ML4621

6

FUNCTIONAL DESCRIPTION

AMPLIFIER

The quantizer has a two stage limiting amplifier with an

1.5V). Maximum sensitivity is achieved through the use

of a DC restoration feedback loop and AC coupling on

the input. The input DC bias voltage is set by an on-chip

network at about 1.9V when it is AC coupled. These

coupling capacitors, in conjunction with the input

impedance of the amplifier, establish a highpass filter

f

C

¥¥

1

2

8000

π

(1)

Two capacitors of equal value are required since the

amplifier has a differential input. One of the coupling

signal driving the input is single ended. The high corner

frequency can also be adjusted by attaching capacitors to

CF1 and CF2. The equation for adjusting this corner is:

f

C

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1

2

425

π

(2)

The offset voltage within the amplifier will be present at

the amplifier’s output even though the input is AC

to reduce this error a DC feedback loop is incorporated.

This negative feedback loop nulls the offset voltage,

used to store the offset voltage. Although the value of

this capacitor is noncritical, the pole it creates can affect

the stability of the feedback loop. The value of this

capacitor should be at least 100 times smaller than the

input coupling capacitors to avoid stability problems

using the ML4621.

The output of the ML4621 amplifier is isolated from the

comparator and made available to the user. This allows

the user to add circuitry between the amplifier and the

comparator for wave shaping and other signal

conditioning.

COMPARATOR

Two types of comparators are employed in the output

section of these quantizers. The high speed ECL

comparator is used to provide the ECL level outputs, and

in turn drives the TTL comparator. The enable pin, CMP

ENABLE, is provided to control the ECL comparator.

When CMP ENABLE is low the comparators function

normally. When it is high it forces ECL+ high, ECL– low,

and TTL OUT high. The CMP ENABLE pin can be

controlled with TTL level signals when the quantizer is

powered by 5V and ground.

LINK MONITOR

This function is implemented by the minimum signal

discriminator and the threshold generator circuits. The

purpose of this function is to monitor the input signal and

provide a status signal indicating when the input falls

below a preset voltage level. This is done by peak

detecting the output of the amplifier section and

Figure 1. ML4621 Configured for 20MHz Bandwidth with TTL Output

VIN–

VIN+

NC

VDC

CF2

CF1

VOUT–

NC

CPEAK

VREF

VTH ADJ

GND

TTL OUT

VCC TTL

25

24

23

22

21

20

19

ML4621

5V

0.5µF

0.1µF

0.001µF

0.1µF

18pF

18pF

5V

470

Ω

5

6

7

8

9

10

11

4

3

2

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12 13 14 15 16 17 18