DiffProbe

Differential Oscilloscope Probe

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Linear Tech Design

This differential oscilloscope probe is based on a Linear Technology application note. The design uses LTC6268-10 op amps which are 4GHz FET input operational amplifiers with extremely low input bias current and low input capacitance.

(PCB Image is from Linear Tech website)

Features

This circuit uses two LT6268 high-frequency amplifiers to realize a fully differential oscilloscope probe
>100MHz bandwidth (dependent on op amp part speed grade selected)
Build options (R1/R3, R2/R5 ratios values)
- 100kilohm input resistance on 1:1 allowing signals up to +/- 2.5V
- 1Megohm input resistance on 10:1 allowing signals up to +/- 25V
The input stage is a modified differential amplifier configuration with 100:1 attenuation on the inputs to allow wide common-mode swings
The inputs are ac-coupled to provide high-voltage dc blocking
The first-stage IC re-amplifies the signal by 10:1 as does the single-ended output stage, for a composite gain of 1:1
A back-termination resistor provides stability when driving a cable to the 1Megohm oscilloscope input port, and is AC-coupled to remove the 2.5V output-stage bias
The circuit is powered from 5V, derived from a 9V battery with an LDO
16.5mA per amplifier
Power Switch for battery operation
Power Indicator LED

Additional Design Notes

Due to the wide bandwidth amplifiers and the relatively high resistances being used, stray capacitance control and careful layout is vital
A 'gimmick' feedback capacitor may be needed to adjust the damping of the amplifiers to avoid oscillation and to achieve best time-response
Can scale the input voltage by selecting different input resistor values