Selecting The Number Of Displayed Digits; Selecting Measurement Rate - Keithley 776 Instruction Manual

indicating that Channel A is now ac coupled. Pressing
the [AC] push-button again turns the AC A light off but
AC B light turns on: indicating that Channel A is dc
coupled and Channel B is ac coupled. Pressing the
button once more. turns both AC A and AC B light on:
indicating that both channels arc now ac coupled.
Pressing the same push-button again turns both lights
off. This now indicates that both channels are set to dc
coupling mode.
3. Use the same procedure to set the slopes. attenuation.
impedance and filtering.
3-12. Selecting The Number Of
Displayed Digits
A major advantage of Model 776 over similar products
is its capability to display a fixed number of digils
regardless of the frequency of the input signal. For
example. with one second of gate time. the Model 776
displays a minimum of 10 digits. This however. may
turn into a disadvantage when measuring the frequent)
of a relatively unstable signal: in which just the most
significant digits are stable and the least significant
digits are "jumping around" with no significant mean-
ing. Model 776 permits truncation of the unstable
least
significant digits. while still preserving its full perform-
ance. To select the number of displayed digits proceed
as follows:
1. Press the [2nd] push-button. Observe that the instru-
ment prompts the following message:
2nd ?
2. Press the [DIGITS] push-button (second function to
]AC A]). The instrument now displays the following:
xx diGit
Where s is the selected number of digits and could range
from 03 to IO digits.
3. Use the [VERNIER fi] to increase the number of the
displayed digits. Conversely. press the [VERNIER u]
to decrease this number.
4. Pressing the [VERNIER ft] and [VERNIER u]
simultaneously. presets ss to IO: indicating that results
will be displayed with up to IO digits of resolution. and
that no digit will be truncated.
5. To return to normal operation of the Model 776
simply press and release the [EXE] push-button (second
function to [2nd]. The instrument than displays the
3-10
processed measurement with the programmed number
of digits.
NOTE
Selecting five
digits
to be displayed.
instead of ten. will eliminate the five
least significant digits and will move
the entire display to the right by five
places. An example of a normal dis-
play reading. and the same display
reading with five digits of resolution
is given as follows.
Display reading w~ith IO digits:
1.234567890
Display reading with selected 5 digits:
1.2345
NOTE
Under certain conditions. Model 776
may display frequency reading with
less than ten digits. This may occur
when the selected gate time is smaller
than 1 .O second. In that case. the in-
strument overrides the programmed
pm-selected number of digits and dis-
plays only as many digits as it can.
When gate time is increased. Model
776 again limits the number of dis-
played digits to the selected value.
3-13 Selecting Measurement Rate
Model 776 offers a choice from three measurement
rates. Two measurement rates are accessible through
front panel programming - normal rate (about 3 readings
per second). and single shot (hold). A fast measurement
rate (up to IO0 reading per second) is also available but
is only accessible from the GPIB interface. The fast
measurement rate will be discussed in details in Section
4. The HOLD light determines front panel measurement
rate. When the indicator is off. the instrument is in its
normal measurement rate. When the HOLD light is on.
the instrument is set to single shot measurements. To
select the measurement raw proceed as follows:
1. Press. in sequence. the [ZndlEXE] and the [HOLD]
push-buttons (second function to IRATIO]). The
HOLD light turns on: indicating lhat the instrument is
now~ armed for a single-shot measurement cycle. Arm-
ing is explained later in this chapter.