I just looked at 5.5 digit meters on ebay, mostly HP, and almost all show 0.5 to 1.5 volts with no probes plugged in. "Tested and working." Sure.
- posted
6 years ago
I just looked at 5.5 digit meters on ebay, mostly HP, and almost all show 0.5 to 1.5 volts with no probes plugged in. "Tested and working." Sure.
Must be all those magical MRI fields circulating around >:-} ...Jim Thompson
-- | James E.Thompson | mens | | Analog Innovations | et |
If you set a DVM to hi-z input impedance, it might read anything.
Given a choice, I prefer Fluke. The Keithley benchtops have high EMI sensitivity, and the HPs with the VF display tend to kick out ugly spikes.
-- John Larkin Highland Technology, Inc lunatic fringe electronics
Just about every LCD based scope has nasty Backlight Inverter hash. Our new Tek MSO3xxx seems to be worse than my Keysight. But you need to be fairly close to the scope to see it.
Cheers
they haven't switched to led back light yet?
I bought one just like that a couple of weeks ago, and it behaves just like that. I believe that's an expected behavior. The manual says that on the lower voltage ranges, the meter input impedance is above 10 GOhms. It takes very little ambient or leakage current to develop an appreciable voltage across that much impedance!
When connected to an actual voltage or current source or resistance, the meter I bought reads correctly to within the accuracy of the references I have available. Short the inputs with a jumper and the reading drops to microvolts.
I haven't yet tried to see if it's a good enough electrometer to sense the static charge in a comb a few inches away from a test probe, but I suspect it will work!
I have a couple of well-aged Keithley 171s that were cheap on eBay. LEDs, 5.5 digits, no worries.
Cheers
Phil Hobbs
3.5 digit meters are usually 10 Mohm (as is the HP 3478A) and they show 0 with no connection.
The HP's I see have either LCD or LED. The Fluke 8842A has VF and I was wondering about that.
The HP 3478A has two flaws Dave EEVBLOG Jones pointed out. Sometimes there's a picture of one that says "SELF TEST OK" while the ERROR segment is turned on. (He doesn't say if maybe that can be a HPIB error when nothing is connected to it.) And their calibration is in battery-backed RAM so if you change the battery it loses cal (the Fluke
8842A uses EEPROM). They don't have a second connector for a temporary battery so you have to hack in one.
That's good. The manual says impedance is 10^7 in the spec summary, but the manual appendix says 10^10 in low voltage ranges.
Somebody on the Net figured out the GPIB commands to read and write the cal RAM data nybbles. Practically the first thing I did with mine was to read the data and make multiple backups... cheap insurance.
any links to the method?
I saved the bookmarks on my computer, don't have access from my tablet while traveling. I'll post them next week.
Thanks!
Excellent.
Here's a bit of simple Python code I wrote to retrieve the cal data and store it. This uses a Python VXI11 library, which is speaking to an ICS8065 VXI11-to-GPIB gateway which then accesses the 3278A,
import vxi11 import signal import sys print("list_devices: ", vxi11.list_devices()) print("list_resources: ",vxi11.list_resources()) print "Opening instrument" instr = vxi11.Instrument("ics8065", "gpib0,23") print "Got instrument ", instr interrupted = 0 def signal_handler(signal, frame): print('You pressed Ctrl+C!') sys.exit(0) signal.signal(signal.SIGINT, signal_handler) print("Reset instrument to H0 home state") instr.write("H0"); calibration = bytearray() print("Fetching calibration array") for offset in range(256): command = bytearray(b'W') command.append(offset) response = instr.ask_raw(command) calibration.append(response) with open("calibration.dat", "wb") as outfile: outfile.write(calibration)
and here's the corresponding restore-the-calibration routine, which I have not actually executed with the CAL switch enabled.
import vxi11 import signal import sys print("list_devices: ", vxi11.list_devices()) print("list_resources: ",vxi11.list_resources()) print "Opening instrument" instr = vxi11.Instrument("ics8065", "gpib0,23") print "Got instrument ", instr interrupted = 0 def signal_handler(signal, frame): print('You pressed Ctrl+C!') sys.exit(0) signal.signal(signal.SIGINT, signal_handler) print("Reset instrument to H0 home state") instr.write("H0"); calibration = bytearray() print("Loading calibration array") with open("calibration.dat", "rb") as infile: calibration = infile.read() if len(calibration) != 256: print("Calibration data isn't exactly 256 bytes!") abort # Warning - the following will overwrite the instrument's # calibration array if the CAL switch is enabled. To allow # this, uncomment the write_raw line. for offset in range(256): command = bytearray(b'X') command.append(offset) command.append(calibration[offset]) # instr.write_raw(command)
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