Datasheet LTC6103 (Analog Devices) - 10

LTC6103
APPLICATIONS INFORMATION Error Sources
supply current can cause an output error if trace resistance between R The current sense system uses an amplifi er and resistors SENSE and +IN is signifi cant (Figure 4). to apply gain and level shift the result. The output is then EOUT(RT_+IN) = (IS • RT/RIN) • ROUT dependent on the characteristics of the amplifi er, such as Trace resistance to the –IN pin will increase the effective bias current and input offset, as well as resistor matching. RIN value, causing a gain error. In addition, internal device Ideally, the circuit output is: resistance will add approximately 0.3Ω to RIN. R Minimizing the trace resistance is important and care V = V OUT OUT SENSE • R should be taken in the PCB layout. Make the trace short IN and wide. Kelvin connection to the shunt resistor pad V = R • I SENSE SENSE SENSE should be used. In this case, the only error is due to resistor mismatch, V+ which provides an error in gain only. However, offset ILOAD RSENSE voltage, bias current and fi nite gain in the amplifi er cause LOAD additional errors. RIN R R T T
Output Error, EOUT, Due to the Amplifi er DC Offset Voltage, VOS
+IN –IN R IS E = V OUT + – OUT(VOS) OS • RIN VS The DC offset voltage of the amplifi er adds directly to the 1/2 OUT value of the sense voltage, V V– SENSE. This is the dominant LTC6103 error of the system and it limits the available dynamic R range. The paragraph, Selection of External Current Sense OUT Resistor provides details. 6103 F04
Output Error, EOUT, Due to Bias Currents Figure 4. Error Due to PCB Trace Resistance
The bias current IB(+) fl ows into the positive input of the internal op amp. I
Output Error, EOUT, Due to the Finite DC Open-Loop Gain,
B(–) fl ows into the negative input.
AOL, of the LTC6103 Amplifi er
EOUT(IBIAS) = ROUT(IB(+) • (RSENSE/RIN) – IB(–)) This error is inconsequential as the AOL of the LTC6103 Since IB(+) ≈ IB(–) = IBIAS, if RSENSE << RIN then: is very large. EOUT(IBIAS) ≈ –ROUT • IBIAS
Design Example:
For instance, if IBIAS is 100nA and ROUT is 1k, then the output error is 0.1mV. If ISENSE range = (1A to 1mA) and: V V OUT
Output Error, E
= 3
OUT, Due to PCB Trace Resistance
I A SENSE 1 The LTC6103 uses the +IN pin for both the positive amplifi er input and the positive supply input for the amplifi er. The 6103f 10