Making Low Level AC Voltage Measurements below 1 mV with the
AgilentTM 34461A Digital Multimeter
A tech note in progress ... revised 6/28/2013
Before showing our latest results for the new Agilent 34461A, it should be emphasized that Agilent (no affiliation) is very clearly defining the bottom of their ACV scales at about 10% of full scale. Therefore, the "official" bottom of the 100 mV scale is 10 mV. That said, as we noted in our first report regarding the hp 34401A, inevitably users of a 6 1/2 digit DMM are going end up working below the bottom of the 100 mV ACV scale. These notes are to caution those users that a reading of 00.000 mV may not be a useful reading when measuring an input AC signal below about 1 mV.
Note that we view the 34461A, 34401A and 34410A DMMs very favorably. This testing is outside of the rated operating envelope of the Agilent 34461A.
In a previous technote, Making Low Level AC Voltage Measurements below 1-5 mV with the AgilentTM 34401A and 34410A Digital Multimeters, we first reported that the hp 34401A DMM AC voltage response below the bottom of the 100 mV ACV scale falls off gradually below 5 mV, then more quickly below 1 mV and finally indicates all zeros when there are still >100 uV of signal present.
Also, it should be noted that use of a high quality x100 or x1000 preamplifier can eliminate the user error readings discussed here.
All that said, as before, we applied about a 1 V AC signal at 1 kHz from a hp 3325B signal generator through a Kay 837 attenuator terminated by a Tektronix model no. 011-0099-005W 50 ohm terminator (it's our most accurate and stable 50 ohm load) to a Stanford SR510 Lock-in amplifier (essentially an AC voltmeter that can measure down to nanovolts because of the synchronization aspect of the measurement technique). The SR510 sync input was coupled to the hp 3325B sync output. The SR510 has a known gain error, which was corrected for. For convenience, we used the 5 mV measurement of the Agilent 34461A to normalize the SR510 gain. To get the last few readings at highest attenuation, we added a series Kay 839 attenuator.
The test signal described above was set to about 5 mV with an attenuation setting of 40 dB. Then from 66 dB attenuation (about 1 mV), we reduced the attenuator setting -1 dB at a time and recorded the AC output reading of the brand new Agilent 34461A DMM, an older Agilent 34410A (2006), a newer 34410A (2010), and a hp 34401A from about 1999.
The purple or magenta colored curve shows the low level AC response of the hp 34401A. The red and green curves are the two 34410A ("10A) DMMs. The light blue curve, which falls in between shows the new 34461A ACV response (at least for our particular 2013 unit). PDF, XLS.
34461A ACV readings begin to fall off quickly below about 300 uV. Artificial zero is achieved by about 150 uV. The reading becomes unstable (wide variation) by about 163 uV and reads all zeros below that. The 34461A is well within specification and these measurements are a decade below the advertised low scale value of 10 mV for the 100 mV scale. However, for those who inadvertently end up in this territory, perhaps this technote will help you to understand what has happened.
It is easier to see the "break points" on a log-log scale:
The 34461A is a really beautiful new instrument with an incredibly user friendly GUI. However for those applications where AC voltage measurements below about 1 mV are needed, the hp 3400A, and the hp 400 series AC volmeters, the hp 400F, hp 400FL, hp 400E, hp 400EL, and hp 400GL analog meters remain the tool of choice. Other types of instruments, such as, for example, some distortion meters, can make audio ACV measurements below 100 uV. Or, add a preamplifier before your late model DMM.
See these Agilent Application notes for more information on making ACV measurements with Agilent DMMs:
Also see this Agilent YouTube video: Digital Multimeter Tutorial Making AC Voltage Measurements
Making Low Level AC Voltage Measurements below 1 mV with the AgilentTM 34461A Digital Multimeter, Joe Geller (no affiliation with AgilentTM) PDF.
COPYRIGHT © 2013 JOSEPH M. GELLER