LTC6102
LTC6102-1/LTC6102HV
14
6102fe
For more information www.linear.com/LTC6102
the largest expected sense voltage gives I
OUT
= 1mA, then
the maximum output dynamic range is available. Output
dynamic range is limited by both the maximum allowed
output current (Note 1) and the maximum allowed output
voltage, as well as the minimum practical output signal. If
less dynamic range is required, then R
IN
can be increased
accordingly, reducing the output current and power dis-
sipation. If small sense currents must be resolved ac-
curately in a system that has very wide dynamic range, a
smaller R
IN
may be used if the max current is limited in
another way, such as with a Schottky diode across R
SENSE
(Figure 3). This will reduce the high current measurement
accuracy by limiting the result, while increasing the low
current measurement resolution. This approach can be
helpful in cases where occasional large burst currents
may be ignored.
applicaTions inForMaTion
the LTC6102, and into R
OUT
via the OUT pin. In order to
minimize gain error, –INS should be routed in a separate
path from –INF to a point as close to R
IN
as possible. In
addition, the higher potential terminal of R
IN
should be
connected directly to the positive terminal of R
SENSE
(or
any input voltage source). For the highest accuracy, R
IN
should be a four-terminal resistor if it is less than 10Ω.
Selection of External Output Resistor, R
OUT
The output resistor, R
OUT
, determines how the output cur-
rent is converted to voltage. V
OUT
is simply I
OUT
• R
OUT
.
In choosing an output resistor, the max output voltage
must first be considered. If the circuit that is driven by the
output does not have a limited input voltage, then R
OUT
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 R
OUT
must be chosen so that I
OUT(MAX)
• R
OUT
is less
than the allowed maximum input range of this circuit.
In addition, the output impedance is determined by R
OUT
. 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 R
OUT
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 R
OUT
,
then the accuracy of V
OUT
will be reduced by 1% since:
VI
RR
OUT OUT
OUT IN DRIVEN
OUT IN DRIVEN
=
+
=
•
()
()
IIR
OUT OUT OUT OUT
•• .• •
100
099=
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.
Figure 3. Shunt Diode Limits Maximum Input Voltage to Allow
Better Low Input Resolution Without Overranging
V
LOAD
D
SENSE
6102 F03
R
SENSE
Care should be taken when designing the PC board lay-
out for R
IN
, especially for small R
IN
values. All trace and
interconnect impedances will increase the effective R
IN
value, causing a gain error. It is important to note that the
large temperature drift of copper resistance will cause a
change in gain over temperature if proper care is not taken
to reduce this effect.
To further limit the effect of trace resistance on gain,
maximizing the accuracy of these circuits, the LTC6102 has
been designed with a Kelvin input. The inverting terminal
(–INS) is separate from the feedback path (–INF). During
operation, these two pins must be connected together.
The design of the LTC6102 is such that current into –INS
is input bias current only, which is typically 60pA at 25°C.
Almost all of the current from R
IN
flows into –INF, through
