System analysis and design

A project log for Active scope probe with no dedicated power supply

Using the probe test point to power a ultra-low capacitance, high-input-impedance amplifier, essential for accurate analog measurements!

Yann Guidon / YGDESYann Guidon / YGDES 04/13/2016 at 10:520 Comments

I have moved this from the details page because it became too large.

I expect the amplifier to draw a milliampere or more. This is not comparable to the DS32KHZ and a different solution is needed because the charge pump will not work at all.

The first question that must be answered is : how much power can the test point provide ? I don't think that there is a standardised design, my cheap scope provides 2V at 1KHz but it doesn't need to be precise because all it has to do is provide clean edges.

This lack of standardisation implies that the DC/DC design must be flexible and accomodate different frequencies, voltages and current ratings.

There are 3 usual methods to step-up :

From this, it appears that a "modified Joule thief" is the most promising solution. The ability to automatically adjust the input current and the output voltage are essential because they will vary all the time ! It can accomodate your brand and make of oscilloscope.

The step-up DC/DC is one thing but it will only work about half of the time. During the peak hours it will try to convert as much energy as possible. This energy is stored in a big capacitor before being used by a linear regulator.

This is where the design borrows from other established systems, in particular the "active PFC", power factor correction circuits that have to deal with the same kind of problem, when the rectified power from the mains (50Hz sine in Europe, 100Hz half-sines) does not provide energy during a significant fraction of the cycle.

The solution is to step-up as much as possible and store in a large capacitor that will provide the necessary energy during the remaining 500µs.

Some desired properties of the capacitor are:

So there is a trade-off between capacitance, ripple, voltage etc...

Note: the probe must be differential (plus a common pin) because some current (with nasty oscillations) will go through the ground-connected shield.

A split-system becomes necessary:

So that's why I went on a quest to find BNC connectors...