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OPA2846 Datasheet(PDF) 10 Page - Texas Instruments |
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OPA2846 Datasheet(HTML) 10 Page - Texas Instruments |
10 / 21 page MPA4609 10 SBOS252D www.ti.com CHANNEL SELECT Two pins are provided to switch between input channels. Pin 3 controls channels B and D while pin 34 controls channels A and C. These are normally tied together for tape preamp applications but can be operated separately if the application requires it. The channel select is a voltage control that switches at approximately 1.25V. Voltages less than 0.9V are guaranteed to select the REV channel while voltages > 1.6V will select the FOR channel. This allows a direct interface to 3.3V (or lower) logic outputs. Virtually no pin current is required in the logic high state while a low 70 µA/ pin out of the pin is required in the logic low state. Left unconnected, the channel select defaults to the forward selection. The channels switch in approximately 50ns typically. With no input present, there is minimal output glitching when switched. CONNECTING TO LOW IMPEDANCE MR HEADS FOR TAPE RECORDERS The MPA4609 is designed to detect and amplify the voltage signal from a Magneto Resistive (MR) head with minimal SNR degradation from the SNR intrinsic to the head. A typical MR head interface is shown in Figure 2. Not consid- ering any postfiltering, the amplifier’s 0.65nV/ √Hz input noise voltage and 90MHz bandwidth combine to produce a very low 7.7 µV RMS input referred noise floor. Even with a conser- vative 20dB S/N margin, input signals as low as 77 µV PP may be detected. With a maximum differential output of 3VPP and a 190V/V gain, a maximum input of 15.7mVPP may be applied. This wide range of inputs translates into 46dB input dynamic range. Since the MR head is biased through exter- nal voltages, it is often required to connect the signal through series blocking capacitors as shown in Figure 2. The high- pass pole created by these capacitors (CB) and the source resistance are selected to pass the lowest frequencies re- quired by the system. TWISTED PAIR RECEIVER The extremely high gain and low input noise of the MPA4609 can provide a very capable, multi-channel, twisted pair re- ceiver. This would be particularly useful for narrowband higher frequency carriers that suffer from significant cable loss. For broadband carriers, a high-pass filter should be included to reduce the low frequency receiver power. Since the MPA4609 provides a gain constant over frequency, the lower frequency region, that does not have as much loss in the twisted pair as the higher frequencies, can easily over- drive the output without the high-pass filter present. HIGH PERFORMANCE QUAD ADC DRIVER The high gain, wide bandwidth, capability of the MPA4609 can provide a very cost effective input stage to quad high performance ADCs. Figure 3 shows an example implemen- tation where a 2nd-order passive low-pass filter has been included in the interface to bandlimit the noise. The filter in this example is set to approximately 18MHz cutoff with a Butterworth (maximally flat) response including the internal 470 Ω output resistors in the design. The distortion perfor- mance of the MPA4609 will support up to 10-bit converters through approximately 10MHz maximum input analog fre- quency. For a 2VPP output, the THD shown in the Typical Characteristic Curves is ≤ 63dBc through 10MHz. The 18MHz Butterworth filter limits the noise power bandwidth for the amplifier output noise to approximately 20MHz. With a 0.65nV/ √Hz input voltage noise and 190V/V nominal gain, this 124nV/ √Hz output noise integrates to approximately 555 µV RMS at the differential input of the converter. This RMS noise is approximately 1/2 of an LSB for a 2VPP full-scale input range. C B +V B C B R BIAS R BIAS MR Element Preamp FIGURE 2. Typical MR Head Interface. FIGURE 3. Quad, ADC Input Driver. +5V 5.9 µH 470 Ω 5.9 µH 1/4 MPA4609 Quad 8 → 10 Bit ADC 1 of 4 Channels Input 67pF 18MHz, 2nd-Order Butterworth Filter 470 Ω |
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