DP8464B datasheet(9/26 Pages) NSC

9 / 26 page

Application Information (Continued)

GAIN CONTROLLED AMPLIFIER

The purpose of the Gain Controlled Amplifier is to increase

the differential input signal to a fixed amplitude while main-

taining the exact shape of the input waveform The Gain

Controlled Amplifier is designed to accept input signals from

20 mVpp to 660 mVpp differential and amplify that signal to

4 Vpp differential The gain is therefore from 6 to 200 and is

controlled by the automatic gain control (AGC) loop The

amplifier output is actually capable of delivering typically 5

Vpp differential output but the parts are only tested and

guaranteed to 4 Vpp

The input to the Gain Controlled Amplifier is shown in

Figure

3 The value of the input capacitors should be selected so

that the pole formed by the coupling capacitor and the 1k

bias resistor is a factor of 10 lower than the lowest signal

frequency These input bias resistors have a g20% toler-

ance and a temperature coefficient of 005% per degree C

When the pulse detector is in the write mode these bias

resistors are automatically shunted by 425X resistors This

allows the input circuit to recover quickly from the large tran-

sients encountered during a write to read transition The

input impedance to the amplifier is therefore 1k during read

operations and 300X during write operations

The output of the Gain Controlled Amplifier is shown in

Fig-

ure 4 The outputs are biased at (12V b (075 mA c 24k)

075V) or 95V Since each output will swing g1V (4 Vpp

differential) each output pin will swing from 85V to 105V If

the total differential load placed on the output is 1k (see

Figure 5 ) then the circuit must supply 2V1k or 2 mA Since

the output is class A external resistors to ground must be

used to provide the sink current In this case in order to sink

2 mA at the lowest voltage then (85V2 mA) or an external

43k resistor from each output to ground is required Note

that the circuit has additional margin since the internal 2 mA

current sources were not included in the calculation Typi-

cally the output impedance of the Gain Controlled Amplifier

is 17X and the b3 dB bandwidth is greater than 20 MHz

TLF5283 – 8

FIGURE 3 Input to Gain Controlled Amplifier

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