Datasheet

LTC6104
10
6104f
APPLICATIONS INFORMATION
From the Electrical Characteristics of the LTC6104, the
output voltage range is 0.3V to 8V.
If the circuit that is driven by the output limits the maxi-
mum output voltage to ≈5V, to achieve maximum dynamic
range, V
OUT
should be 0.3V for –1mA I
OUT
and 5V for
1mA I
OUT
.
R
VV
mA
k
VV
OUT
REF
==
=+ =
503
2
235
03
503
2
265
–.
.,
.
–.
.
A standard 2.5V reference could be used in this example.
With I
OUT
= ±1mA and R
OUT
= 2.2k, the output voltage
range would equal 0.3V to 4.7V
V
REF
Considerations
V
REF
as shown in Figure 1, provides a positive offset so
that the output can swing above and below this point. It is
recommended that this is an accurate voltage reference.
Most voltage references will work in this application as
long as they are able to sink and source current. Make sure
that the device maintains the required voltage accuracy as
the current varies through its entire range.
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:
VVIR V
R
R
VR
OUT REF OUT OUT SENSE
OUT
IN
SENSE
–• •,==
=
SSENSE SENSE
I•
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 section, 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.
ERI
R
R
I
OUT IBIAS OUT B
SENSE
IN
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.
Output Error, E
OUT
, Due to the Finite DC Open-Loop
Gain, A
OL
, of the LTC6104 Amplifi er
This error is inconsequential as the A
OL
of the LTC6104
is very large.
Example:
If an I
SENSE
range = (±1mA to ±1A) and
V
I
V
A
OUT
SENSE
=
3
1
Then, from the Electrical Characteristics of the
LTC6104:
R
V
I
mV
A
m
SENSE
SENSE MAX
SENSE MAX
≈==
()
()
500
1
500 Ω
GGain
R
R
V
V
V
mV
OUT
IN
OUT MAX
SENSE MAX
== = =
()
()
3
500
66