Low frequency/DC oscilloscope probes - Page 1
Forum section: electronics / projects

author: IanB
author_rank: Super Contributor
timestamp: January 01, 2012, 12:26:23 am
content: I've been reading up a bit on probe design and understanding why oscilloscope probes are designed the way they are. Mostly this seems to be about working well at RF frequencies and higher.But if I am interested in sampling low frequency signals, say in the 10's to 1000's of Hz on a DSO, is there any reason to continue using a specially constructed probe? Suppose I just used a pair of separate leads like on a multimeter, and perhaps put a 100k resistor across the input to reduce noise picked up by the leads? This would be for use above the sampling frequency of a typical multimeter, but much lower than traditional oscilloscope territory.It's often much easier to connect a separate pair of test leads to a circuit than an oscilloscope probe with a short ground clip. Are there any downsides to this that I have not foreseen?

author: amspire
author_rank: Super Contributor
timestamp: January 01, 2012, 12:32:15 am
content: As long as the DC voltages are low enough to be safe for the oscilloscope, there is no reason why you can't directly connect the oscilloscope to the circuit under test without any probe for 1KHz and lower.It is exactly what the x1 setting on an oscilloscope probe is doing anyway.Richard

author: Psi
author_rank: Super Contributor
timestamp: January 01, 2012, 12:50:07 am
content: I use x10 all the time, unless i have a reason to need x1.With x10 your scope front-end is protected a bit more.For example, if you were probing a circuit and got 1000V of back emf from a relay because the diode was open.If it's on x10 then it's only 100V to the scope but if it's on x1 then it's the full 1000V into the input channel.The rigol, for example, is rated to 300VRMS max on the front-end.

author: IanB
author_rank: Super Contributor
timestamp: January 01, 2012, 01:26:21 am
content: I see. So instead of making a test lead to BNC adapter, I could make a probe to banana socket adapter. My adapter would have two pins to clip the oscilloscope probe on to, connected to banana sockets for the test leads.

author: amspire
author_rank: Super Contributor
timestamp: January 01, 2012, 01:45:30 am
content: Or buy some adapters:http://www.ebay.com.au/itm/1PCS-BNC-Male-Dual-Binding-Posts-POST-Banana-Connector-Plug-TEST-Adapter-BBP-/30063403723But Psi's warnings are correct. One mistake in a circuit with inductors, and your scope is dead. Using a X10/X1 probe has the advantage that at least you can start at x10 and check that nothing bad is happening.You could add your own protection like back to back zeners across the scope input and an extra 10K HV resistor going to the test circuit. It will reduce all the voltages to the scope by 1%.

author: IanB
author_rank: Super Contributor
timestamp: January 01, 2012, 02:15:12 am
content: That eBay link doesn't seem to work for me. But I did look up BNC adapters from the likes of Pomona through mainstream suppliers and they were crazy expensive.My idea was to keep the standard probe on the X10 setting and put the adapter on the end of the probe rather than attach it directly to the scope. That way I would keep the degree of input protection offered by the probe and still have the convenience of test leads.

author: amspire
author_rank: Super Contributor
timestamp: January 01, 2012, 02:36:36 am
content: I somehow deleted the last character of the address. Try this link:http://www.ebay.com/itm/1PCS-BNC-Male-Dual-Binding-Posts-POST-Banana-Connector-Plug-TEST-Adapter-BBP-/300634037237

author: IanB
author_rank: Super Contributor
timestamp: January 01, 2012, 02:49:39 am
content: Thanks, I see, super low price. But an item such as that does not heed the warning about connecting directly to the scope input in the event of accidental voltage spikes. Also, a rigid adapter like that runs the risk of mechanical stress on the scope socket unless I put a short coax cable between it and the scope.Having considered the feedback in this thread, I won't rush to plug test leads directly into the scope.

author: Unknown
author_rank: Unknown
timestamp: January 01, 2012, 04:09:58 pm
content: Using flying leads increases the loop area between the two wires. This will make it more susceptible to pick up noise. Especially if you don't low-pass filter the signal.

author: bfritz
author_rank: Regular Contributor
timestamp: January 01, 2012, 04:28:24 pm
content: Quote from: Psi on January 01, 2012, 12:50:07 amI hear this as a reason very commonly, but think it is not well thought out. For example, to get 1000V of back EMF into the scope, would require the switching voltage of the relay to be 500V. The worst you can do with the back emf from a relay or switching device is twice the normal voltage.

author: Unknown
author_rank: Unknown
timestamp: January 01, 2012, 04:44:46 pm
content: Why would inductive kick from the relay be limited to two times its switching voltage? There's a constant current flowing through the inductor, then the circuit is broken so the current drops to zero. V = L dI/dt, since dI/dt has a very large magnitude (it drops almost immediately to zero), V will also be very large until something starts conducting.

author: Zero999
author_rank: Super Contributor
timestamp: January 01, 2012, 05:59:26 pm
content: Yes, there's no theoretical limit to the inductive kickback from a relay coil, in a perfect world it'd be an infinite voltage, in practice it depends on the current, internal resistance and shunt capacitance of the coil.Another reason to use a x10 probe, even at low frequencies is the probe capacitance and impedance will be lower than a long piece of co-axil cable connecting it to the oscilloscope input.

author: Unknown
author_rank: Unknown
timestamp: January 01, 2012, 06:34:09 pm
content: Capacitance is not really a big deal for <= 1 kHz. A 2 meter piece of unterminated RG-58 might have a capacitance of 200 pF, representing an impedance of a little under 1 Mohm. In parallel with the scope input, the total impedance would be around 400 kohm. Most circuits should have no problem driving this load, unless the circuit is very high impedance.

author: IanB
author_rank: Super Contributor
timestamp: January 01, 2012, 06:51:33 pm
content: Quote from: alm on January 01, 2012, 04:09:58 pmYes, I've seen that. Simply powering up a switching regulator near the scope with an open probe (just the standard Rigol probe) causes a visible fattening of the noise bar on the trace.I wanted my flying leads for near-DC signals, but I've since found the Rigol's built in digital filtering is disabled at scan rates slower than a few ms per division. It doesn't say that in the manual.

author: w2aew
author_rank: Super Contributor
timestamp: January 03, 2012, 06:56:53 pm
content: Good advice here regarding the voltage protection given by the 10x probes. One more thing to think about - the 10x probes will provide 10x higher resistive loading than direct (1x) connection - it may matter for some circuits, particularly very high impedance circuits. If you can afford the extra capacitance (likely with <1KHz signals), using a coaxial connection for 1x is vastly preferable to flying leads. If you want, check out my video on the basics of 1x and 10x probes...