Re: Just got a 475, nothing showing on display


David Kuhn
 

" It makes no mention of being able to measure ripple values with a DMM"

Would not a DVm measure ripple in RMS mode? I always tend to measure
ripple in Peak to Peak mode with a scope. So any ripple you can measure
with a DVM you would multiply by 2.828??

Dave

On Wed, Jul 29, 2020 at 2:29 AM Brian Cockburn <
brian.cockburn.1959@...> wrote:

Hi,

Jake's point about having an issue or uncertainty about measuring ripple
with a multimeter reminded me about a similar question that's been in my
mind for a while.

In the service manual of the 2245A (the 'scope I have), Table 4-1 calls
for a 'DMM DC Volts range: 0 to 140 V. DC voltage accuracy: ±0.15%. 4½
digit display. e.g. Tektronix DM 501A'. It makes no mention of being able
to measure ripple values with a DMM, suggesting instead that a 'scope is
used. So is measuring ripple with a DMM valid? Is it useful given that
maximum allowable ripple is given for two distinct bands: 60-150 Hz and
20-40 kHz (which I take to mean 20 kHz to 40 kHz, rather than 20 Hz to 40
kHz). Do I need a DMM that 'goes' to at least 40 kHz on ACV to make a
useful measurement here?

For instance, my 5½ digit 220,000 count bench DMM claims to have a 3 dB
bandwidth of 300 kHz. On the 200 mVAC range it clearly doesn't meet the
required ±0.15% accuracy figure, but it's what I've got. (Accuracy is
given as %rdg + counts: 20 Hz to 50 Hz => 1.00 + 100, 50 Hz to 10 kHz =>
0.35 + 100, 10 kHz to 20 kHz => 0.6 + 200, 20 kHz to 50 kHz => 1.5 + 250,
50 kHz to 100 kHz => 5 + 400.) Do I just need to make the calculations
about my DMM's worst case error in each (of its) bands and make sure that
my measured value including the error is below the required (specified)
level?

Thanks, Brian.



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