Datasheet

LTC6103
10
6103f
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:
VV
R
R
VRI
OUT SENSE
OUT
IN
SENSE SENSE SENSE
=
=
•
•
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
OUT
, Due to the Amplifi er DC Offset
Voltage, V
OS
EV
R
R
OUT VOS OS
OUT
IN
()
•=
The DC offset voltage of the amplifi er adds directly to the
value of the sense voltage, V
SENSE
. 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, E
OUT
, Due to Bias Currents
The bias current I
B
(+) fl ows into the positive input of the
internal op amp. I
B
(–) fl ows into the negative input.
E
OUT(IBIAS)
= R
OUT
(I
B
(+) • (R
SENSE
/R
IN
) – I
B
(–))
Since I
B
(+) ≈ I
B
(–) = I
BIAS
, if R
SENSE
<< R
IN
then:
E
OUT(IBIAS)
≈ –R
OUT
• I
BIAS
For instance, if I
BIAS
is 100nA and R
OUT
is 1k, then the
output error is 0.1mV.
Output Error, E
OUT
, Due to PCB Trace Resistance
The LTC6103 uses the +IN pin for both the positive amplifi er
input and the positive supply input for the amplifi er. The
supply current can cause an output error if trace resistance
between R
SENSE
and +IN is signifi cant (Figure 4).
E
OUT(RT_+IN)
= (I
S
• R
T
/R
IN
) • R
OUT
Trace resistance to the –IN pin will increase the effective
R
IN
value, causing a gain error. In addition, internal device
resistance will add approximately 0.3Ω to R
IN
.
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.
APPLICATIONS INFORMATION
Figure 4. Error Due to PCB Trace Resistance
+
–
+IN
OUT
V
S
I
S
1/2
LTC6103
–IN
R
IN
R
T
R
T
R
SENSE
V
+
V
–
R
OUT
6103 F04
LOAD
I
LOAD
Output Error, E
OUT
, Due to the Finite DC Open-Loop Gain,
A
OL
, of the LTC6103 Amplifi er
This error is inconsequential as the A
OL
of the LTC6103
is very large.
Design Example:
If I
SENSE
range = (1A to 1mA) and:
V
I
V
A
OUT
SENSE
=
3
1