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MPA4609IPFBT Datasheet(PDF) 9 Page - Texas Instruments |
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MPA4609IPFBT Datasheet(HTML) 9 Page - Texas Instruments |
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9 / 21 page  MPA4609 9 SBOS252D www.ti.com APPLICATIONS INFORMATION DIFFERENTIAL FIXED GAIN AMPLIFIER OPERATION The MPA4609 consists of four pairs of two-channel, low- noise, differential input stages that can be selected using a pair of digital control pins. These two pins, F/R, are intended to switch 4 differential preamps from the forward tape head to the reverse tape head in a tape preamp application. This multiplexing capability can be used in other applications as well. Where a high gain, quad differential output is required, one set of inputs can be used for signal transmission while the other set can be applied as a reference signal input or as a simple look-away during high overdrive conditions on the signal input. Channel switching is a fast 50ns. Figure 1 shows the internal configuration for 1 of 4 channels along with a simplified external configuration used for test. Since the MPA4609 is such a high gain device, the input signal is typically attenuated for test purposes at the input through a resistive divider network that matches the source and provides a 65 Ω differential source impedance looking out of the inputs. Those are not shown in Figure 1, but some input signal interface components were typically present when the Typical Characteristics curves were taken. All input pins are DC-biased from an internal 2.1V bandgap reference through 3k Ω resistors. In application, the total AC source impedance is dominated by the low source imped- ance of the head, giving a minimal gain for the input current noise terms. For a low output DC offset, very low input voltage offset and bias current offset terms are required. The ±0.8mV maximum input offset voltage along with the ±0.25µA typical input bias current offset give a low total output DC differential offset voltage. AC loading for the source is mini- mal given the low 3.5pF differential input capacitance looking into each of the 8 inputs. Channel select is provided in the 1st stage of the amplifier. The unselected input stage is biased down. The output stage includes a common-mode control loop referenced to the 2.1V internal bandgap reference. Output swings specified in the Specifications and Characteristics are at the internal nodes (V ′ O) prior to the series 470Ω resistors provided in each output leg. These resistors provide an easy way to adjust the overall differential gain by including an external RL. However, it is the internal output swing (V ′ O) that matters in setting the limits to performance. The 2.1V I/O DC bias voltage is intended to retain maximum differential output voltage swing when the nominal +5V single supply drops as low as +4.5V. Each output can swing ±0.75V around this bias giving a maximum 3VPP differential signal at V ′ O. This 3VPP available swing is adequate to support most differential ADC input ranges. The series 470 Ω output resis- tors also provide an easy means to bandlimit at the input of the ADC with a single capacitor. Input signals that require DC coupling can override the input common-mode reference over the specified 1.4 to 2.3V input common-mode range limits at 25 °C. MPA4609 1 of 4 Channel Pairs V I 3k Ω 470 Ω 470 Ω 3k Ω 3k Ω +5V V S 2.2 µF + 3k Ω V CM G = 190V/V 2.1V V CM 2.1V Channel Select R S = 65Ω 1:1 G = 190V/V Reverse Forward V I R S = 65Ω 1:1 F/R 0.1 µF V O ′ R L V CM 2.1V V O FIGURE 1. Typical Test Circuit for 1 of 4 Channel Pairs. |
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