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, VOUT should be 0.3V for –1mA IOUT and 5V for
1mA IOUT.
R
VV
mA
k
VV
OUT
REF
==
=+
=
50 3
2
235
03
50 3
2
265
–.
.,
.
–.
.
A standard 2.5V reference could be used in this example.
With IOUT = ±1mA and ROUT = 2.2k, the output voltage
range would equal 0.3V to 4.7V
VREF Considerations
VREF 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 amplier and resistors
to apply gain and level shift the result. The output is then
dependent on the characteristics of the amplier, such as
gain and input offset, as well as resistor matching. Ideally,
the circuit output is:
VV
I
R
V
R
VR
OUT
REF
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 nite gain in the amplier cause
additional errors.
Output Error, EOUT, Due to the Amplier DC Offset
Voltage, VOS
EV
R
OUT VOS
OS
OUT
IN
()
=
The DC offset voltage of the amplier adds directly to the
value of the sense voltage, VSENSE. 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, EOUT, Due to the Bias Currents, IB
+ and
IB
–
The bias current IB
+ ows into the positive input of the
internal op amp. IB
– ows into the negative input.
ER
I
R
I
OUT IBIAS
OUT
B
SENSE
IN
B
()
–
=
+
Since IB
+ ≈ IB– = IBIAS, if RSENSE << RIN then:
EOUT(IBIAS) ≈ –ROUT IBIAS
For instance if IBIAS is 100nA and ROUT is 1k, the output
error is 0.1mV.
Output Error, EOUT, Due to the Finite DC Open-Loop
Gain, AOL, of the LTC6104 Amplier
This error is inconsequential as the AOL of the LTC6104
is very large.
Example:
If an ISENSE 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
≈=
=
()
500
1
500
G
Gain
R
V
mV
OUT
IN
OUT MAX
SENSE MAX
==
=
()
3
500
6