Filter
Top Products
60
Service
Theory of Operation
Differential Probe
The probe contains a two-path differential amplifier with unity gain. It is implemented
on a double-sided surface-mount PC board with the high-frequency path on one side and
the low-frequency path on the other. The two paths are split directly after the differential
input connections.
High-Frequency Path
The positive and negative inputs are ac-coupled at 33 Hz into identical impedance
converters. The HF CMRR adjustment balances the gain at the outputs of the impedance
converters. The impedance feed a semi-custom differential amplifier. An additional
negative input to the differential amplifier brings in the sum of the low-frequency and
feedback signals. The FREQ COMP adjustment provides variable high-frequency peaking
of the differential amp. The output amp provides two signals. A feedback signal is
summed with the low-frequency signals and the output signal is the final output of the
probe. Overall probe gain is set by the HIGH FREQ GAIN adjustment.
The entire signal portion of the high-frequency path is implemented on a hybrid IC.
Support circuitry includes bias for the impedance converters and a bias supply for
current sources on the hybrid.
Low-Frequency Path
The bandwidth of the low-frequency path is approximately 75 KHz. The positive and
negative inputs are dc coupled into identical inverting op-amps with gain of 0.5. They
provide a precision 1 M Ω input impedance for the probe. The LOW FREQ BANDWIDTH
adjustments match the gain and phase of the low-frequency path to that of the feedback
from the probe output. One of the two adjustments is set to match the properties of the
feedback and the other is to match the two low-frequency paths. These adjustments
affect the CMRR quality of the probe. The inputs of the inverting op-amps include
protection for ESD and over-voltage conditions.
The inverting op-amps feed a precision differential amplifier with unity gain and a single-
ended output. The output is fed to the summing amp and to the control module for use
in the dc reject circuit.
Summing Amp
The summing amp combines the feedback signal, the low-frequency signal, and the offset
signal. The LOW FREQ GAIN adjustment matches the gain of the low-frequency path to
the overall gain.
Control and Power Module
The control and power module provides offset functions, local and remote control, and
power to the probe system.