LT1187CS8 datasheet(8/12 Pages) LINER

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LT1187

APPLICATIO S I FOR ATIO

The primary use of the LT1187 is in converting high speed

differential signals to a single-ended output. The LT1187

video difference amplifier has two uncommitted high input

impedance (+) and (–) inputs. The amplifier has another

set of inputs which can be used for reference and feed-

back. Additionally, this set of inputs give gain adjust and

DC control to the difference amplifier. The voltage gain of

the LT1187 is set like a conventional operational amplifier.

Feedback is applied to pin 8, and it is optimized for gains

of 2 or greater. The amplifier can be operated single-ended

by connecting either the (+) or (–) inputs to the +/REF (pin

1). The voltage gain is set by the resistors: (RFB + RG)/RG.

Like the single-ended case, the differential voltage gain is

set by the external resistors: (RFB + RG)/RG. The maximum

input differential signal for which the output will respond

is approximately

±0.38V.

Power Supply Bypassing

The LT1187 is quite tolerant of power supply bypassing.

In some applications a 0.1

µF ceramic disc capacitor

placed 1/2 inch from the amplifier is all that is required. In

applications requiring good settling time, it is important to

use multiple bypass capacitors. A 0.1

µF ceramic disc in

parallel with a 4.7

µF tantalum is recommended.

Calculating the Output Offset Voltage

Both input stages contribute to the output offset voltage at

pin 6. The feedback correction forces balance in the input

stages by introducing an Input VOS at pin 8. The complete

expression for the output offset voltage is:

VOUT = (VOS + IOS(RS) + IB(RREF)) × (RFB+RG)/RG + IB(RFB)

RS represents the input source resistance, typically 75Ω,

and RREF represents the finite source impedance from the

DC reference voltage, for VREF grounded, RREF = 0Ω. The

IOS is normally a small contributor and the expression

simplifies to:

VOUT = VOS(RFB+RG)/RG + IB(RFB)

If RFB is limited to 1k the last term of the equation

contributes only 2mV, since IB is less than 2µA.

ILT1187 • AI02

+ 1 REF

RREF

RFB

350

µA

345

µA

–

7V +

1.1k

4V –

Figure 1. Simplified Input Stage Schematic

V +

LT1187

V –

OUT

–

S/D

AV = +

RFB +

RFB

LT1187

OUT

–

IN DIFF

S/D

RFB

LT1187 • AI01

LT1187

+/REF

–/FB

OUT

–

S/D

VO =

RFB +

RFB

LT1187

OUT

–

S/D

AV = –

RFB +

RFB

(

( VIN DIFF

RFB

( ( VIN

+/REF

–/FB

+/REF

–/FB

+/REF

–/FB

VO = (V

RFB +

IN DIFF + VIN)

IN DIFF

–

V +

V –

V +

V –

V +

V –

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