Dear all,

Can anyone explain me clearly why digital multimeters have an upper limit of frequency when measuring RMS value ?
Generally after 1MHz frequency, DMM's stop measuring.
I have made some researchs and till now, i have found that it is related with impedance as in the attachment.
Impedance is inversely proportional with frequency.
But why impedance is related with error ?

Thanks in advance...

 

Everything, but everything, has an upper frequency limit. Including resistors.
Every component, every wire, has stray capacitance and stray inductance. As the frequency rises the effects of these strays increases and, in the case of a DMM, reduces the accuracy. A true RMS meter must use some active circuitry to make this measurement further limiting the available bandwidth.

 

Dear all,

Can anyone explain me clearly why digital multimeters have an upper limit of frequency when measuring RMS value ?
Generally after 1MHz frequency, DMM's stop measuring.
I have made some researchs and till now, i have found that it is related with impedance as in the attachment.
Impedance is inversely proportional with frequency.
But why impedance is related with error ?

Thanks in advance...

My first ever DMM had an op-amp front end - that was blown, which how I rescued it from a bin and got it for free.

Can't remember the op-amp type, but it was a "common or garden variety" that I had no difficulty sourcing from the junk box. For that; it should be no surprise that it doesn't get far past audio frequencies.

The ADC design also has a bearing - they might not bother using a flash ADC if the bandwidth up front isn't too impressive.