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Current Sensing with LTC2945

Dear Sir I have questions about LTC2945. As the simplified schematic block shown in the data sheet, the delta-Vsense (current sensing) voltage is amplified by 20 and then is converted into digital bits which are stored in the register. The host MCU can read the register and obtain the digital bits, and then MCU needs a equation to convert this digital bits into a current value (current flowing in the external Rsense). However, I can't find this conversion equation in this data sheet. thanks a lot

  • The LSB read back in delta-Vsense is 25uV. You don't need to compensate for the 20x internal amplification yourself, as it is already accounted for in the 25uV value of the LSB.

    Current across the shunt can be calculated using Ohm's law: I=V/R.  So if you have 1A across the 0.02Ω shunt, the IC would measure 20mV for delta-Vsense. (delta-Vsense would be roughly 800 in this 1A example, read from IC in binary as 1100100000 and multiplied by 25uV to get the resulting 20mV).

  •  Dear mvrablic   

    Thanks for your reply!

    Some other questions about LTC2945.

    1. Is LTC2945 bidirectional or unidirectional in current sensing application?  In addition,.

    2.Do I need to connect a shunt diode(or normal diode) in parallel with Rsense so as to protect voltage drop cross  Vsense+/-  port from over-ranging ?

    3.What the maximum frequency of ac current can be measured with high reliability?

    Thanks a lot!

    1. The sense ADC alone is not bidirectional, but page 34 of the datasheet gives an example of how to use ADIN and SENSE to measure bidirectional current/power using an input current sense amplifier such as LT6105.
    2. If you expect to see current surges which create more than +/-1v across the shunt, or have an external shunt with long wires to LTC2945, then consider adding a RC low pass filter to prevent transients from exceeding the sense+ and sense- absolute maximums.  This is not necessary in most applications.  See the DC1697A schematic for a typical example with the shunt on the PCB.
    3. In continuous scan mode you have an effective refresh rate of 7.5hz.  This will be the greatest limitation for AC current that is able to be measured.