LTC6104I View Datasheet(PDF) - Linear Technology
Part Name
Description
Manufacturer
LTC6104I Datasheet PDF : 16 Pages
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LTC6104
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.
ROUT
=
5V – 0.3V
2mA
=
2.35k,
VREF
=
0.3 +
5
– 0.3
2
=
2.65V
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 amplifier and resistors
to apply gain and level shift the result. The output is then
dependent on the characteristics of the amplifier, such as
gain and input offset, as well as resistor matching. Ideally,
the circuit output is:
VOUT
–
VREF
=
IOUT
•
ROUT
=
VSENSE
•
ROUT
RIN
,
VSENSE = RSENSE • ISENSE
In this case, the only error is due to resistor mismatch,
which provides an error in gain only. However, offset
voltage, bias current and finite gain in the amplifier cause
additional errors.
Output Error, EOUT, Due to the Amplifier DC Offset
Voltage, VOS
EOUT(VOS)
=
VOS
•
ROUT
RIN
The DC offset voltage of the amplifier 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+ flows into the positive input of the
internal op amp. IB– flows into the negative input.
EOUT(IBIAS)
= ROUT
⎛⎝⎜IB+
•
RSENSE
RIN
– IB–⎞⎠⎟
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 Amplifier
This error is inconsequential as the AOL of the LTC6104
is very large.
Example:
If
an
ISENSE
range
=
(±1mA
to
±1A)
and
VOUT
ISENSE
=
3V
1A
Then, from the Electrical Characteristics of the
LTC6104:
RSENSE
≈
VSENSE(MAX)
ISENSE(MAX)
=
500mV
1A
=
500mΩ
Gain = ROUT = VOUT(MAX) = 3V = 6
RIN VSENSE(MAX) 500mV
6104f
10
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