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ADS2806Y Datasheet(PDF) 9 Page - Burr-Brown (TI) |
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ADS2806Y Datasheet(HTML) 9 Page - Burr-Brown (TI) |
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9 / 19 page  ADS2806 SBAS178B 9 www.ti.com APPLICATION INFORMATION THEORY OF OPERATION The ADS2806 integrates two high-speed CMOS ADCs and an internal reference. The ADCs utilize a pipelined converter architecture consisting of 11 internal stages. Each stage feeds its data into the digital error correction logic, ensuring excellent differential linearity and no missing codes at the 12-bit level. The output data becomes valid after the rising clock edge (see Timing Diagram). The pipeline architecture results in a data latency of 6 clock cycles. The analog input of the ADS2806 consists of a differential track-and-hold circuit. The differential topology along with tightly matched poly-poly capacitors produce a high level of AC performance at high sampling rates and in some under- sampling applications. Both inputs (IN, IN) require external biasing using a com- mon-mode voltage that is typically at the mid-supply level (+VS/2). DRIVING THE ANALOG INPUTS The analog inputs of the ADS2806 are very high impedance and should be driven through an R-C network designed to pass the highest frequency of interest. This prevents high- frequency noise in the input from affecting SFDR and SNR. The ADS2806 can be used in a wide variety of applications and deciding on the best performing analog interface circuit depends on the type of application. The circuit definition should include considerations of input frequency spectrum and amplitude, single-ended or differential drive, and avail- able power supplies. For example, communication (fre- quency domain) applications process frequency bands not including DC. In imaging (time domain) applications, the input DC component must be maintained into the ADC. Features of the ADS2806 include full-scale select (SEL), external reference, and CM output, providing flexibility to accommodate a wide range of applications. The ADS2806 should be configured to meet application objectives, while observing the headroom requirements of the driving ampli- fiers, to yield the best overall performance. The ADS2806 input structure allows it to be driven either single-ended or differentially. Differential operation of the ADS2806 requires an in-phase input signal and a 180 ° out- of-phase part simultaneously applied to the inputs (IN, IN ). The differential operation offers a number of advantages that, in most applications, will be instrumental in achieving the best dynamic performance of the ADS2806: • The signal swing is half of that required for the single- ended operation and, therefore, is less demanding to achieve while maintaining good linearity performance from the signal source. • The reduced signal swing allows for more headroom in the interface circuitry and, therefore, a wider selection of the best suitable driver op amp. • Even-order harmonics are minimized. • Improves the noise immunity based on the converter’s common-mode input rejection. Using 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 equal in terms of their impedance and performance, except that applying the signal to the complementary input (IN) instead of the IN input will invert the input signal relative to the output code. For example, in the case when the input driver operates in inverting mode, using IN as the signal input will restore the phase of the signal to its original orientation. Time-domain applications may benefit from a single-ended interface configuration and its reduced circuit complexity. Driving the ADS2806 with a single-ended signal will result in a reduction of the distortion performance, while maintaining good Signal-to-Noise Ratio (SNR). Employing dual-supply amplifiers and AC-coupling will usually yield the best re- sults, while DC-coupling and/or single-supply amplifiers impose additional design constraints due to their headroom requirements, especially when selecting the 3Vp-p input range. However, single-supply amplifiers have the advan- tage of inherently limiting their output swing to within the supply rails. Alternatively, a voltage limiting amplifier, like the OPA688, may be considered to set fixed-signal limits and avoid any severe over-range condition for the ADC. The full-scale input range of the ADS2806 is defined by the reference voltages. For example, setting the range select pin to SEL = LOW, and using the internal references (REFT = +3.0V and REFTB = +2.0V), the full-scale range is defined as: FSR = 2 • (REFT – REFB) = 2Vp-p. The trade-off of the differential input configuration versus the single-ended is its higher complexity. In either case, the selection of the driver amplifier should be such that the amplifier’s performance will not degrade the ADC’s perfor- mance. The ADS2806 operates on a single power supply that requires a level shift for ground-based bipolar input signals to comply with its input voltage range requirements. The input of the ADS2806 is of a capacitive nature and the driving source needs to provide the current to charge or discharge the input sampling capacitor while the track-and- hold is in track mode. This effectively results in a dynamic input impedance that depends on the sampling frequency. In most applications, it is recommended to add a series resistor, typically 20 Ω to 50Ω, between the drive source and the converter inputs. This will isolate the capacitive input from the source, which can be crucial to avoid gain peaking when using wideband operational amplifiers. Secondly, it |
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