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TIDA-00879_4.5 Digit Low Power Handheld (DMM)

martin b
Intellectual 560 points
Other Parts Discussed in Thread: TIDA-00879, OPA333

Hi.

I have a newbie type question in relation to one of TI's design projects, "TIDA-00879_4.5 Digit Low Power Handheld (DMM)".

Referring to the schematic "tidrmi4.pdf", what is the purpose of U3, an OPA333. It seems to me if I try to measure 60V between J6, and J8, then current would flow through the low output impedance of the opamp? and 60V applied to the cpu, or have I missed something? How does this also influence current measurement.

Thanks

  • 0
    TI__Prodigy 70 points

    The purpose of U3, R6, and R8 is to set the DMM measurement reference voltage to ½ AVCC (or ~1.2V). This reference voltage is also connected to J6 which then serves as the common reference point between the DMM and the voltage being measured.  When 60V is applied between J6 and J8, the voltage at J6 remains at 1.2V relative to the DMM ground, and J8 thus becomes -58.8V with respect to DMM ground.  The 60V between J6 & J8 is scaled appropriately by the resistor divider network (with most of the voltage appearing across R19), so that the voltage present at pin 1 of U4 is within the operating voltage range of the system.  I’m not sure I understand your current measurement question, but the same basic reference point principle applies in this case as well.