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DP8464BV-3 Datasheet(PDF) 11 Page - National Semiconductor (TI) |
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DP8464BV-3 Datasheet(HTML) 11 Page - National Semiconductor (TI) |
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11 / 26 page  Application Information (Continued) AUTOMATIC GAIN CONTROL (AGC) The Automatic Gain Control holds the signal level at the Gate Channel Input at a constant level by controlling the gain of the Gain Controlled Amplifier This is necessary be- cause the amplitude of the input signal will vary with track location variations in the magnetic film and differences in the actual recording amplitude The Gain Controlled Amplifi- er is designed for a maximum 4 Vpp differential output To prevent the Gain Controlled Amplifier from saturating the VREF level must be set so the maximum amplifier output voltage is 4 Vpp The AGC will force the differential peak-to- peak signal on the Gate Channel Input to be four times the voltage applied to the VREF pin Normally some kind of filter is connected between the Gain Controlled Amplifier’s output and the Gate Channel Input Typically this filter hasa6dB insertion loss in its pass band Since the AGC holds the amplitude at the Gate Channel Input constant this 6 dB loss through the Gate Channel filter will cause the Gain Con- trolled Amplifier’s output to be 6 dB larger than the Gate Channel Input The AGC loop starts out in the high gain mode When the input signal is larger than expected the AGC loop will quick- ly reduce the amplifier gain so the peak-to-peak differential voltage on the Gate Channel Input remains four times the voltage on VREF If the input amplitude suddenly drops the AGC loop will slowly increase the amplifier gain until the differential peak-to-peak Gate Channel Input voltage again reaches four times VREF The AGC loop requires several peaks to react to an increased input signal In order to re- cover the exact peak timing during this transition the VOUT level must be set somewhat lower than the maximum of 4 Vpp For instance if the VREF is 05V and if the loss in the gate channel filter is 6 dB then the Amp Output is 4 Vpp If the Amp Input suddenly increases 30% the amplifier may saturate and the timing for a few peaks may be disturbed until the AGC reduces the amplifier gain If the peak detec- tion is critical during this time the system may fail The prop- er operation for this example is to set the VREF at 035V so the amplifier will not saturate if the input suddenly increases 30% A simplified circuit of the AGC block is shown in Figure 6 When the full wave rectified signal from the Gate Channel Input is greater than VREF the voltage on the collector of transistor T1 will increase and charge up the external ca- pacitor CAGC through T2 The typical available charging cur- rent is 25 mA Conversely if this input is less than VREF transistor T2 will be off so the capacitor CAGC will be dis- charged by the base current going into the Darlington T3 and T4 This discharge current is approximately 1 mA The voltage across CAGC controls the gain of the Gain Con- trolled Amplifier This voltage will vary from typically 34V at the highest gain to 45V at the lowest gain When the AGC circuit has not received an input signal for a long time the base current of the Darlington will discharge the external CAGC to 34V The amplifier will now be at its highest gain When a large signal comes in the external CAGC will be charged up with the 24 mA from T2 thereby reducing the gain of the amplifier The formula I e C c (dVdt) can be used to calculate the time required for the amplifier to go from a gain of 200 to a gain of 6 For in- stance if CAGC e 001 mf the charging current I is 24 mA and the dV required for the amplifier to go through its gain range is 11V then dt e (001 mF c 11V)(24 mA) or 46 ms In reality the gain does not change this quickly since the CAGC would only be charging during a portion of the input waveform By using the same argument the time required to increase the amplifier gain after the input has been suddenly reduced can be calculated This time the discharging current is only 1 maso dt e (001 mF c 11V)1 mA) or 11 ms TLF5283 – 11 FIGURE 6 Simplified AGC Circuit 11 |
Similar Part No. - DP8464BV-3 |
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Similar Description - DP8464BV-3 |
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