LTC6103 Linear Integrated Systems, LTC6103 Datasheet - Page 10

• Manufacturer Part Number: LTC6103
• Description: High Side Current Sense Amplifier
• Manufacturer: Linear Integrated Systems

APPLICATIONS INFORMATION
LTC6103
Error Sources
The current sense system uses an amplifi er and resistors
to apply gain and level shift the result. The output is then
dependent on the characteristics of the amplifi er, such as
bias current and input offset, as well as resistor matching.
Ideally, the circuit output is:
In this case, the only error is due to resistor mismatch,
which provides an error in gain only. However, offset
voltage, bias current and fi nite gain in the amplifi er cause
additional errors.
Output Error, E
Voltage, V
The DC offset voltage of the amplifi er adds directly to the
value of the sense voltage, V
error of the system and it limits the available dynamic
range. The paragraph, Selection of External Current Sense
Resistor provides details.
Output Error, E
The bias current I
internal op amp. I
Since I
For instance, if I
output error is 0.1mV.
Output Error, E
The LTC6103 uses the +IN pin for both the positive amplifi er
input and the positive supply input for the amplifi er. The
10
E
E
E
V
V
OUT(IBIAS)
OUT(IBIAS)
OUT VOS
OUT
SENSE
B
(
(+) ≈ I
=
OS
=
V
SENSE
)
R
=
= R
≈ –R
B
SENSE
OUT
OUT
OUT
(–) = I
V
BIAS
OS
B
OUT
B
, Due to the Amplifi er DC Offset
, Due to Bias Currents
, Due to PCB Trace Resistance
•
(+) fl ows into the positive input of the
(–) fl ows into the negative input.
OUT
R
•
•
(I
R
is 100nA and R
R
BIAS
OUT
I
B
R
IN
SENSE
• I
OUT
(+) • (R
IN
BIAS
, if R
SENSE
SENSE
SENSE
www.DataSheet4U.com
. This is the dominant
<< R
/R
OUT
IN
) – I
IN
is 1k, then the
then:
B
(–))
supply current can cause an output error if trace resistance
between R
Trace resistance to the –IN pin will increase the effective
R
resistance will add approximately 0.3Ω to R
Minimizing the trace resistance is important and care
should be taken in the PCB layout. Make the trace short
and wide. Kelvin connection to the shunt resistor pad
should be used.
Output Error, E
A
This error is inconsequential as the A
is very large.
Design Example:
If I
OL
IN
E
SENSE
I
, of the LTC6103 Amplifi er
value, causing a gain error. In addition, internal device
SENSE
OUT(RT_+IN)
V
OUT
Figure 4. Error Due to PCB Trace Resistance
range = (1A to 1mA) and:
SENSE
=
3
1
A
V
OUT
= (I
and +IN is signifi cant (Figure 4).
LOAD
, Due to the Finite DC Open-Loop Gain,
I
1/2
LTC6103
S
S
I
LOAD
• R
V
S
T
+ –
/R
R
+IN
R
SENSE
R
T
OUT
IN
OUT
) • R
V
+
R
R
–IN
IN
T
OUT
V
6103 F04
–
OL
of the LTC6103
IN
.
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