Tektronix 1A5 Instruction Manual page 9

OPER A TI N G INSTRUCTIONS
Introduction
This section contains a description of differential ampli ­
fiers in general, of the Type 1A5 Differential Amplifier con ­
trols, connectors and indicators, and of the Type 1A5 Differ ­
ential Amplifier operation.
GENERAL DIFFERENTIAL
AMPLIFIER INFORMATION
An osc i lloscope with a differential amplifier is a device that
ampli fi es and displays a voltage difference that exists at
every ins tant between s i gnals applied to its two input chan ­
nels. The following conclusions can be drawn from this
definition when two s i gnals are applied to the input of a
differential amplifier.
1. If two s i gnals are in phase and of equal amplitude
( herea fter called common-mode), the output will be zero.
2. If the two s i gnals are in phase but of different ampli ­
tudes the output will equal the amplitude dif
3. If the two s i gnals are out of phase and of equal
amplitude the output will be the phasor diffe
the two s i gnals. (S in u sod ial signa
4. If the two s i gnals are out of phase and of different
amplitude the output s i gnal is a complex quan
from both amplitude and phase dif
Common Mode Rejection
The definition of the term "di
a rejection of equal ampli t ude, coincident s
cation is correct. However, the degree of rejec
prim a r i ly on the s y mm et ry of the amplifier inpu
amount of diffe r ence s i gnal contributed by a particular ampli ­
fier at a spec i fic frequency is documented with a mathe ­
matical relationship that is called the common-mode re ­
jection ratio (CMRR). This ratio and ass
defined as follows:
Common Mode: R e fers to s
amplitude and time. It is also used to iden
par t s of two s i gnals that are identical in amplitude and time.
Common-Mode R e j ect i on: The ability of a differential
amplifier to reject common-mode signals.
Common-Mode R e j ect ion R
ampli t ude of the common-mode input s
input s i gnal which would produce the s
the CRT screen.
NOTE
Since the differential amplifier is part of an os
losc o pe, the output s ig
CMRR is measured from
VOL TS/ CM switch s ett
amplifier that produces a .001 volt output when
SECTION 2
f e r ence.
r ence between
l s . )
t ity derived
fer ences.
ff erential amp l ifier" impl
i gnals. This impli ­
t ion depends
t s. The
oci ated terms are
i gnals that are identical in both
t ify the respective
ati o (CMRR): The ratio of the
i gnal to the diffe r ence
a me deflection on
cil ­
nal used to calculate the
the CRT s cr een and
ing. Thus, a differential
driven by a 10 volts peak to peak common-mode
signal has a CMRR of 10 — .001 or 10,000:1.
Amplitude and Common-Mode Rejection
In the text which follows, the term "Input Signal Range "
means the common-mode operating range of voltage through
which the amplifier will produce a useable output.
should not be confused with the maximum (non-destructive)
input voltage, which is related to the breakdown limits of
the amplifier compone
Factors Which Affect CMRR
F re quency: S i nce the common-mode output voltage is a
factor of phase di ff e
the frequency of the input common-mode s
direct bearing on the CMRR. Generally, as the frequency of
the input s i gnal increases, the CMRR decreases. (Exception:
with AC-coupled input the CMRR will become higher as fre ­
quency is increased from DC to over 100 Hz.)
S o urce Impedance: The spec
points being measured have identical s
The s o urce impedance and the amplifier input impedance
form an RC divider which determines the portion of the
s i gnal that appea r s across the amplifier input, and the ap ­
parent effect on CMRR. See F
The us e r may desire to cons
versus frequency for specific appl
or s i gnal transporting lead impedances are unbalanced.
i es F i g. 2-4 is therefore included
S i gnal T r ans por ting L
maximum CMRR is that the s
two inpu t s with no change in phase or amplitude. Sight
dif fe r ences in attenuation facto
two input attenuato r
Attenuator probes extend the us
differential amplifier by reducing the input signal level
below the maximum common-mode input voltage.
ever, a reduction in the apparent CMRR will us
because of component value dif
F o r example, F i g. 2-5 illustra
ent) due to × 10 probes that are within 1, 2, and 3% of
their attenuation value.
In measurements where attenuator probes must be used
because of high voltage levels, and at the s
( a bove 100:1) CMRR must be m
P 6 023 P ro be is recommended.
this × 10 low capacitance probe is adjustable over a
±2.5 % range.
Ground Connecti o
generated from ground loop curren
elec tr ic a lly connect the probe or s
at the probe body or s
ment ground. See F
nt s.
r ences as well as gain between channels,
i gnal has a
i fied CMRR assumes that the
o urce impedance.
i g. 2-2 and 2-3.
t ruct a s i m i lar graph of CMRR
i ca ti ons where the source
.
e ads: A principle requirement for
i gnals arrive at the ampli
r s, or phase s h ift between
s may reduce the CMRR 20% or more.
a ble voltage range of a
f e r ences within the probes.
t es the change in CMRR (appar ­
a me time a high
a intained, the Tek
The attenuation factor of
ns: P ro per grounding reduces signals
t s. It is us ua
i gnal lead shields together
i gnal s o urce, but not to the ins
i g. 2-6.
Type 1A5
This
f ier ' s
How ­
ual ly occur
t ronix Type
lly best to
t ru ­
2-1