DiffProbe

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Differential Oscilloscope Probe

DiffProbe-720px-P580.jpg

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.

LTC6268-10_isoSPI_DiffAmp.png

(Card Image is from Linear Tech website)

Features

  • This circuit uses two LT6268 high-frequency amplifiers to realize a fully differential oscilloscope probe
  • >100MHz bandwidth
  • 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
  • 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

Other Ap Notes

Linear Tech Application Note

LTC6268-10_isoSPI_DiffAmpSchematic.jpg

(Image from the Linear Tech website)

Board Layout

DiffProbe-FrontAndRear-350pxV.png

DiffProbe-Rear-720px-P583.jpg

Connectors

J1 - Power

  • 6-9V DC Input power
  • 2-pin, 5mm pitch terminal block connector
  • Pinout
  1. +VCC
  2. GND

J2 BNC Output (to Scope)

  • Female BNC connector
  • Use male-male coupler or BNC-to-BNC cable to connect to scope

J3 - Input

  • 2-pin, 5mm pitch terminal block connector
  • Pinout
  1. + (Plus side of differential probe)
  2. - (Minus side of differential probe)

Assembly Sheet

Differential Probe Assembly Sheet

Differential Probe Assembly Sheet Rev X1