DP8464B datasheet(4/26 Pages) NSC

4 / 26 page

Pulse Pairing Set Up

Transformer (T1) is

Tektronix CT-2 current

probe or equivalent

TLF5283 – 3

DP8464B

f e 25 MHz

VIN e 40 mVpp differential

VREF e 050V

CD e 50 pF

RD e 430X

Filter

R1 e 240X

R2 e 680X

C1 e 15 pF

C2 e 100 pF

L1 e 47 mH

This is a 3 pole Bessel with the corner frequency at 75

MHz

TLF5283 – 4

Pulse Pairing Measurement

Connect a scope probe to pin 14 (Encoded Data Out) and

trigger off its positive edge Adjust the trigger holdoff so the

scope first triggers off the pulse associated with the positive

peak and then off the pulse associated with the negative

peak (as shown in the scope photo below) Pulse pairing is

displayed on the second pair of pulses on the display If the

second pulses are separated by 4 ns then the pulse pairing

for this part is g2 ns

Circuit Operation

The output from the readwrite amplifier is AC coupled to

the Amp Input of the DP8464B The amplifier’s output volt-

age is fed back via an external filter to an internal fullwave

rectifier and compared against the external voltage on the

VREF pin The AGC circuit adjusts the gain of the amplifier

to make the peak to peak differential voltage on the Gate

Channel Input four times the DC voltage on VREF Typically

the signal on Amp Out will be set for 4 Vpp differential

Since the filter usually hasa6dB loss the signal on the

Gate Channel Input will be 2 Vpp differential The user

should therefore set 05V on VREF which can be done with a

simple voltage divider from the a12V supply

The peak detection is performed by feeding the output of

the Amplifier through an external filter to the differentiator

The differentiator output changes state when the input pulse

changes direction generally this will be at the peaks How-

ever if the signal exhibits shouldering (the tendency to re-

turn to the baseline) the differentiator will also respond to

noise near the baseline To avoid this problem the signal is

also fed to a gating channel which is used to define a level

either side of the baseline This gating channel is comprised

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