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ADS808Y/250 bảng dữ liệu(PDF) 8 Page - Texas Instruments |
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ADS808Y/250 bảng dữ liệu(HTML) 8 Page - Texas Instruments |
8 / 24 page ADS808 8 SBAS179C www.ti.com APPLICATION INFORMATION THEORY OF OPERATION The ADS808 is a high-speed, high-performance, CMOS ADC built with a fully differential, 9-stage pipeline architec- ture. Each stage contains a low-resolution quantizer and digital error-correction logic, ensuring excellent differential linearity and no missing codes at the 12-bit level. The conversion process is initiated by a rising edge of the external convert clock. Once the signal is captured by the input track-and-hold amplifier, the bits are sequentially en- coded starting with the MSB. This process results in a data latency of five clock cycles, after which the output data is available as a 12-bit parallel word either coded in a straight binary or binary two’s complement format. The analog input of the ADS808 consists of a differential track-and-hold circuit, as shown in Figure 1. The differential topology produces a high level of AC-performance at high sampling rates. It also results in a very high usable input bandwidth that is especially important for IF, or undersampling applications. Both inputs (IN, IN) require external biasing up to a common-mode voltage that is typically at the mid-supply level (+VS/2). This is because the on-resistance of the CMOS switches is lowest at this voltage, minimizing the effects of the signal dependent nonlinearity of RON. The track-and-hold circuit can also convert a single-ended input signal into a fully differential signal for the quantizer. For ease of use, the ADS808 incorporates a selectable voltage reference, a ver- satile clock input, and a logic output driver designed to interface to 3V or 5V logic. particularly suited for communication systems that digitize wideband signals. Features on the ADS808, like the input range selector or the option of an external reference, provide the needed flexibility to accommodate a wide range of applications. In any case, the analog interface/driver require- ments should be carefully examined before selecting the appropriate circuit configuration. The circuit definition should include considerations on the input frequency spectrum and amplitude, single-ended versus differential driver configura- tion, as well as the available power supplies. Differential versus Single-Ended The ADS808 input structure allows it to be driven either single-ended or differentially. Differential operation of the ADS808 requires an input signal that consists of an in-phase and a 180 ° out-of-phase component simultaneously applied to the inputs (IN, IN). Differential signals offer a number of advantages that in many applications will be instrumental in achieving the best harmonic performance of the ADS808: • The signal amplitude is half of that required for the single- ended operation and is, therefore, less demanding to achieve while maintaining good linearity performance from the signal source. • The reduced signal swing allows for more headroom of the interface circuitry and, therefore, a wider selection of the best suitable driver amplifier. • Even-order harmonics are minimized. • Improves the noise immunity based on the converter’s common-mode input rejection. For the single-ended mode, the signal is applied to one of the inputs while the other input is biased with a DC voltage to the required common-mode level. Both inputs are identical in terms of their impedance and performance except that apply- ing the signal to the complementary input (IN) instead of the IN-input will invert the orientation of the input signal relative to the output code. For example, if the input driver operates in inverting mode using IN as the signal input, it will restore the phase of the signal to its original orientation. Time- domain applications may benefit from a single-ended inter- face configuration and a reduced circuit complexity. Driving the ADS808 with a single-ended signal will result in a trade- off of the excellent distortion performance, while maintaining a good signal-to-noise ratio (SNR). The trade-off of the differential input configuration over the single-ended is its increase in circuit complexity. In either case, the selection of the driver amplifier should be such that the amplifier’s perfor- mance will not degrade the A/D converter’s performance. Input Full-Scale Range versus Performance Employing dual-supply amplifiers and AC-coupling will usually yield the best results. DC-coupling and/or single-supply ampli- fiers impose additional design constrains due to their head- room requirements, especially when selecting the 2Vp-p input range. The full-scale input range of the ADS808 is defined either by the settings of the reference select pins (SEL1, SEL2) or by an external reference voltage (see Table I). T&H C IN C IN S 1 S 2 S 3 S 4 S 6 S 5 IN IN Tracking Phase: S 1, S2, S3, S4 Closed; S5, S6 Open Hold Phase: S 1, S2, S3, S4 Open; S5, S6 Closed ADS808 FIGURE 1. Simplified Circuit of Input Track-and-Hold Amplifier. DRIVING THE ANALOG INPUTS Types of Applications The analog input of the ADS808 can be configured in various ways and driven with different circuits, depending on the application and the desired level of performance. Offering a high dynamic range at high input frequencies, the ADS808 is |
Số phần tương tự - ADS808Y/250 |
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Mô tả tương tự - ADS808Y/250 |
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