What is it?

• Caution -This is a PCB Only with a lot of surface mount soldering of fine pitch parts
• This circuit uses two LT6268 high-frequency amplifiers to realize a fully differential oscilloscope probe
• Greater than 100MHz bandwidth
• 100kilohm input resistance on 1:1 allowing signals up to +/- 2.5V (build time option)
• 1Megohm input resistance on 10:1 allowing signals up to +/- 25V (build time option)
• 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

Schematic

Why did you make it?

Sometimes you need to test circuits that can't be connected through your scope chassis to earth ground. Commercial differential scope probes are extremely expensive.

What makes it special?

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.

Part 1 of 4 part Video series that describes my failures along the way to the final success. This was a difficult board to get the layout just right.