Sniffing a proprietary data line at 47.5V

Question

I need to sniff the physical layer of a proprietary data line with a logic analyser, but my analyser is limited to 5V input. Unfortunately the line runs at 47.5V, which would fry everything. I considered a simple voltage divider, but I'm concerned that I might pump a lot of current through it, or affect the data on the line.

Any suggestions on a way to do this? It doesn't need to be robust - it's just a one-time hack for a single purpose.

EDIT: To answer some of the questions asked: I don't know if it's 47.5V DC offset or 47.5V p-p, as I only have a single figure given to me as part of the spec. It's an old system produced by a company that no longer exists, so getting original specs and info is practically impossible. The data rate is 2Mbps, which to me implies that it shouldn't exceed 4MHz at the physical layer.

Answer 1

Two options.

Option 1. If data is fully swinging from 0V to 47.5V then a voltage divider using two resistors is fine. Choose values of resistors that are not too low so that excessive power is dissipated. Check also to see how much the line can be loaded with. If necessary, make a pseudo scope input from 1Mohm and 100k potential divider - this will produce about 4.3Vp-p BUT, you may need to put 22pF across the 1M and 220pF across the 100k to keep "edge" information in the data stream clean.

Option 2. If the data is 5Vp-p superimposed on a dc level of 47.5V then it's more complex - this is like "phantom power data" and to "read" the 5Vp-p data you need to remove the DC by using a capacitor - this leaves you with an AC data signal that is 5Vp-p but undulating up and down with the average mark-space ratio of the signal.

If the data is encoded in such a way that there are always data edge transitions then that's fairly easy - connect the dc-removed signal into a signal comparator that has a 0V/5V output then feed that into your analyser. However, if the data is similar to UART data, there will be long periods when the signal is 1 or 0 and this is a problem that you may never solve easily.

Answer 2

Here's a simple inverting circuit that should work, provided you can safely connect the grounds together. It presents a very light load (120K) to the signal, and should work okay up to maybe 50 or 100kHz. If you need higher frequency response, shunt R1 with a few pF.

^{simulate this circuit – Schematic created using CircuitLab}

This assumes the input signal swings from 0 to +47.5V so sets the transition at around 20V. As Andy says, if your signal does not swing 0/47.5 then other means must be used.

Note that it is inverting, so output is low for 47.5 in and high for 0V in.

Answer 3

Even though this is most likely a DC offset scenario, you can eliminate the peak/peak possibility for your sanity. Take an appropriately sized resistor, and an LED, and connect this between the signal and its reference (ground). If the LED is constantly lit, it is an offset scenario. If the LED flickers, it is a peak/peak scenario. (If you are swinging 48 volts then at some point that signal will be below ground and the LED will be off.) Hopefully your ground team can manage this much.

So, let's assume this is a DC offset scenario where the voltage could swing significantly around 47.5V. I propose using a level shifter circuit, using two resistors and an NMOS, to bring the voltage down to a 5V range. You can adjust the range by adjusting the resistor ratio. I played with this a bit in LTSpiceIV and the idea seems to hold up.

http://husstechlabs.com/wp-content/uploads/2010/09/Level-shifter.jpg

However there will still be some DC offset present, and the swing won't be optimal for a TTL UART bus, so you'll need to condition further. A single instrumentation amplifier should be able to difference the signal with a DC level, and gain it properly to 0-5 V.

http://en.wikipedia.org/wiki/Instrumentation_amplifier

Also the level shifter might give a spiky output - use a decoupling cap before the input to the instrumentation amplifier to mitigate this.