Selection of External Output Resistor, ROUT
參數(shù)資料
型號: LTC6101HVBIS5#TRMPBF
廠商: Linear Technology
文件頁數(shù): 4/22頁
文件大小: 0K
描述: IC AMP CURRENT SENSE TSOT23-5
標準包裝: 500
放大器類型: 電流檢測
電路數(shù): 1
增益帶寬積: 200kHz
電流 - 輸入偏壓: 100nA
電壓 - 輸入偏移: 150µV
電流 - 電源: 350µA
電流 - 輸出 / 通道: 1mA
電壓 - 電源,單路/雙路(±): 5 V ~ 100 V
工作溫度: -40°C ~ 85°C
安裝類型: 表面貼裝
封裝/外殼: SOT-23-5 細型,TSOT-23-5
供應商設備封裝: TSOT-23-5
包裝: 帶卷 (TR)
LTC6101/LTC6101HV
12
6101fh
APPLICATIONS INFORMATION
Selection of External Output Resistor, ROUT
The output resistor, ROUT, determines how the output cur-
rent is converted to voltage. VOUT is simply IOUT ROUT.
In choosing an output resistor, the max output voltage
must rst be considered. If the circuit that is driven by
the output does not limit the output voltage, then ROUT
must be chosen such that the max output voltage does
not exceed the LTC6101 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 useful output impedance will be accept-
able. However, if the driven circuit has relatively low input
impedance, 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:
V
OUT =IOUT
R
OUT RIN(DRIVEN)
R
OUT + RIN(DRIVEN)
=I
OUT ROUT
100
101
=0.99 I
OUT ROUT
Error Sources
The current sense system uses an amplier and resistors
to apply gain and level shift the result. The output is then
dependent on the characteristics of the amplier, such as
gain and input offset, as well as resistor matching.
Ideally, the circuit output is:
V
OUT = VSENSE
R
OUT
R
IN
;V
SENSE = RSENSE ISENSE
In this case, the only error is due to resistor mismatch,
which provides an error in gain only. However, offset
voltage, bias current and nite gain in the amplier cause
additional errors:
Output Error, EOUT, Due to the Amplier DC Offset
Voltage, VOS
EOUT(VOS) = VOS (ROUT/RIN)
The DC offset voltage of the amplier adds directly to the
value of the sense voltage, VSENSE. This is the dominant
error of the system and it limits the available dynamic
range. The paragraph “Selection of External Current Sense
Resistor” provides details.
Output Error, EOUT, Due to the Bias Currents,
IB(+) and IB(–)
The bias current IB(+) ows into the positive input of the
internal op amp. IB(–) ows 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 100nA and ROUT is 1kΩ, the output
error is 0.1mV.
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 below. Under both conditions:
EOUT(IBIAS) = ± ROUT IOS; IOS = IB(+) – IB(–)
LTC6101
ROUT
VOUT
6101 F04
RIN
V+
LOAD
RSENSE
RIN
+
+
RIN
+ = RINRSENSE
V+
V
OUT
–IN
+IN
Figure 4. Second Input R Minimizes
Error Due to Input Bias Current
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