Operating the Probe
1
Differential Amplifiers and CMRR
1141A and 1142A User’s Guide
25
Differential Amplifiers and CMRR
The 1141A 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 ratio (CMRR) is the measurement of this ability. It
is expressed as the ratio between the amplitudes of the common
mode and differential signals which product equal outputs. For
example, if a common mode signal of 1V and 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 to balance circuit parasitics and
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 1141A Differential Probe is specified at the input
of the probe and cannot be affected expect 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 the
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 source
forms a network with the input impedance of the connections 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.
Summary of Contents for 1141A
Page 4: ...4 1141A and 1142A User s Guide Contents...
Page 58: ...58 1141A and 1142A User s Guide 3 Adjusting the Probe Attenuator Adapter Adjustment...
Page 76: ...76 1141A and 1142A User s Guide 4 Service Replaceable Parts Figure 44 Cabling Diagram...
Page 86: ...86 1141A and 1142A User s Guide 4 Service Replaceable Parts...
