coming in a moment ... on air now ...
THe HP / Agilent 34401 has been around for a long time and is considered by many to be the 'gold standard' of the 6 1/2 digit class meters. It only shares this class with the Keithley 2001 for its time frame.
The newer generation machines like hte 34410 and 34411 share with the Fluk 88xx series ( see my other teardown )
These machines are only surpassed by the 3458A 8 1/2 digit meter that is in a league of it's own. even though Keithley has the model 2002 it doesn't come close in terms of throughput. The 3458 is the calibration lab's workhorse and used in many an ATE setup for mass volume testing due to its very high digitizing speed. THe 3458 is, literally, the machine used to calibrate the calibrators...
The frontpanel is a familiar sight for machines of this product line. Rubber mebrane keyboard and a starbusrst type 14 segment VFD display
The front panel has its own 87c51 to scan the keypad and refresh the display. A Sipex high voltage column driver does the VFD interfacing. The communication is level shifted on a 18 volts common rail. Mine is open here since two segments were shorted. i had to replace the driver chip.
Decent test equipment is always galvanically isolated and this machine is no different. An opticvally isolated inguard/outguard design we find two optocouplers bridging the domains.
The outguard consists of a 87c51 and a mp9914 GPIB controller.
THe brains of the machine are formed by an intel 80c196 16 bits processor and an asic common to machines of this lineage. The asic does frontpanel operations, contains uarts and has the a/d control logic on board. A static ram and eprom complete the computer part.
This machine uses a charge balancing technology vcalled Multislope III.
I Have given extensive explanation on its working in another review of a power supply that uses the same approach.
The truerms converter uses the classical AD637 chip. This particular machine has a problem with a fried tantalum cap. Tantalum caps are notorious for this kind of behavior. Any tantalum capacitor , treated with kindenss and respect, may , for whatever reason, spontaneously combust for whatever reason it feels like. that, and they may as well call them unobtainium capacitors. the worlds largest tantalum mines ( australia ) have shut down , opened again , shut down again and the future of this mineral is unclear. There have been shortages and the prices of these parts wildly fluctuates... That's why they are being phased out ...
The entire analog portion of the system occupies most of the circuit board and is normally hidden under a metal shield cover. Precision and stability is derived from a thermally stabilized buried zener of the LM199 type. ( whit round thingie )
The entire precision analog part is contained in two custom ASIC's made by agilent in their own fabs.
The large ceramic part is a precision hybrid containing the trimmed resistors for all ranges.
The PLCC device holds the analog switches and current sources. there is a smaller plcc device to the right that has some other amplification and switching circuitry. I still need to clean off the flux of the PLCC and the two opamps. This machine suffered catastrophic failure but it's up and running again now. Not bad for something fished out of the garbage skip ...
I collected the asic from a multimeter module out of an 34970 that had its brains fried ... someone plugged it on 220 while the switch was on 110... Those machines have the same multimeter guts as a 34401 but in a different form factor.
The input section holds a real quality mechanical switch to select between front and back and has latching relays for certain range switching as well as a fully shielded COTO reed relay ( big red can ). Plenty of protection is available in the form of MOV's ( blue drums ) and fat diodes ( left of yellow relays: the bridge and two big diodes above ).