EEVblog Electronics Community Forum
Products => Test Equipment => Topic started by: PaulSchmidt on May 17, 2022, 09:43:43 pm
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Greetings, all
I have been studying the schematic for the Fluke 8020A DMM, as part of a small "let's settle this argument" project resulting from the often stated "The Heathkit IM-2215" is just a kit version of the Fluke 8020A". For the record, making this comparison, it is very clear that they are not the same design in spite of their considerable similarities in outward appearance.
Even though the schematic in the Fluke manual for this meter leaves something(s) to be desired in terms of clarity and draftsmanship, I can mostly figure out all aspects of how the Fluke design works.
However, I have so far been scratching my head over the "AC Converter" (precision op-amp rectifier) part of the circuit. I have attached a sketch of this sub-circuit, and I hope that it is visible to others on this forum.
At issue is the first op-amp in the circuit (BTW, this is a dual op-amp with dual power sources above and below circuit common, which is actually just a reference point established between the + and - sides of the 9V battery). If the meter is in DC mode, the conditioned signal from the input switching and scaling circuit bypasses the AC Converter, but when in the AC mode, the conditioned signal is applied to the + input of the first op-amp via a switch, a series capacitor and a series resistor. The op-amp has direct negative feedback, so I am thinking, OK, just a voltage follower, and it buffers both AC signals as well as any DC component that might somehow be there. Then an output series capacitor has a positive feedback path through another resistor.....I don't understand how positive feedback here does anything desirable. So, that's one thing.
The first op-amp's output then goes to a resistor voltage divider, comprised of an "AC CAL" trim potentiometer and a fixed resistor (both in series with the signal) and another resistor to circuit common, feeding into the (-) input of the second op-amp. OK, the first op-amp buffers the signal which is then attenuated before being presented to the second op-amp.
The second op-amp is easier for me to understand, as it is obviously configured as a precision rectifier with two diodes in the negative feedback path. There is also a positive R/C feedback path, which appears to me to add some filtering to smooth the output. So far, so good.
But then there is ANOTHER feedback path, from the second op-amp's (-) input back, through a series resistor, to the input of the whole AC Converter circuit, but this particular feedback is only there when the meter is in DC mode. In other words, when we are not even using the AC Converter to process the signal, this feedback path is added. My head scratching has not so far conjured up why this is being done, with my best guess being it is there to give the first op-amp some kind of input when it would otherwise be left floating while the meter is in DC mode.
So, I don't 'get' the positive feedback of the first op-amp, and don't feel I understand the full/correct reason for the DC mode feedback path.
Another thing that is a bit interesting about this circuit is how, instead of just having the A/D converter's differential "IN"puts (IN HI & IN LO) measuring between circuit common and the diode-to-resistor junction of one of the two negative feedback paths of the second op-amp, as is commonly done in many DMM circuits, Fluke decided to use that differential input to measure between both of the feedback diode-to-resistor junctions....not sure what they gained by doing this.......?
Any insight on the above will be welcome.