Rigol DS1074z Inaccurate Voltage Measurement - 800 mV offset - Page 1
Forum section: electronics / testgear

author: radian
author_rank: Contributor
timestamp: January 29, 2017, 05:49:25 am
content: Hi. I'm trying to use my oscilloscope to measure a DC voltage, but even without a probe connected, the voltage on channel 3 is reports 800 mV. When the probe is connected and measuring a 5 V from a linear PSU, the oscilloscope reports 5.8 V, for 10 V it reports 10.8 V, etc. There is a consist offset of 800 mV.Comparing to the other channels, Channel 1 has the least offset, followed by channels 2 and 4. Channel 3 has the most voltage offset.I've tried performing a "self calibration", but this did not help.Is there something wrong with my scope? It's 2 years old but has been used maybe a dozen times. Is there a way to manually "zero" the voltage on a channel?Thanks----------------------------------Edit: Might have figured this out... the oscilloscope datasheet specifies a DC offset accuracy of "+/- 0.1 div, +/- 2 mV, +/- 1%". For higher vertical scales this offset becomes more apparent. I was trying to cram a lot on the screen, so I set channel 3 to 10 V / div, which leads to very noticeable offset of around 500 mV. Decreasing the vertical scale to a lower value reduces the offset. Interestingly 20 V/div has less apparent offset than 10 V/div... hmmm

author: tggzzz
author_rank: Super Contributor
timestamp: January 29, 2017, 09:08:27 am
content: Quote from: radian on January 29, 2017, 05:49:25 amThat's using a hammer to insert a screw (which is done by professionals, except for the last quarter turn!).But congratulations for working out the problem and actually RTFMing; if more people did that there would be fewer questions.Scopes are like analogue voltmeters: great for indicating changes and trends, but less good where accuracy is required.

author: jpb
author_rank: Super Contributor
timestamp: January 29, 2017, 12:56:33 pm
content: It can be very useful to use a scope to measure voltages and I am surprised how accurate my scope (WaveJet) is.For example I've been repairing an HP10811 OCXO and the repair manual suggests using a scope, but more importantly you need to measure both AC (at 10MHz) and dc and look at the waveforms and it is much easier to do it with just one probe.Of course the scope isn't particularly accurate but it gives a good reading to within around 1% which is good enough. You also need a scope that writes the number out rather than trying to count divisions!

author: tggzzz
author_rank: Super Contributor
timestamp: January 29, 2017, 01:33:45 pm
content: Quote from: jpb on January 29, 2017, 12:56:33 pmNeed? Only if you never learned your *2 and *5 tables. It really isn't that difficult - although I reserve the right to change my answer as I start to lose my mental facultiesWorking out the frequency is a little more tricky, but frequently mental arithmetic can give a sufficiently useful answer.BTW, congrats to the OP for not trusting their instrument (any instrument) implicitly - a valuable attribute to cultivate.

author: David Hess
author_rank: Super Contributor
timestamp: January 29, 2017, 04:36:34 pm
content: Quote from: jpb on January 29, 2017, 12:56:33 pmOn screen cursors are useless. Why did they ever include them?Also, screw slide-back measurements. Who needs an analog oscilloscope which has 0.1% precision and accuracy without cursors or a readout?Vernier caliper measurements are useless also.

author: vk6zgo
author_rank: Super Contributor
timestamp: January 30, 2017, 02:32:45 am
content: Quote from: tggzzz on January 29, 2017, 09:08:27 amTechs quite often use Oscilloscopes to measure DC voltages-----we seldom have the time to waste farting about,swapping from 'scope to DMM & back again for different measurements,as the Boss wants the job donenow!It seems EEs can be more leisurely about their troubleshooting.Oscilloscopes have the advantage that you can look at a DC supply,then check its ripple without changing anything but the 'scopes control settings.You can compare two DC voltages using the A-B setting,watch adjustable DC voltages as they vary,to check for smoothness of adjustment.QuoteIn many cases a few % is quite adequate.

author: tggzzz
author_rank: Super Contributor
timestamp: January 30, 2017, 08:56:45 am
content: Quote from: vk6zgo on January 30, 2017, 02:32:45 amAll very true, provided you respect the limitations of the tool.Unfortunately, many "digital" tools are perfectly happy to give readouts with unjustified number of significant digits - and the unwary just believe them. That's less true with analogue meters and with halfway decent DVMs.