LTC6104
9
6104f
APPLICATIONS INFORMATION
Selection of External Input Resistor, RIN
The external input resistor, RIN, controls the trans-
conductance of the current sense circuit.
Since I
transconductance g
OUT
m
==
V
R
SENSE
IN
,
1
R
For example if R
then I
V
IN
OUT
S
,,
==
100
E
ENSE
OUT
SENSE
or
I
mA for V
mV
100
1
100
.
=±
RIN should be chosen to allow the required resolution
while limiting the output current. At low supply voltage,
IOUT may be as much as ±1mA. By setting RIN such that
the largest expected sense voltage gives IOUT = ±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 RIN 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 RIN than the maximum current specication
allows may be used if the maximum current is limited in
another way, such as with a Schottky diode across RSENSE
(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 RIN value, causing
a gain error, especially for small RIN values. In addition,
internal device resistance will add approximately 0.3
Ω
to RIN.
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, ROUT
The output resistor, ROUT, determines how the output cur-
rent is converted to voltage. VOUT is simply IOUT ROUT +
VREF. In choosing an output resistor, the maximum output
voltage range must rst be considered. If the circuit that
is driven by the output does not limit the output voltage
range, then ROUT 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 ROUT must be chosen so that VOUT is in 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 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 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:
VV
I
RR
OUT
REF
OUT
IN DRIVEN
OUT
IN DRI
–
()
(
=
+
V
VEN
OUT
IR
)
.
==
100
101
099
Selection of External Voltage Reference, VREF
Selection of external reference voltage should be consid-
ered together with selection of ROUT.
Example:
Given the conditions: IOUT = –1mA to 1mA, VS = 12V.
Figure 3. Shunt Diodes Limit Maximum Input Voltage to Allow
Better Low Input Resolution Without Overranging
DSENSE
RSENSE
BATTERY
6104 F03
LOAD