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TSA7887ARMZ Datasheet(PDF) 11 Page - Touchstone Semiconductor Inc |
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TSA7887ARMZ Datasheet(HTML) 11 Page - Touchstone Semiconductor Inc |
11 / 20 page TSA7887 TSA7887DS r1p0 Page 11 RTFDS and performance will degrade. Figure 6 illustrates how the TSA7887’s harmonic performance as a function of frequency is affected by different source impedances. The TSA7887’s Internal 2.5-V Reference Using the REF bit (the DB5 bit) in the TSA7887’s Control Register, th e TSA7887’s internal 2.5-V reference can be enabled (DB5 cleared to “0”) or disabled (DB5 set to “1”). If enabled (the default condition), the internal voltage reference can be used in applications for other purposes and, if this is desired, the reference should be buffered by an external, precision op amp. If an external, precision voltage reference is to be used instead of the TSA7887’s internal reference, the internal reference is automatically overdriven. In this case, the TSA7887’s internal reference should be disabled by setting the REF bit in the control register. When the internal reference is disabled, switch SW1 as shown in Figure 7 opens and the input impedance seen at the AIN1/VREF pin is the reference buffer ’s input impedance, approximately in the gigaohm range (GΩ). When the internal reference is enabled, the input impedance at the AIN1/VREF pin is typically 10kΩ. When the TSA7887 is configured for two- channel operation, the TSA7887 ’s reference is set internally to VDD. TSA7887’s Power-Down Operating Modes The TSA7887 provides flexible power management to allow the user to achieve the best power performance for a given throughput rate. The four power management options are selected by programming the TSA7887’s power management bits ( “PM” Bits PM1 and PM0) in the control register as summarized in Table 6. When the PM bits are programmed for either of the auto power-down modes (PM Mode 3 or 4), the TSA7887 is powered- down on the 16th low-to-high SCLK transition after a high-to-low CS transition. The first high-to-low SCLK transition after a high-to-low CS transition powers- up the TSA7887 again. When the TSA7887 is programmed in PM Mode 1 (i.e., [PM1,PM0] = [0,0], the default condition), the TSA7887 is powered down on a low-to-high CS transition and powers up from shutdown on a high-to-low CS transition. If the CS pin is toggled low-to-high during the conversion in this operating mode, the ADC is immediately powered down. Cold-Start and Standby Power-Up Delay Times When VDD is first applied to the TSA7887 (in other words, from cold start-up), the ADC powers up in PM Mode 1 ([PM1,PM0] = [0,0]). Upon a subsequent high-to-low CS transition, the TSA7887’s power-up delay time is approximately 5μs When using an external voltage reference in single-channel operation or when the TSA7887 is powered up from standby mode (PM Mode 4), its power-up delay time is approximately 1μs because the internal reference has been either disabled (refer to Control Register DB5) or the internal reference has remained powered up (via PM Mode 4). Since the TSA7887’s power-up delay time PM Mode 4 is very short, powering up the ADC and executing a conversion with valid results in the same read/write operation is feasible. TSA7887 Power Consumption vs. Throughput Rate Considerations In operating the TSA7887 in auto-shutdown mode (PM Mode 3), in auto-standby mode (PM Mode 4), or in PM Mode 1, the average power drawn by the TSA7887 decreases at lower throughput rates. As shown in Figure 8, the average power drawn from Figure 7: TSA7887’s Integrated 2.5-V VREF Circuitry. THROUGHPUT RATE - ksps 1 0.1 0.01 0 10 30 50 20 10 Figure 8: TSA7887 Power Consumption vs Throughput Rate VDD = 3V SCLK = 2MHz VDD = 5V SCLK = 2MHz 40 |
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