency) goes
to µP via optical isolators.
6.3 Microprocessor Sys-
tem
Refer to Drawing No. E160.20.1000. U1, containing six schematic
diagrams.
Refer to Schematic Diagrams 1 through 3 (QA-90; Integrated Key-
board QA-90; and CPU QA-90). The processor system is divided into
5 function blocks comprising a CPU, memory, display, printer inter-
face, serial and keypad interface, and keypad board.
The QA-90's CPU and keypad board are located behind the front pan-
el. The unit comprises a processor system, display, control compo-
nents, connection to the metering board, an RS-232 port, printer port,
and a port for the bar code pen.
The processor controls the measurement process in the QA-90. Meas-
ured analogue values are converted to an A/D frequency that is