LT1251CS datasheet(13/24 Pages) LINER

13 / 24 page

LT1251/LT1256

APPLICATIONS INFORMATION

In low gain applications, R1 and R2 are small compared to

the feedback resistors and therefore we can simplify the

equation to:

sR C

()( )

−

()

()( )

−

()

Note that the denominator causes a gain error due to the

open-loop gain (typically 0.1% for frequencies below

20kHz) and for mismatches in RF1 and RF2. A 1% mis-

match in the feedback resistors results in a 0.25% error at

K = 0.5.

If we set RF1 = RF2 and assume ROL >> RF1 (a 0.1% error

at low frequencies) the above equation simplifies to:

and

VKV A

K V A

where A

=+ −

()

This shows that the output fades linearly from input 2,

times its gain, to input 1, times its gain, as K goes from

0 to 1.

If only one input is used (for example, V1) and Pin 14 is

grounded, then the gain is proportional to K.

Similarly for the inverting case where the noninverting

inputs are grounded and the input voltages V1 and V2 drive

the normally grounded ends of RG1 and RG2, we get:

General Equation for the Inverting Amplifier Case

sR C

=−









−

()

















−

()









Note that the denominator is the same as the noninverting

case. In low gain applications, R1 and R2 are small

compared to the feedback resistors and therefore we can

simplify the equation to:

sR C

=−

−

()

−

()

Again, if we set RF1 = RF2 and assume ROL >> RF1 (a 0.1%

error at low frequencies) the above equation simplifies to:

and

VKV A

K V A

where A

=−

+ −

()

[]

The 4-resistor difference amplifier yields the same result

as the inverting amplifier case, and the common mode

rejection is independent of K.

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