ModemHead, on your attached image, I assume the green circuitry is part of current source, and R2 and E1 are protection for voltage input? Correct me if I'm wrong.
Yes, the protection circuit marked in green is basically part of the current source, which comes out through the divider resistors and goes out to the unknown resistance through R1. (The divider resistors are re-purposed as reference resistors in ohms configuration.) As noted by Kleinstein, this has to be a low-impedance path back to the ASIC to develop enough test current with a low supply voltage, so this node has the extra protection. Note that there is a voltage sense input for this node as well (RRS=reference resistor sense). The ratio of this voltage to the one across the input terminals (OHS), and the reference resistor value, yields the unknown resistance.
So you are saying that in ohms range, if we connect 1000V to DMM, the only thing that protects it on pin 20(10) is R2 and internal diodes?
The voltage sense for the unknown resistance on the input terminals (OHS) is protected by E1 and R2, although E1 would only conduct if a massive voltage overload is present, not rated voltage. So yes, it looks like R2 and internal diodes are key.
Home
Help
Search
Login
Register
