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Operating the Probe
Differential amplifiers and CMRR
Differential amplifiers and CMRR
The 1153A Differential Probe is a high-impedance differential amplifier. A characteristic
of differential amplifiers is the ability to reject signals that are common to the two inputs.
The common mode rejection ration (CMRR) is the measurement of this ability. It is
expressed as the ratio between the amplitudes of the common mode and differential
signals which produce equal outputs. For example, if a common mode signal of 1V and
a differential signal of 1 mV both produce outputs of 1 mV, the CMRR is 1000:1.
The ability to reject common mode signals is dependent on the balance designed into
the differential amplifier. At higher frequencies it becomes harder balance circuit
parasitics and the parameters of devices so CMRR degrades as frequency increases. Also,
stray coupling increases with frequency and coupling may vary between the two
differential paths.
The CMRR of the 1153A Differential Probe is specified at the input of the probe and
cannot be affected except by adjustments in the probe. However, the way the probe is
connected into the circuitry being tested can have a big influence in the overall result of
measurement, especially at high frequencies.
The following things can affect the effective CMRR of a test setup.
The connection to the circuit under test
The method used to connect the probe is
important because it involves the symmetry of the differential input circuitry. For
example, using different lengths of wire to connect the circuit to the two probe inputs
unbalances the inductance and capacitance at the inputs. The effective CMRR will be
reduced, especially at high frequencies. Additionally, coupling from adjacent circuitry
will be less balanced.
The impedance of the source
This is another instance where the symmetry of the
differential circuit is important. The impedance of the source forms a network with the
input impedance of the connection and the probe. This network determines the
frequency response for the measurement. If each side of the differential source has a
different impedance, the frequency response of each side will be different and the
unbalance is reflected in a reduced CMRR. Of course, lower source impedances have less
effect on the frequency response of the measurement.
The ground connection
A poorly located ground connection allows ground loops to
add to the common mode signal.
Frequency
Frequency is the most important factor in CMRR only because of all the
factors mentioned above are frequency dependent. The unbalances of capacitance and
inductance are more important as frequency increases. Therefore, good high-frequency
practice is important when using a high impedance differential probe.
On the other hand, if the differential probe is ac coupled to the circuit under test (the
ac adapter is being used) the CMRR will be degraded below a certain frequency; the
lower the frequency the worse the CMRR. This is because unbalance in the series
capacitances of the ac coupler becomes more significant the lower the frequency.
Summary of Contents for 1153A
Page 4: ...5 1 General Information ...
Page 12: ...13 General Information General Characteristics Figure 1 3 1153A Differential Probe Dimensions ...
Page 13: ...15 2 Operating the Probe ...
Page 23: ...25 3 Service ...
Page 34: ...36 Service Replaceable Parts Figure 3 7 1153A Differential Probe Parts ...
