Rate; Common-Mode Voltages; Connection; Guarding Information - HP 3455A Operating And Service Manual

Section Hl
frequency, and use of the Auto-Cal and High Resolu-
tion modes. When the Sample Rate buttons are pressed,
the display rate of the reading are changed. By depress-
ing the Decrease + 2 button on the front panel, the
display rate can be decreased. Each time this button is
pressed, the display rate is divided by two. The rate may
be divided a maximum of six times for a display rate of
1/64 of the maximum rate. The 3455A can be reset to
the maximum rate by depressing the maximum button,
after the display rate has been decreased. Table 3-1 gives
the maximum number of readings the instrument can
display on the front panel, in local operation.
Table 3-1. Maximum Front Panel Reading Rates.
Func High Auto Maximum Sample Rate
Function Resolution | Calibration | Maximum Sample Rate
DC Volts 3 readings/sec (60 Hz)
2.5 readings/sec {50 Hz)
5 readings/sec (60 Hz)
3.5 readings/sec {50 Hz)
6 readings/sec (60 Hz)
5 readings/sec (50 Hz)
24 readings/sec (60 Hz)
22 readings/sec (50 Hz)
2 readings/sec (60 Hz)
1.8 reading/sec (50 Hz)
45 readings/sec (60 Hz}
4 readings/sec (50 Hz}
3 readings/sec (60 Hz)
2.5 readings/sec (50 Hz)
12 readings/sec (60 Hz}
11 readings/sec (50 Hz)
1.3 readings/sec (60 Hz)
1.1 readings/sec (50 Hz)
1.3 readings/sec (60 Hz}
1.1 readings/sec (50 Hz)
4.5 readings/sec (60 Hz)
3.5 readings/sec (50 Hz)
13 readings/sec {60 Hz)
12 readings/sec (50 Hz)
ON
OFF
Not
Applicable
Not
Applicable
Not
Applicable
Not
Applicable
3-38. Auto Range.
3-39. The AUTO RANGE feature of the 3455A can be
used to automatically uprange and downrange the in-
strument to the optimum range. This action takes place
when an input measurement is taken. Upranging is done
when the reading is 150% of full scale and downranging
at 14% of full scale. The Auto Range operation can be
observed by applying 1.4 volts to the input of the
3455A. The range selected by the instrument is the | V
range. When the input voltage exceeds 1.5 volts, the
3455A upranges to the 10 V range. When the input
voltage is decreased below 1.4 volts, the 1 V range is
again selected. The uprange points, the downrange'
points, and the accuracy of the instrument should be
kept in mind when making a measurement. Time-
3-6
Model 3455A
varient inputs may cause the 3455A to constantly
uprange and downrange. If this happens, manually set
the instrument to the higher range.
3-40. Measurement time may also change, when the in-
strument is in the Auto Range mode. If the instrument is
not on the optimum range, a reading is taken and the
3455A will either uprange or downrange. Another
reading is then taken and if the optimum range has been
found the reading will be displayed. If not, the instru-
ment continues to uprange or downrange. A reading is
taken on all intermediate non-optimum ranges until the
correct range is found. The measurement time on each
range should be added to the total measurement time.
3-41, GUARDING.
3-42. Common-Mode Voltagas.
3-43. Common-mode voltages are those which are
generated between the power line ground point of the
source and the LO input and power line ground point of
the 3455A. Currents caused by common-mode voltage
can be included in the measurement circuit, causing
measurement errors.
3-44. Guard Connection.
3-45, Figure 3-3 illustrates three methods of connecting
the 3455A Guard terminal to reduce errors caused by
common-mode voltages. In example A, Guard is at
nearly the same potential as the LO measurement ter-
minal so that currents caused by common-mode voltage
flows through Guard and not the measurement circuit.
In example B, the 3455A guard switch is closed connec-
ting guard to the LO input terminal. This allows
common-mode current to flow through lead resistance
Rp causing some measurement error. This connection
may be used if common-mode voltages are not expected
to be a problem. Example C is similar to A with the ex-
ception that connecting guard in this manner allows any
common-mode current generated between the source
low and powerline ground to flow in the measurement
circuit.
NOTE
The front panel quard pushbutton applies
only for front panel inputs. Be sure to wire
rear panel guard connections yourself, if-
using the rear panel input terminals.
3-46. Guarding Information.
3-47. More detailed information on purpose and
methods of guarding may be found in -hp- Application
Note No. 123, ''Floating Measurements and
Guarding''. This application note is available through
your nearest -hp- Sales and Service Office.