4

Rev:A Date: 3/18/04

SP8126B High Speed Differential APC Amplifier

© Copyright 2004 Sipex Corporation

The

±6dB Gain adjustment is therefore done

directly in the TIA, by adjusting the active

feedback blocks proportional with the gain set-

ting resistor. The external resistor is not directly

in the signal path, and therefore any parasitic

from the off-chip connections does not affect

the signal quality. The value of the active feed-

back is controlled tightly over supply and tem-

perature changes through a Control Block with

active feedback circuitry.

Gain control is proportional with the external

duce the -6dB gain adjustment and the highest

value will produce the +6dB gain adjustment.

Please consult the specification table for the

Buffer

This stage buffers the differential signal from

the internal reference voltage. A balanced cur-

rent feedback amplifier is used for this purpose

to achieve high slew rate and fast settling.

The buffer is designed to drive high capacitive

loads. The maximum load is 50pF bulk. The

actual load is typically a flexible printed circuit

(FPC) that acts like a transmission line. This

presents a distributed capacitive load plus in-

ductance and resistance. In this case care should be

taken to match the characteristic impedance of the

line at the far end to avoid standing waves and

ringing. The buffer is designed to drive 1k

Ω to

ground. However, this resistor can be adjusted in

value to accommodate the characteristic imped-

ance of the signal trace. The output buffer ampli-

fier is designed to be stable without load and with

loads up to 50pF lumped capacitance.

THEORY OF OPERATION: Continued

180M

160M

140M

120M

100M

80M

60M

40M

0.0

250

500

750

1.0k

1.25K

1.5k

-9.5dB

0.0dB

-6.0dB

6 .0dB

TABLE 1: APC SYSTEM TARGET GAIN SET POINTS.

Gain

Sensitivity

Ts 1%

BW

Pin

(dB)

(V/W)

(

(nS)

(MHz)

(

+6

6000

1230

12

68

335

0

3000

510

7

119

666

-6

1500

225

5.5

160

1331

7.0k

6.0k

5.0k

4.0k

3.0k

2.0k

1.0k

0.0k

0.0

250

500

750

1.0k

1.25K

1.5k