Tektronix 2A61 Instruction Manual page 11

Differential Voltage Measurements
Differential voltage measurements are made with the
INPUT SELECTOR switch set to A~B and the signals applied
to the A and B terminals of the INPUT connector. Under this
condition, only the voltage difference of the two signals is
amplified and displayed on the CRT. Common mode signals
(signals that are common in amplitude, frequency and phase)
are rejected and not displayed (see Fig. 2-2). Maximum com-
mon-mode rejection occurs when the MILLIVOLTS/DIV switch
is in the higher sensitivity settings (.5 through .01 positions).
Before making critical differential voltage measurements,
be sure to check the adjustment of the DIFF BAL control as
described under Setting the DIFF BAL Control with a signal
as nearly identical as possible in frequency and wave shape
to the signal you wish to reject.
Severe distortion may result from exceeding the maximum
input voltage ratings of the Type 2A61.
In differential measurements, the impedance of the signal
source is of primary importance in determining the resultant
Common Mode Rejection Ratio. The source impedance and
input impedance of the Type 2A61 form voltage dividers
which, if unbalanced, will cause loss of Common Mode Rejec-
tion Ratio. For instance, with a measurement situation such
as that of Fig. 2-3, the Common Mode Rejection Ratio with
respect to VCM is only 1000.1.
Frequency Response
Upper and lower frequency limits of the Type 2A6] are
variable with the FREQ RESPONSE controls. This feature is
useful in improving the signal to noise ratio of the display
(see Fig. 2-4). However, if the FREQ RESPONSE controls are
not properly set, attenuation or distortion of the signal of
interest may result.
A square wave, for example, contains
a wide
range of frequency components.
The effect of the
FREQ RESPONSE controls may appear to be more pro-
nounced on a square wave than with sinsoidal signals. The
chart of Fig. 2-1
gives the effect of the FREQ RESPONSE
controls at various frequencies.
The line frequency filter provides selective attenuation
to signals at or near the line frequency.
The FILTER switch
should be used with the same precaution as the FREQ
RESPONSE
controls.
When the FILTER
switch
is set to IN,
avoid the tendency to apply too large a line frequency sig-
nal to the INPUT of the Type 2A61. Maximum applied volt-
ages for the Type 2A61
are given in Section 1, Characteristics,
and are applicable regardless of the setting of the FILTER
switch.
Applications
®
Operating Instructions—Type 2A61
The Type 2A61 is also suitable for direct measurement of
outputs from dynamic strain measurement systems. It will
display outputs from resistive, inductive or capacitive bridges.
Inputs from two bridges (one applied to A input and the
other to B input) will allow measurement of strain at either of
two points or the differential strain between two points.
oor
ML 0.52 to coo:
~t. +—
Fig. 2-4. Typical CRT display showing improvement of signal to noise
through use of the FREQ RESPONSE controls.
2-5