Assessing Cmrr Error; Input Impedance Effects On Cmrr; Differential-Mode Rejection; Channel Isolation - Tektronix TriMode TDP7700 Series Technical Reference

Assessing CMRR error

Input impedance effects
on CMRR

Differential-mode rejection

Channel isolation

TDP7700 Series TriMode Probes Technical Reference
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.
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.
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:
| |
S SC21
20log + 6dB
S SD21
for the measured common mode response. The 6 dB term in the AC DMRR
equation gives the voltage-referenced response.
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 other input, and this effect increases with frequency. Channel
isolation is a measure of how much crosstalk occurs between the two probe
inputs. The channel isolation is defined with S-parameter measurements below,
where:
A input = S1, B input = S2, Output = S3
A ISOLATION = 20 log (S
B ISOLATION = 20 log (S
A typical isolation plot for the TDP7700 Series probes using a TekFlex solder tip
is shown in the following figure. When the probe is used with TekFlex solder
tips, note that channel isolation performance is highly dependent on probe tip
attachment lead length. Good channel isolation requires keeping the interconnect
lead length for both signal and ground connections very short.
/S ) | A Mode
31 32
/S ) | B Mode
32 31
Reference
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