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LT6350

6350f

APPLICATIONS INFORMATION

Because the input bias current into op amp 2 is much larger

than the offset current, choosing R+IN2 to be RINT/2 greatly

reduces the offset contribution of op amp 2’s input currents

on all units. With R+IN2 = RINT/2, VOSCM reduces to:

VOSCM = VOS2 – (IOS2/2) • RINT

VOSCM is trimmed to within ±125μV with a 499Ω resistor

installed at +IN2.

The value of VOS1 is trimmed to bring VOSDIFF to ± 125μV.

Because linear modulation of VOS1 with input common

mode could degrade the common mode rejection ratio

speciﬁcation of op amp 1, and nonlinear modulation of

VOS1 could cause nonlinear gain error (distortion), VOS1

is trimmed to a low constant value over as wide an input

common mode range as possible. A precision, two-point

trim algorithm is used that results in VOS1 within ±125μV

over the input range V– + 1.3V ≤ V+IN1 ≤ V+ and VOS1

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 VOS1 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 (RG and RF) 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 non-inverting gain-of-two

conﬁguration 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+IN2

RINT

in1

en1

enRF

enRG

enRS

eno1

eno2

–IN1

+IN1

+IN2

OP AMP 1

OP AMP 2

6350 F08

enRINT

en2

enR+IN2

in2

eno

–

Figure 8. Noise Model

Note that the parallel combination of gain-setting resis-

tors RF and RG behaves like the source resistance, RS,

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,

eno = √(4 • [e2n1 + (in1RS)2 + e2nRS](1 + (RF /RG))2 + 4 • (in1RF)2 + 4e2nRF (1 + (RF /RG)) + 4e2n2 + 4e2nR+IN2 +

2e2nRINT + (in2RINT)2 + 4 • (in2R+IN2)2 )

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