Tektronix Trimode TDP7700 Series, Техническое описание

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The 6 dB term in the AC CMRR equation gives the voltage-referenced response. CMRR generally is highest (best) at DC
and degrades with increasing frequency. A typical CMRR plot for a TDP7700 Series probe and a flex circuit-based solder tip
is shown.
Figure 25: Typical CMRR
Assessing CMRR error
The CMRR of the TDP7700 Series probes is shown in graphs assuming a sinusoidal common-mode signal. A quick way to
assess the magnitude of CMRR error when the common-mode signal is not sinusoidal is to connect both leads to the same
point in the circuit. The oscilloscope displays only the common-mode component that is not fully rejected by the probe.
While this technique might not yield accurate measurements, it allows you to determine if the magnitude of the common-
mode error signal is significant. When using the solder-in tips, keep the tip leads the same length to maximize the probe
CMRR.
Input impedance effects on CMRR
The lower the input impedance of the probe relative to the source impedance, the lower the CMRR for a given source
impedance imbalance. Differences in the source impedance driving the two inputs lowers the CMRR. Note that single-
ended measurements generally result in asymmetric source impedances which tend to reduce the differential mode CMRR.
Differential-mode rejection
When making common-mode signal measurements ((A+B)/2 – GND) with the TriMode probe, it is desirable to reject the
differential-mode signal present between the two inputs. This rejection is expressed as the Differential-Mode Rejection Ratio
(DMRR).
AC DMRR for the probe is defined using 3-port, mixed-mode S-parameters as:
for the measured common mode response. The 6 dB term in the AC DMRR equation gives the voltage-referenced
response.
Channel isolation
Under ideal conditions when taking single-ended measurements with a differential probe, no part of a signal applied to one
input of the probe would appear on the other input. In reality some portion of the signal on one input does “bleed” over to the
Reference
TDP7700 Series TriMode™ Probes Technical Reference 21