Datasheet

LTC6104
9
6104f
APPLICATIONS INFORMATION
Selection of External Input Resistor, R
IN
The external input resistor, R
IN
, controls the trans-
conductance of the current sense circuit.
Since I transconductance g
OUT m
==
V
R
SENSE
IN
,
1
RR
For example if R then I
V
IN
IN OUT
S
,,==100Ω
EENSE
OUT SENSE
or
I mA for V mV
100
1 100
Ω
.=± =±
R
IN
should be chosen to allow the required resolution
while limiting the output current. At low supply voltage,
I
OUT
may be as much as ±1mA. By setting R
IN
such that
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 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 maximum output current and
power dissipation. If low sense currents must be resolved
accurately in a system that has very wide dynamic range,
a smaller R
IN
than the maximum current specifi cation
allows may be used if the maximum 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.
Care should be taken when designing the printed circuit
board layout to minimize input trace resistance (to Pins
5, 6, 7 and 8). Trace and interconnect impedances to the
–IN terminals will increase the effective R
IN
value, causing
a gain error, especially for small R
IN
values. In addition,
internal device resistance will add approximately 0.3Ω
to R
IN
.
Trace and interconnect impedances to the +INB terminal will
have an effect on offset error. These errors are described
in more detail later in this data sheet.
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
+
V
REF
. In choosing an output resistor, the maximum output
voltage range must fi rst be considered. If the circuit that
is driven by the output does not limit the output voltage
range, then R
OUT
must be chosen such that the maximum
output voltage range does not exceed the LTC6104 maxi-
mum output voltage range (see Electrical Characteristics).
If the following circuit is a buffer or ADC with limited input
range, then R
OUT
must be chosen so that V
OUT
is in 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 imped-
ance, then almost any useful output impedance will be
acceptable. However, if the driven circuit has relatively low
input impedance, 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:
VVI
RR
RR
OUT REF OUT
OUT IN DRIVEN
OUT IN DRI
–•
•
()
(
=
+
VVEN
OUT OUT OUT OUT
IR IR
)
•• .••==
100
101
099
Selection of External Voltage Reference, V
REF
Selection of external reference voltage should be consid-
ered together with selection of R
OUT
.
Example:
Given the conditions: I
OUT
= –1mA to 1mA, V
S
= 12V.
Figure 3. Shunt Diodes Limit Maximum Input Voltage to Allow
Better Low Input Resolution Without Overranging
D
SENSE
R
SENSE
BATTERY
6104 F03
LOAD