LT6350CDDPBF datasheet(14/28 Pages) LINER

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LT6350

6350f

INPUT AMPLIFIER (OP AMP 1) CHARACTERISTICS

Figure 5 shows a simpliﬁed schematic of the LT6350’s input

ampliﬁer. The input stage has NPN and PNP differential

pairs operating in parallel. This topology allows the inputs

to swing all the way from the negative supply rail to the

positive supply rail. Both differential pairs are operational

when the common mode voltage is at least 1.3V from

either rail. As the common mode voltage swings higher

than V+ – 1.3V, current source I1 saturates, and current

in PNP differential pair Q1/Q4 drops to zero. Feedback is

maintained through the NPN differential pair Q2/Q3, but the

input stage transconductance, gm, is reduced by a factor

of 2. A similar effect occurs with I2 when the common

mode voltage swings within 1.3V of the negative rail. A

precision, two-point algorithm is used to maintain near

constant offset voltage over the entire input range (see

Offset Considerations).

Negative input bias current ﬂows into the +IN1 and –IN1

inputs when the input common mode is centered between

the rails. The magnitude of this current increases when

the input common mode voltage is within 1.3V of the

negative rail and only Q1/Q4 are active. The polarity of the

current reverses when the input common mode voltage is

APPLICATIONS INFORMATION

within 1.3V of the positive rail and only Q2/Q3 are active.

Typical total change in input bias current over the entire

input common mode range is approximately 4μA. These

changes in input bias current will generate corresponding

changes in voltage across the source and gain-setting

resistors. Because the LT6350 input offset current is less

than the input bias current, matching the effective source

and feedback resistances at the input pins will reduce total

offset errors generated by changes in input bias current

and will keep distortion to a minimum.

INPUT AMPLIFIER (OP AMP 1) FEEDBACK

COMPONENTS

When feedback resistors are used to set gain in op amp 1,

care should be taken to ensure that the pole formed by the

feedback resistors and the total capacitance at the inverting

input, –IN1, does not degrade stability. For instance, to

set the LT6350 in a differential gain of +4, RF and RG of

Figure 3 could be set to 1kΩ. If the total capacitance at

–IN1 (LT6350 plus PC board) were 3pF, a new pole would

be formed in the loop response at 106MHz, which could

lead to ringing in the step response. A capacitor connected

across the feedback resistor and having the same value

DIFFERENTIAL

DRIVE

GENERATOR

–

Q10

Q11

DESD2

DESD4

DESD3

DESD1

+IN1

–IN1

DESD5

DESD6

VBIAS

V+

V–

V+

V–

V+

V–

6350 F05

OUT1

Figure 5. Input Ampliﬁer (Op Amp 1) Simpliﬁed Schematic

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