METRON QA-ES, User & Service Manual, Page: 33 / 55

The measuring device uses a 12-bit A/D converter from linear,
Ul. This circuit measures both RMS values, and positive and
negative peak voltage. Each of the signals is multiplexed into
the A/D converter via the U24 switch.
The sampling speed depends on the Processor Board's reading
speed. Schottky diode D7 protects the A/D converter from in-
coming negative signals.
7. Interface Towards Load Board and Processor Board
The measuring device and load resistances in the QA-ES are
isolated from the processor unit and the housing by a galvanic
shield. This protects the user if a fault in the QA-ES produces
ground currents during the measurements. This interface is
based upon opto-couplers and various mains transformers for
the user interface and the measuring device in the QA-ES.
Data is transferred digitally between the Sampling Unit and the
Processor Board in serial form via J3 to simplify the optical in-
terface. Opto-couplers U5, U6, U7 and U8 are used in transmit-
ting signals from the processor system to the Sampling Unit.
The Opto-coupler U9 transfers the measurement from the Sam-
pling Unit back to the processor system on the Processor Board.
The relays used in choosing load values on the Load Board are
also controlled via terminal J3 on the Sampling Unit. The com-
mands are then transmitted from the Sampling Unit to the Load
Board via terminal J2.
8. High Voltage Protection
Diodes D8-D15 are a protection device against high voltage sig-
nals entering the Sampling Unit.
6.3 Load Board
(Refer to QA-ES Load Board Component and Schematic Diagrams).
The Load Board is located inside the internal protection box in the
QA-ES. The card is fixed vertically inside the box. The unit in-
cludes a power supply, load resistances with corresponding control
relays, and interface towards the processor system Sampling Unit, a
measurement resistor with attenuators and measuring Transformer
for the scope output.
The Load Board forms the load for the ESU being tested. The load
can vary from 10 ohms to 5200 ohms, or be fixed to 200 ohms. A
high voltage relay inside the Load Board (K1) connects the load to
or from the ESU.
1. Power Supply
The Load Board is supplied with 2 x 9 VAC from the Trafo in
the front part of the protection box via J14. The voltage is con-
verted with diode D9, and filtered through Cl and C2. U1 is a
voltage regulator, providing the relay d5 V. U1 also
supplies Sampling Unit opto-couplers U10, U11 and U12 with 5
5