Datasheet

LT6350
19
6350fc
For more information www.linear.com/LT6350
applicaTions inForMaTion
Because the input bias current into op amp 2 is much larger
than the offset current, choosing R
+IN2
to be R
INT
/2 greatly
reduces the offset contribution of op amp 2’s input currents
on all units. With R
+IN2
= R
INT
/2, VOS
CM
reduces to:
VOS
CM
= VOS2 – (IOS2/2) • R
INT
VOS
CM
is trimmed to within ±125µV with a 499Ω resistor
installed at +IN2.
The value of V
OS1
is trimmed to bring V
OSDIFF
to ± 125µV.
Because linear modulation of V
OS1
with input common
mode could degrade the common mode rejection ratio
specification of op amp 1, and nonlinear modulation of
V
OS1
could cause nonlinear gain error (distortion), V
OS1
is trimmed to a low constant value over as wide an input
common mode range as possible. A precision, 2-point
trim algorithm is used that results in V
OS1
within ±125µV
over the input range V
–
+ 1.3V ≤ V
+IN1
≤ V
+
and V
OS1
within ±300µV over the input range V
–
≤ V
+IN1
≤ V
+
. A
negative supply below –1.3V can be used to extend the
input range for which V
OS1
is within ±125µV all the way
down to ground.
As a result of the trim procedure, the lowest offsets, both
common mode and differential mode, will occur with a
499Ω resistor at +IN2. This resistor can be bypassed with
a capacitor to eliminate its noise contribution. The gain-
setting resistor network (R
G
and R
F
) impedance should
be matched to that of the source to minimize op amp 1’s
input bias current contributions to the offsets.
NOISE CONSIDERATIONS
A model showing the sources of output noise in the LT6350
is shown in Figure 8. The total output noise resulting from
all contributors is governed by the equation:
The LT6350 uses very low noise op amps, resulting in a
total differential output spot noise at 10kHz of 8.2nV/√Hz
when the LT6350 is in the noninverting gain-of-two con
-
figuration shown in Figure 2. This is equivalent to the
voltage noise of a 1015Ω resistor at the +IN1 input. For
source resistors larger than about 1k, voltage noise due to
the source resistance will start to dominate output noise.
Source resistors larger than about 13k will interact with
the input current noise and result in output noise that is
resistor noise and amplifier current noise dominant.
–
+
+
–
R
G
R
S
R
+IN2
R
F
R
INT
R
INT
i
n1
i
n1
e
n1
e
nRF
e
nRG
e
nRS
e
no1
e
no2
–IN1
+IN1
+IN2
OP AMP 1
OP AMP 2
6350 F08
e
nRINT
e
nRINT
e
n2
e
nR+IN2
i
n2
i
n2
+
e
no
–
Figure 8. Noise Model
Note that the parallel combination of gain-setting resis-
tors R
F
and R
G
behaves like the source resistance, R
S
,
from the point of view of noise calculations, and the value
should be kept below about 1k to avoid increasing the
output noise. Lower-value gain and feedback resistors,
e
no
= √(4 • [e
2
n1
+ (i
n1
R
S
)
2
+ e
2
nRS
](1 + (R
F
/R
G
))
2
+ 4 • (i
n1
R
F
)
2
+ 4e
2
nRF
(1 + (R
F
/R
G
)) + 4e
2
n2
+ 4e
2
nR+IN2
+
2e
2
nRINT
+ (i
n2
R
INT
)
2
+ 4 • (i
n2
R
+IN2
)
2
)