This repository describes how to build your own differential oscilloscope probe. In the respective folders you can find all necessary files to have the pcb produced, drawings on how to prepare the enclosure for final built-in, and detailed datasheets on the used components.
A couple of years ago I built a basic differential probe, limited to 1MHz/700Vpp input signals. The design was entirely done with through-hole components on a single-sided pcb. As technology and material have significantly improved over the years, I decided to build the next version with better specifications, based on modern SMD components.
The below "What is it all about" section will explain why you would actually want to build a differential probe, how it works, and what it can be used for in your lab.
The initial reason for building these differential probes concerned troubleshooting of serial industrial communication equipment (mainly RS422/485).
Being the proud owner of both the Horowitz & Hill's "The Art of Electronics" and the Linear Technology's "Analog Circuit Design" trilogy, I started reading to find a good solution for a state-of-art roll-your-own differential oscilloscope probe. Soon I found out that Linear Technology was not only having the right components, but was also having some very enlightening application notes, almost completely covering what I needed. As the schematic for a differential amplifier is very well-documented in LT's design note collection, I could start designing the pcb almost right away. However, a lot of attention was needed for the specific design constraints, as explained in the respective paragraph of the opamp's datasheet on page 16. As a cherry on the cake, Linear Technology has been so kind to sample some chips for the prototype. The key components for this project concern two LTC6268-10 4GHz FET-input operational amplifiers with extremely low input bias current and low input capacitance, and an LT1761 low noise, LDO micropower regulator. An extended [design note](pdf-files/designnotes - LT1761.pdf) concerning the LDO can be found in Analog Circuit Design Volume 3, chapter 166, or on the LT website. More in-depth theory on this particular LDO is also available in Analog Circuit Design Volume 1, section 3, chapter 12.
A differential probe is actually an accessory to simplify the measurement of differential signals with an oscilloscope. Before explaining the reasons why to use this accessory, let's first try to understand what differential signals are about.
Differential signals are the opposite of single-ended signals. In single-ended signalling technique, the signal is a voltage between the signal line and the ground (which is common between the transmitter and receiver circuitry). A differential signal consists of two complementary signals on two wires, which are mirrored around a virtual zero volt axe. At the receiver side, the two signals are subtracted. As any interference on the communication lines will affect either of the lines equally, no more "noise" will be present in the subtracted signal. For that matter it is important for both lines (or wires) to be physically identical, and to follow the same routing. For longer wiring, this usually implies the use of twisted pair cables. When applying differential signal paths in PCB's, it might be a good idea, to use the concerning functions which are built in most software packages. In order to obtain a good interference filtering, both (mirrored) complementary signals should cross the virtual zero-line at the exact same moment. For that matter, it is also very important that both signal lines have the exact same length. In electronics, the efficiency of this filtering is referred to as common-mode rejection ratio.
Now, why building this accessory for your oscilloscope ?
Nowadays, most oscilloscopes do have built-in functionality to subtract the signals between two channels. However, most oscilloscopes unfortunately only have two channels. So when during troubleshooting, you would like to compare the differential signal to anything else in your electronics circuit, a third channel (or second oscilloscope) would be required.
Todo : more info is coming...
The chip supplier for this project is also having a SPICE (Simulation Program with Integrated Circuit Emphasis) in his catalogue : LTspice®️, available for both Windows and MAC OS's (free downloads). A specific demo circuit for simulation and calculations for the LTC6268-10 can be downloaded here. It concerns a file with extension ".asc" which can be opened with the LTspice:registered: software.
- have a need
- define the technical specifications
- decide on what is should look like
- create drawings for the housing
- test and validate the concept
- publish the schematic and explain the electronics
- publish production files for the electronics board board
- write end-user documentation
- 1:1 active differential probe
- (over) 100MHz of bandwith
- 100kΩ input resistance
- max 700Vpp or 2500Vpp (depending on C1,C2)
As explained above, the heart of the circuit is based on two Linear Technology LT6268-10 high-frequency amplifiers, IC1/IC2, to obtain a fully differential oscilloscope probe with interesting specifications. Next, a detailed circuit description from Linear Technology, extended with component names :
The input stage, around IC1, is a modified differential amplifier configuration with 100:1 attenuation on the inputs to allow wide common-mode swings. Additionally, the inputs are ac-coupled, by C1/C2, to provide high-voltage dc blocking. The first-stage, IC1, re-amplifies the signal by 10:1 as does the single-ended output stage IC2, for a composite gain of 1:1. A back-termination resistor, R12, provides stability when driving a cable to the 1MΩ oscilloscope input port, and is AC-coupled, C6, to remove the 2.5V output-stage bias.
The LT1761 low noise LDO micropower regulator IC3 outputs a stable 5v to power the entire circuit, coming from a standard 9V block battery.
| Part | Value | Package | Description |
|---|---|---|---|
| R1 | 2k2 | 1206 | resistor |
| R1, R2 | 49.9kΩ | 1206 | resistor |
| R3, R5 | 549Ω | 1206 | resistor |
| R4, R7, R8 | 10kΩ | 1206 | resistor |
| R6 | 4.99kΩ | 1206 | resistor |
| R9 | 191kΩ | 1206 | resistor |
| R10 | 499Ω | 1206 | resistor |
| R11 | 54.9Ω | 1206 | resistor |
| R12 | 48.7Ω | 1206 | resistor |
| C1, C2 | 2.2nF/1kV (1) | C050-025X075 | capacitor |
| C3, C4, C5, C9 | 10nF | 1206 | capacitor |
| C6, C7 | 1µF | 1206 | capacitor |
| C8 | 10µF/10V | Panasonic type B | polarised capacitor |
| IC1, IC2 | LTC6268HS6-10 | SOT95P280X100-6N | op-amp |
| IC3 | LT1761ES5-5 | SOT95P280X100-5N | LDO voltage regulator |
| S1 | SPDT | MINI SWITCH 90 | mini toggle switch |
| CON1 | 9V | soft clip | 9V battery clip |
| CON2-1, CON2-1 | 4mm (2) | banana socket | 4mm red/black banana socket |
| CON3 | BNC | BNC-F-90-4 | female BNC 90° (PCB) |
| H1 | 1593LGY | enclosure | Hammond 1593L enclosure |
(1) 1kV or 3kV, see building instructions
(2) use appropriate double isolated banana sockets for measurements on lines >48V (also see building instructions)
The prototype has been tested in several real-life measurement setups, and shows correct functionality.
...more info on the actual test setups will follow later...
The bases for the housing is an abs handheld instrument enclosure from the 1593 series, fabricated by Hammond manufacturing, more specifically 1593LGY (grey-coloured, with two removable end-plates).
The pcb for the probe has been designed in such a way that it can be fixed with four screws to the bottom of the enclosure. On the side of the pcb, cut-outs have been foreseen for the holes that allow to screw the bottom and top of the enclosure together. The large rectangle cut-out fits a 9V block battery which serves as power supply for the probe. A T-shaped groove on the input-side protects the circuitry from arcing and creep when higher input voltages are applied (please implement all safety precautions as explained in the electronics and building paragraphs when working with potential differences which are higher than 48V-to-earth).
In order to be able to mount the input and output connectors, as well as for power switch operation, several holes need to be made in the frontplates. The easiest way is to use the templates for laser-cutting available in the respective folder (instructions included in the readme-file). A cutting template is available for both the input panel and the output panel. The templates have been drawn on scale in Coreldraw X6, and as such, can also be used as a guide to drill the required holes by hand.
The best way to populate the PCB, in case of hand-soldering, is to start with the chips IC1,IC2,IC3, as you will need access to the surroundings to remove the excess solder with some braided shielding wire. Next all 1206 package SMD components can be mounted (resistors and capacitors). Then you can also mount the aluminium capacitor C8 (look at the footprint for the correct orientation). After having finished with the SMD components, through-hole capacitors C1,C2 need to be mounted. Next, push the BNC connector's mechanical pins firmly in the pcb (the holes are very tight). Switch S1, which is soldered sideways on the component side should be positioned in order for the metal housing to be just behind the frontplate of the Hammond enclosure (for positioning, first only solder the middle pin, put the pcb in the enclosure, and slide in the frontplate to check the position). Finally, the 9V clip wires can be soldered (mind polarisation). Finish by preparing two wires (ideally using different colours), which will be connected to the two signal-input banana connectors during final mounting.
As for the incoming signal, there are several options, depending on the target application of the probe. In case the probe will only be used for low voltages (below 48V), any type of test-lead and banana socket can be used. When working with voltages above this level, appropriate sockets and test leads should be installed, and capacitors C1,C2 should be dimensioned accordingly. On the current pcb both 1kV and 3kV types will fit (both commercially available). As for the banana sockets, double isolated types should be used (see below image). Depending on the voltage to apply, you should also consider putting electrical isolation on the back of the sockets, and replace the pcb screws with nylon versions.
If you are having any good suggestions, just drop me a line 📧. If feasible, I'll be happy to implement any proposed improvements. And if you are having lots of time, I'll be happy to share the work with you ;-).
When you create your own version, don't forget to send us some nice pictures of your construction. We'll be happy to publish them in the 🎊Hall of Fame🎊.
At this moment, there is no specific license attached to this project.
So, today, if you like it, have fun with it (at your own risk of course :-D), and especially, be creative.
Oh, and when using anything from this repository, it is highly appreciated if you mention its origin.
If you would like to use any of this work (or the whole project) for commercial use, first contact us 📧, so we can add the appropriate license, which best fits your business.