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This is a autoranging DMM circuit. Specifically I would like to know what the lower op amp is for. It seems to be a non-inverting configuration with the (+) terminal at ground, but I don't what it does. Also, what are the symbols with the dashed circle? Why does the + input of the opamp go to one of them?

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  • \$\begingroup\$ This app note may help (see ohms converter block p.2.): www.intersil.com/data/an/an028.pdf The Op Amp generates a constant voltage for the ladder network. \$\endgroup\$ – pat Oct 26 '12 at 4:59
  • \$\begingroup\$ That "311" op-amp circuit uses a single supply. The + terminal reference is generated by a voltage divider without any bypassing. What is with that? \$\endgroup\$ – Kaz Oct 27 '12 at 3:52
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Yes it is a confusing schematic. THere are at least 3 poles on the rotary function switch which each have arrows to indicate the poles and the position on the DC connector changes to connect to the current source on the lower side. Meanwhile there are reed relays with logical switch control names to show when it is closed.

The 8007 is used as a constant current source driven by IGSS of FET. The output drives the probes while the ADC senses the voltage drop across the probes using sense resistors of 1K, 10K, 100K, 1000K to generate a voltage proportional to reistance controlled by Reed Relays with under and overflow detection. The binary control of A,B,C,D relays determine the value of the ladder network and also used for gain control on AC,DC input voltage.

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