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LTC6102HDD-PBF Datasheet(PDF) 11 Page - Linear Technology |
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LTC6102HDD-PBF Datasheet(HTML) 11 Page - Linear Technology |
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11 / 20 page  LTC6102/LTC6102HV 11 6102fb must be chosen such that the max output voltage does not exceed the LTC6102 max output voltage rating. If the following circuit is a buffer or ADC with limited input range, then ROUT must be chosen so that IOUT(MAX) • ROUT is less than the allowed maximum input range of this circuit. In addition, the output impedance is determined by ROUT. If the circuit to be driven has high enough input impedance, then almost any output impedance will be acceptable. However, if the driven circuit has relatively low input imped- ance, or draws spikes of current, such as an ADC might do, then a lower ROUT value may be required in order to preserve the accuracy of the output. As an example, if the input impedance of the driven circuit is 100 times ROUT, then the accuracy of VOUT will be reduced by 1% since: Error Sources The current sense system uses an amplifier and resistors to apply gain and level shift the result. The output is then dependent on the characteristics of the amplifier, such as gain and input offset, as well as resistor matching. Ideally, the circuit output is: VOUT = VSENSE • ROUT RIN ;VSENSE =RSENSE •ISENSE In this case, the only error is due to resistor mismatch, which provides an error in gain only. Output Error, EOUT, Due to the Amplifier DC Offset Voltage, VOS EOUT(VOS) = VOS • (ROUT/RIN) The DC offset voltage of the amplifier adds directly to the value of the sense voltage, VSENSE. This error is very small (3μV typ) and may be ignored for reasonable values of RIN. For very high dynamic range, this offset can be calibrated in the system due to its extremely low drift. APPLICATIONS INFORMATION Figure 4. Second Input R Minimizes Error Due to Input Bias Current VOUT =IOUT • ROUT •RIN(DRIVEN) ROUT +RIN(DRIVEN) =I OUT •ROUT • 100 101 = 0.99 •I OUT •ROUT –INF VREG 0.1μF –INS 6102 F04 RIN + = RIN– – RSENSE LTC6102 ROUT VOUT V+ LOAD RSENSE V+ V– OUT +IN RIN – RIN + Output Error, EOUT, Due to the Bias Currents, IB(+) and IB(–) The input bias current of the LTC6102 is vanishingly small. However, for very high resolution, or at high temperatures where IB increases due to leakage, the current may be significant. The bias current IB(+) flows into the positive input of the internal op amp. IB(–) flows into the negative input. EOUT(IBIAS) = ROUT((IB(+) • (RSENSE/RIN) – IB(–)) Since IB(+) ≈ IB(–) = IBIAS, if RSENSE << RIN then, EOUT(IBIAS) ≈ –ROUT • IBIAS For instance if IBIAS is 1nA and ROUT is 10k, the output error is –10μV. Note that in applications where RSENSE ≈ RIN, IB(+) causes a voltage offset in RSENSE that cancels the error due to IB(–) and EOUT(IBIAS) ≈ 0. In applications where RSENSE < RIN, the bias current error can be similarly reduced if an external resistor RIN(+) = (RIN – RSENSE) is connected as shown in Figure 4. Under both conditions: EOUT(IBIAS) = ± ROUT • IOS; IOS = IB(+) – IB(–) Adding RIN+ as described will maximize the dynamic range of the circuit. For less sensitive designs, RIN+ is not necessary. |
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