268XA
Service Manual
2-18
Stallion Chip
The Stallion IC (A3U30) is a Fluke-designed 100-pin CMOS device that performs the
following functions under control of the A/D microprocessor (A3U5):
•
Input signal routing
•
Input signal conditioning
•
A/D buffer amplifier range switching
•
Frequency measurements
•
Active filtering of ac voltage measurements
The Stallion IC design is taken from the NetDAQ design and contains the A/D
conversion function using a multi-slope technique.
Two separate signal paths are used. One path is for the functions dcv/ohms/temperature,
and other path is used for ac voltages/frequency.
Input Protection
Input protection is provided by series hold-off resistors A3R111, A3R110, A3R138
(thermistor), and A3R132, and related transistor switches used as clamp devices.
Excessive voltages develop a current through the resistors that is sensed by the
corresponding transistor, which turns on to provide a signal path to ground. For example,
an excessive input on the LO SENSE line is sensed by A3R132 (100 k
Ω
, 3w) and
clamped to ground by A3Q17.
Input Signal Conditioning
Each analog input is conditioned and/or scaled to a dc voltage (3 volts or less) for input to
the buffer amplifier (A3U27, A3U28 and related devices), which scales the voltage to
approximately 3 V Full Scale for measurement by the multi-slope A/D converter
circuitry. The gains of the buffer amplifier are x1, x4.021, x10, and x32.168. Accuracy is
derived by software calibration constants.
AC volts signal conditioning consists of conversion of an ac level to a scaled and
corresponding dc level. The ac level is scaled by resistor network A3Z6 and switches
A3Q10 to A3Q16, and is processed by A3U29. Input protection is via A3Z6 and A3CR5.
DC voltages below 3 V can be applied directly to the Stallion IC, while higher dc input
voltages are scaled by A3Z7. Ohms inputs are converted to a dc voltage, and ac inputs are
first scaled then converted to a dc voltage. Noise rejection is provided by the A/D for dc
inputs and an active filter for ac inputs.
Function Relays
For both the PAI and FAI, the function relays A3K25, A3K26, and A3K27 route the
input signal to the correct measurement path. They are latching relays and switched when
a 6 ms pulse is applied to the set or reset coils. The A/D microprocessor (A3U5) controls
the relay drive pulses by putting a data word on the bus and latching it into F/F A3U10.
The drive pulses are sent by A3U10 to the appropriate coils.
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