LTC6104CMS8

 High Voltage, High Side, Bi-Directional Current Sense Amplifi er ( 16 Page)

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 10 page LTC6104106104fAPPLICATIONS INFORMATIONFrom the Electrical Characteristics of the LTC6104, theoutput voltage range is 0.3V to 8V.If the circuit that is driven by the output limits the maxi-mum output voltage to ≈5V, to achieve maximum dynamicrange, VOUT should be 0.3V for –1mA IOUT and 5V for1mA IOUT.RVVmAkVVOUTREF===+=50 322350350 32265–..,.–..A standard 2.5V reference could be used in this example.With IOUT = ±1mA and ROUT = 2.2k, the output voltagerange would equal 0.3V to 4.7VVREF ConsiderationsVREF as shown in Figure 1, provides a positive offset sothat the output can swing above and below this point. It isrecommended that this is an accurate voltage reference.Most voltage references will work in this application aslong as they are able to sink and source current. Make surethat the device maintains the required voltage accuracy asthe current varies through its entire range.Error SourcesThe current sense system uses an ampliﬁer and resistorsto apply gain and level shift the result. The output is thendependent on the characteristics of the ampliﬁer, such asgain and input offset, as well as resistor matching. Ideally,the circuit output is:VVIRVRRVROUTREFOUTOUTSENSEOUTINSENSE–••,=== SSENSE SENSEI•In this case, the only error is due to resistor mismatch,which provides an error in gain only. However, offsetvoltage, bias current and ﬁnite gain in the ampliﬁer causeadditional errors.Output Error, EOUT, Due to the Ampliﬁer DC OffsetVoltage, VOSEVRROUT VOSOSOUTIN()•=The DC offset voltage of the ampliﬁer adds directly to thevalue of the sense voltage, VSENSE. This is the dominanterror of the system and it limits the available dynamicrange. The section, Selection of External Current SenseResistor, provides details.Output Error, EOUT, Due to the Bias Currents, IB+ andIB–The bias current IB+ ﬂows into the positive input of theinternal op amp. IB– ﬂows into the negative input.ERIRRIOUT IBIASOUTBSENSEINB()–•–=⎛⎝⎜⎞⎠⎟+Since IB+ ≈ IB– = IBIAS, if RSENSE << RIN then:EOUT(IBIAS) ≈ –ROUT • IBIASFor instance if IBIAS is 100nA and ROUT is 1k, the outputerror is 0.1mV.Output Error, EOUT, Due to the Finite DC Open-LoopGain, AOL, of the LTC6104 AmpliﬁerThis error is inconsequential as the AOL of the LTC6104is very large.Example:If an ISENSE range = (±1mA to ±1A) andVIVAOUTSENSE=31Then, from the Electrical Characteristics of theLTC6104:RVImVAmSENSESENSE MAXSENSE MAX≈==()()5001500ΩGGainRRVVVmVOUTINOUT MAXSENSE MAX====()()350066