4 Assembly Level Troubleshooting
118 Keysight 3458A Assembly-Level Repair Manual
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.
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