Multimeter frequency response

[gjpollitt]

Hi, after connecting a function gen to a scope and configuring for 50KHz at 20 VP-P and then measuring the ac on a multimeter I read approx 38mv. If I drop the frequency then I can measure the ac amplitude fine on the meter.

Is this because the meter cannot measure rms amplitude at higher frequency signals?

thanks

 

[Nigel Goodwin]

Hi, after connecting a function gen to a scope and configuring for 50KHz at 20 VP-P and then measuring the ac on a multimeter I read approx 38mv. If I drop the frequency then I can measure the ac amplitude fine on the meter.

Is this because the meter cannot measure rms amplitude at higher frequency signals?

It's because the meter only works at low frequency AC, specifically it's designed for mains frequencies, 50Hz or 60Hz, but generally work a good bit higher. The actual spec will be given in the meters instruction leaflet, I just checked a Velleman meter I have here, and it's spec says 40Hz to 400Hz, and 400Hz is a LONG way short of 50,000Hz!.

 

[gjpollitt]

excellent, thanks for your help

my function gen only arrived today and I thought it had a fault at higher freq until I checked on the scope.

I never thought the meter was freq limited but then I wouldnt have done until now !!

 

[Dean Huster]

Digital multimeters vary widely in their ability to deal with AC voltage measurements. My Tektronix DM501 and DM502 both have compensated attenuators and they are specified at both 10KHz and 20KHz (±0.7% and ±1.6% respectively, as I recall) and says that it's useable to 100KHz, begin down only around 5%.

I've checked some of Flukes better handheld DMMs and found them to have very poor frequency response much over 1KHz. On the other hand, I've checked Simpson 260 analog VOMs and found them not too back up to around 100KHz. In fact, those "lousy analog meters" were actually more accurate at measuring 50KHz AC voltages than were any of the handheld multimeters that we had available.

Most handheld DMMs do not have compensated attenuators. The better bench DMMs do.

Dean

 

[_3iMaJ]

If you look in the manual for the DMM it show you percent accuracy vs frequency.

When I teach this lab the DMM has a lower cut off frequency than the analog meter. Typically the DMM is accurate up to about 100 KHZ and the analog meter is good up to about 2 MHz.