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ADS807E bảng dữ liệu(PDF) 9 Page - Texas Instruments |
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ADS807E bảng dữ liệu(HTML) 9 Page - Texas Instruments |
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9 / 21 page  ADS807 9 SBAS072A www.ti.com • 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 case 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 con- figuration and its reduced circuit complexity. While maintain- ing good SNR, driving the ADS807 with a single-ended signal will result in a reduction of the distortion performance. Employing dual-supply amplifiers and AC-coupling will usu- ally yield the best results, 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 advantage of inherently limiting their output swing to within the supply rails. Alternatively, a voltage-limiting ampli- fier, like the OPA688, may be considered to set fixed-signal limits and avoid any severe over-range condition for the A/D converter. The full-scale input range of the ADS807 is defined by the reference voltages. For example, setting the range select pin to FSSEL = LOW, and using the internal references (REFT = +3.0V and REFTB = +2.0V), the full-scale range is defined to: 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 A/D converter’s performance. The ADS807 operates on a single power supply, which requires a level shift to a ground-based bipolar input signals to comply with its input voltage range require- ments. The input of the ADS807 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 which depends on the sampling frequency. It 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 will create a 1st-order, low-pass filter in conjunction with the specified input capacitance of the ADS807. Its cutoff fre- quency can be adjusted even further by adding an external shunt capacitor from each signal input to ground. The opti- mum values of this R-C network depend on a variety of factors which include the ADS807 sampling rate, the se- lected op amp, the interface configuration, and the particular application (time domain versus frequency domain). Gener- ally, increasing the size of the series resistor and/or capacitor APPLICATION INFORMATION THEORY OF OPERATION The ADS807 is a high-speed, CMOS A/D converter which employs a pipelined converter architecture consisting of 12 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 Dia- gram). The pipeline architecture results in a data latency of 6 clock cycles. The analog input of the ADS807 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 undersampling 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 ADS807 are a very high impedance. They 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 ADS807 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 (frequency domain) applications process frequency bands not including DC. In imaging (time domain) applications, the input DC component must be maintained into the A/D converter. Features of the ADS807, including full-scale select (FSSEL), external reference, and CM output provide flexibility to ac- commodate a wide range of applications. The ADS807 should be configured to meet application objectives while observing the headroom requirements of the driving amplifi- ers to yield the best overall performance. The ADS807 input structure allows it to be driven either single-ended or differentially. Differential operation of the ADS807 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 which, in most applications, will be instrumental in achieving the best dynamic performance of the ADS807: • 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 |
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Mô tả tương tự - ADS807E |
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