Fluke NetDAQ 2640A, Service Manual

Page: 75 / 260

Theory of Operation
Detailed Circuit Description
2
2-33
Stallion Chip 2-56.
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 based on the Mercury A/D Chip used in Fluke 45 and Hydra,
except it does not contain the A/D conversion function, that is now done using discrete
components 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 2-57.
Input protection is provided by series hold-off resistors A3R111, A3R110, A3R138 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 2-58.
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 3V Full Scale for measurement by the multi-slope A/D converter
circuitry. The scalings 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 3V 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 2-59.
For both the 2640A and 2645A, 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.