DMM Low Level AC Voltage Scales
Or, Why don't DMMs have ACV scales below 100 mV?
A tech note in progress ... begin 10/13/2013
In past writings1,2, we noted that most bench DMMs (including and especially industry standard 6 1/2 digit DMMs) are generally rated to a tenth of their lowest AC voltage 100 mV scale, or about 10 mV. Many such DMMs continue to respond to 1 mV or in some cases to 100 uV. Some DMM's gently round down to a "quiet" zero display in under-range scenarios.
So, what was different in the analog AC voltmeters from the 1960's to the mid 80's, such as, for example, the hp 400GL and related 400 series and the hp 3400A/B thermocouple meters that allowed them to be far more sensitive (low end readings of 10 uV or 100 uV)?
My impression is that there are two major differences. First, the old analog meters had many more scales, with a low full scale range of 1 mV or even 100 uV (hp 400GL, F, FL). So, in a sense, the 10% of full scale value has always been the rule of thumb. For example, the hp 400GL 100 uV scale ranges from 10 uV to 100 uV. Similarly, the hp 3400A 1 mV scale ranges from 100 uV to 1 mV.
So, why don't DMM manufacturers simply add more amplification and give us a few more low end ACV scales? Undoubtedly, there are cost concerns, with more switches, amplifiers, and passives required. However, I suspect that cost is not the main issue, or we might see more low end ACV scales on the higher end models. I suspect that the second reason is that the signal to noise ratio below the 100 mV scale precludes high resolution measurements. It might be the case that using a typical input bandwidth on the order of 20 Hz to 300 kHz, that only about 3 1/2 digits would result on a 10 mV scale, perhaps degrading further on a 1 mV scale. 2 1/2 digits might be the result for a 100 uV scale.
The lack of low end ACV scales (e.g. <100 mV full scale) might also be a reflection of low customer demand. It might be that with the decline of general interest in audio and analog design in general, low level ACV measurement has become a niche market. While it certainly remains true that every digital measurement system has some sort of analog front end, perhaps few DMM users make direct measurements at the lowest signal levels.
Well, how did the analog meters pull it off then? Analog meters are generally taken as equivalent to 3 1/2 digits at best. While it would be incredibly useful in many applications to have even 2 1/2 to 3 1/2 digits on a very sensitive bench DMM ACV scale, perhaps DMM makers do not want to deal with the inevitable criticism of why a 6 1/2 digit DMM has only two or three useful digits on its lowest (most sensitive) scale.
hp 400 GL full scale values (switch selectable): (8 ranges) 100 uV, 1 mV, 10 mV, 100 mV, 1 V, 10 V, 100 V, 1000 V
hp 400 F full scale values (switch selectable): (14 ranges) 100 uV, 300 uV, 1 mV, 3 mV, 10 mV, 30 mV, 100 mV, 300 mV, 1 V, 3 V, 10 V, 30V, 100 V, 300 V (14 ranges)
hp 3400A full scale values (switch selectable): (12 ranges) 1 mV, 3 mV, 10 mV, 30 mV, 100 mV, 300 mV, 1 V, 3 V, 10 V, 30V, 100 V, 300 V
Adding a relatively wide bandwidth front end amplifier before a bench DMM, enables the relatively low resolution measurements mentioned above. In many applications, such 2 1/2 to 3 1/2 digit resolution measurments are plenty good enough.
However, for higher resolution measurements, the input amplifier bandwidth is typically limited, sometimes severely limited to a narrow bandwidth of interest (e.g. frequency selective voltmeter techniques).
At the lowest ranges (highest gain), low noise front end amplifier design considerations come in to play to reduce the analog input noise floor.
Other common techniques include ultra-low level ACV measurements made down to nV using synchronization techniques (generally to a reference generated high level ACV signal) such as synchronized demodulators, lock-in amplifiers, and related correlation techniques. Related DSP spectral techniques useful with digitized waveforms include FFT, HRFFT, and the filter diagonalization method (FDM).
COPYRIGHT © 2013 JOSEPH M. GELLER