LT1886CS8 데이터 시트보기 (PDF) - Linear Technology

부품명

상세내역

제조사

LT1886CS8

Dual 700MHz, 200mA Operational Amplifier

Linear Technology 

LT1886

APPLICATIO S I FOR ATIO

For example, reducing RBT by a factor of n = 4, and with an

amplifer gain of (1 + RF/RG) = 10 requires that RP2/RP1

=␣ 12.3.

Note that the overall gain is increased:

[ ] [ ] Vo

Vi

=

RP2 / (RP2 + RP1)

(1+ 1/ n) / (1+ RF /RG) − RP1 / (RP2 + RP1)

A simpler method of using positive feedback to reduce the

back-termination is shown in Figure 8. In this case, the

drivers are driven differentially and provide complemen-

tary outputs. Grounding the inputs, we see there is invert-

ing gain of –RF/RP from –Vo to Va

Va = Vo (RF/RP)

and assuming RP >> RL, we require

Va = Vo (1 – 1/n)

solving

RF/RP = 1 – 1/n

So to reduce the back-termination by a factor of 3 choose

RF/RP = 2/3. Note that the overall gain is increased to:

Vo/Vi = (1 + RF/RG + RF/RP)/[2(1 – RF/RP)]

ADSL Driver Requirements

The LT1886 is an ideal choice for ADSL upstream (CPE)

modems. The key advantages are: ±200mA output drive

RP2

RP1

+

Vi

VP

–

RF

RG

Va RBT Vo

RL

1886 F07

FOR RBT = RL

n

( ) ( ) 1 + RF

RG

RP1

1

=1–

RP1 + RP2

n

Vo =

Vi

RP2/(RP2 + RP1)

1 + 1/n

( ) 1 + RF

RG

– RP1

RP2 + RP1

Figure 7. Back-Termination Using Positive Feedback

with only 1.7V worst-case total supply voltage headroom,

high bandwidth, which helps achieve low distortion, low

quiescent supply current of 7mA per amplifier and a

space-saving, thermally enhanced SO-8 package.

An ADSL remote terminal driver must deliver an average

power of 13dBm (20mW) into a 100Ω line. This corre-

sponds to 1.41VRMS into the line. The DMT-ADSL peak-to-

average ratio of 5.33 implies voltage peaks of 7.53V into

the line. Using a differential drive configuration and trans-

former coupling with standard back-termination, a trans-

former ratio of 1:2 is well suited. This is shown on the front

page of this data sheet along with the distortion perfor-

mance vs line voltage at 200kHz, which is beyond ADSL

requirements. Note that the distortion is better than

–73dBc for all swings up to 16VP-P into the line. The gain

of this circuit from the differential inputs to the line voltage

is 10. Lower gains are easy to implement using the

compensation techniques of Figure 5. Table 2 shows the

drive requirements for this standard circuit.

The above design is an excellent choice for desktop

applications and draws typically 550mW of power. For

portable applications, power savings can be achieved by

reducing the back-termination resistor using positive feed-

back as shown in Figure 9. The overall gain of this circuit

is also 10, but the power consumption has been reduced

to 350mW, a savings of 36% over the previous design.

Note that the reduction of the back-termination resistor

has allowed use of a 1:1 transformer ratio.

Vi +

–

Va RBT

RF

RG

RP

RP

RG

RF

–

+

–Vi

RBT

–Va

Vo

FOR RBT = RL

n

RL

n= 1

1 – RF

RP

RL

1 + RF + RF

Vo =

Vi

RG RP

( ) 2 1 – RF

RP

–Vo

1886 F08

Figure 8. Back-Termination Using Differential Positive Feedback

12

Share Link: