Assembly Level Troubleshooting 4
Keysight 3458A Assembly-Level Repair Manual 117
Inguard section
The inguard circuitry consists of all the analog, measurement circuitry (DC
circuitry, AC converter, etc.), analog-to-digital (A/D) converter, inguard controller,
and power supply. The inguard circuitry performs all instrument measurements
and converts the analog measurement data into digital measurement data. This
digital measurement data is transferred to the outguard circuitry through the
isolation logic. The inguard circuitry is explained as follows.
Input switching and signal conditioning
This circuitry connects the signal path from the front/rear input terminals to the
appropriate DC, AC, or Ohms circuitry. The circuitry also provides the means for an
Autozero measurement. In an Autozero measurement, the input of the DC and AC
amplifiers is connected to ground for an offset measurement. This measurement is
stored in memory and subtracted from the subsequent measurement taken at the
front/rear terminals.
DC ranging, DC amplifier, and autocal
The A/D converter requires the same full-scale input voltage for full-scale inputs
on all ranges. For example, a 10 V full scale input is required for all full scale
inputs on the 0.1 V, 1 V, 100 V, and 1000 V ranges. This requirement is met by the
DC circuitry (DC amplifier in conjunction with the DC ranging circuitry). For the
lower 1 V and 0.1 V ranges, the circuitry amplifies the input voltage by a gain of
X10 and X100, respectively. For the 10 V range, the gain is 1. For the higher 100 V
and 1000 V ranges, the circuitry attenuates the inputs by 100. This results in
full-scale voltages of 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.
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