Datasheet

LTC6101/LTC6101HV
12
6101fh
APPLICATIONS INFORMATION
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 fi rst be considered. If the circuit that is driven by
the output does not limit the output voltage, then R
OUT
must be chosen such that the max output voltage does
not exceed the LTC6101 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 useful output impedance will be accept-
able. 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:
V
OUT
=I
OUT
•
R
OUT
•R
IN(DRIVEN)
R
OUT
+ R
IN(DRIVEN)
=I
OUT
•R
OUT
•
100
101
= 0.99 •I
OUT
•R
OUT
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
gain and input offset, as well as resistor matching.
Ideally, the circuit output is:
V
OUT
= V
SENSE
•
R
OUT
R
IN
;V
SENSE
= R
SENSE
•I
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
E
OUT(VOS)
= V
OS
• (R
OUT
/R
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 the Bias Currents,
I
B
(+) and I
B
(–)
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, the output
error is 0.1mV.
Note that in applications where R
SENSE
≈ R
IN
, I
B
(+) causes
a voltage offset in R
SENSE
that cancels the error due to
I
B
(–) and E
OUT(IBIAS)
≈ 0. In applications where R
SENSE
<
R
IN
, the bias current error can be similarly reduced if an
external resistor R
IN
(+) = (R
IN
– R
SENSE
) is connected as
shown in Figure 4 below. Under both conditions:
E
OUT(IBIAS)
= ± R
OUT
• I
OS
; I
OS
= I
B
(+) – I
B
(–)
LTC6101
R
OUT
V
OUT
6101 F04
R
IN
–
V
+
LOAD
R
SENSE
R
IN
+
–
+
R
IN
+
= R
IN
–
– R
SENSE
V
+
V
–
OUT
–IN+IN
Figure 4. Second Input R Minimizes
Error Due to Input Bias Current