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ML4621CQ

Data Quantizer

Micro Linear Corporation 

FUNCTIONAL DESCRIPTION (Continued)

VOUT+

VOS

VOUT–

ML4621

VREF

R1

VTHADJ

R2

REF

THRESHOLD

GENERATOR

Figure 2.

Figure 3.

The equation which determines the droop rate of the peak

detector is:

dV = IISET

(3)

dt C

In this equation C is the peak capacitor at CPEAK. On the

ML4621 the droop rate of the peak detector can be

adjusted two ways:

1) By adjusting the value of the peak capacitor at CPEAK.

2) By adjusting the charge current into the peak capacitor

at ISET.

The charge current, IISET, can be controlled externally by

connecting a resistor, REXT, between ISET and VCC. IISET

will then be:

IISET

=

VCC – 0.7

REXT + 1700

(4)

For convenience an on-chip current source of 125µA is

available by connecting INOM to ISET.

The threshold generator level-shifts the reference voltage

at VTHADJ through a circuit which has a temperature

coefficient matching that of the limiting amplifier. The

relationship between VTHADJ and VTH (the minimum

peak voltage at the input which will trigger the link

monitor) is:

b g VTHADJ = 600 ¥ VTH + 0.7

(5)

The on-chip reference voltage, VREF, can be tied directly

to VTHADJ to set the threshold level. This will set the

minimum input signal on the ML4621 at about 3mV

(peak). A lower threshold level can be set by dividing

down VREF with a resistor string, as in Figure 3.

Since the ML4621 has a relatively low input impedance

of 6.8kΩ and is offset by one diode drop, the equation

which accounts for the load and offset is:

cb b gg b gh VTHADJ

=

R2

¥ 6800 ¥

6800 ¥ R1

VREF

+ R2

+ 0.7 ¥ R1

+ R1 ¥ R2

(6)

THRESHOLD ADJUSTMENT EXAMPLE

To make the link monitor trigger when the received

optical power goes below 1µW (-30dBm), you first need to

calculate the resultant voltage at VIN+ and VIN–. If a

Hewlett-Packard HFBR-24X6 fiber-optic receiver with a

responsive level of 8mV/µW is used, the peak-to-peak

voltage would be:

1µW

¥

8mV

µW

=

8mVP-P

(7)

Then the link monitor should trigger at some point slightly

lower than 4mV peak. Setting VTH in Equation 5 to 3mV

and solving for VTHADJ yields:

VTHADJ = (600 × 0.003) + 0.7 = 2.5V

(8)

This is a convenient value since the reference voltage

supplied by the quantizer, VREF, is 2.5V.

The link monitor has about 0.4mV (peak) hysteresis

built-in. More hysteresis can be induced by connecting a

resistor between TTL LINK MON and VTHADJ creating a

positive feedback loop.

Refer to Micro Linear’s Application Note 6 for more

detail.

7

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