72 Chapter 4 Assembly Level Troubleshooting
1 V and 10 V for the 100 V and 1000 V ranges, respectively. The 1 V and
10 V amplifier gains will then be used for these ranges.
The DC autocal constants are also generated by the Autocal circuitry in
conjunction with the DC ranging and DC amplifier circuitry.
AC Ranging, AC
Amplifier, AC to DC
Converter, and Autocal
The A/D converter also requires the same full-scale input voltage for
full-scale inputs on all AC volts and AC + DC volts ranges. In addition, the
inputs to the A/D converter must be DC volts and not AC volts. Both of
these requirements are met using the AC circuitry (AC ranging, AC amplifier,
and AC to DC converter). The AC and AC + DC volts to DC volts conversion
method used is True RMS. This method is used since the converter must
convert both DC and AC voltages.
The AC autocal constants are also generated by the Autocal circuitry in
conjunction with the AC ranging, AC amplifier, and AC to DC converter
circuitry.
The AC circuitry is also used for track/hold measurements. Although these
measurements are DC volts measurements, the AC circuitry is used due to
its higher bandwidth. Track/hold requires a higher bandwidth than the DC
circuitry provides.
Current Ranging The 3458A measures current by measuring the voltage across a shunt resistor.
This voltage and the shunt resistor value are then used to calculate the current.
The outguard circuitry calculates the correct current value.
The current ranging circuitry is composed of the shunt resistors for the
different current ranges and the switches to select the ranges. Depending on
the type of current measured (AC or DC current), the circuit is also used to
apply the measured voltage to the appropriate circuitry (AC or DC circuitry).
Ohms Current Source
and Autocal
Resistance measurements are made by applying a known current to the
unknown resistor and then measuring the voltage drop across it. From this
the correct resistance is then calculated by the outguard.
The ohms current source circuitry generates the different currents for
resistance measurement ranges. The DC voltage measurement across the
unknown resistance is made by the DC ranging and DC amplifier circuitry.
The ohms autocal constants are also generated by the Autocal circuitry in
conjunction with the ohms current source.
Reference The reference circuit serves two functions. It provides a very stable reference
voltage for the A/D converter and the Ohms current source. This voltage is
necessary for accurate analog-to-digital conversion and to generate accurate
currents for resistance measurements. The reference voltage is generated by
a diode selected at the factory for excellent long-term stability.
Analog-to-Digital (A/D)
Converter
The A/D converter converts the applied DC voltage to digital data. It uses a
multi-slope conversion process to convert analog voltages to digital data.
The digital data is sent to the outguard through the isolation logic circuitry.
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