Reference
This section defines common terms used to specify the instrument.
Differential measurements
A differential probe is optimized to measure high speed differential signals. Differential signals are formed from two
complementary signals with a common reference voltage. (See
Figure 24: Simplified model of a differential amplifier
on page 20.)
Devices designed for differential measurements avoid problems presented by single-ended systems. Differential signal
transmission improves signal fidelity by doubling the effective signal amplitude compared to single-ended signal
transmission. Differential signaling also improves signal fidelity by removing common mode noise due to the CMRR of the
receiver or measurement device.
A differential probe is basically a differential amplifier used to make differential measurements that reject any voltage
common to the inputs and amplifies any difference between the inputs. Voltage that is common to both inputs is often
referred to as the common-mode voltage (V
CM
) and voltage that is different as the differential mode voltage (V
DM
).
Figure 24: Simplified model of a differential amplifier
Common-mode rejection ratio
Differential amplifiers cannot reject all of the common-mode signal. The ability of a differential amplifier to reject the
common-mode signal is expressed as the common-mode rejection ratio (CMRR).
The DC CMRR is the differential-mode gain (A
DM
) divided by the common-mode gain (A
CM
). It is expressed either as a ratio
or in dB:
AC CMRR for the probe is determined using 3-port, mixed-mode S-parameters for the measured differential mode
response, where A input = Port 1, B input = Port 2 and Output = Port 3 is defined as:
Reference
TDP7700 Series TriMode™ Probes Technical Reference
20
