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ADS808Y/250 bảng dữ liệu(PDF) 11 Page - Texas Instruments |
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ADS808Y/250 bảng dữ liệu(HTML) 11 Page - Texas Instruments |
11 / 24 page ADS808 11 SBAS179C www.ti.com attenuate some of the wideband noise. The actual compo- nent values would need to be tuned for the individual appli- cation requirements. As a guideline, resistor values are typically in the range of 10 Ω to 100Ω, capacitors in the range of 10pF to 200pF. In any case, the RIN and CIN values should have a low tolerance. This will ensure that the ADS808 sees closely matched source impedances. AC-Coupled, Differential Interface with Gain The interface circuit example presented in Figure 6 employs two OPA685s, (current-feedback op amps), optimized for gains of 8V/V or higher. The input transformer (T1) converts the single-ended input signal to a differential signal required at the amplifier’s inverting inputs, that are tuned to provide a 50 Ω impedance match to an assumed 50Ω source. To achieve the 50 Ω input match at the primary of the 1:2 transformer, the secondary input must see a 200 Ω load impedance. Both amplifiers are configured for the inverting mode resulting in close gain and phase matching of the differential signal. This technique, along with a highly sym- metrical layout, is instrumental in achieving a substantial reduction of the 2nd-harmonic, while retaining excellent 3rd- order performance. A common-mode voltage (VCM) is ap- plied to the noninverting inputs of the OPA685. Additional series of 43.2 Ω resistors isolate the output of the op amps from the capacitive load presented by the 22pF capacitors and the input capacitance of the ADS808. This 43.2 Ω/22pF combination sets a pole at approximately 167MHz and rolls off some of the wideband noise. REFERENCE REFERENCE OPERATION Integrated into the ADS808 is a bandgap reference circuit including some logic that provides a +0.5V, +0.75V, or +1V reference output by selecting the corresponding pin-strap configuration. Table I gives a complete overview of the possible reference options and pin configurations. +5V –5V OPA685 22pF 22pF +5V –5V 600 Ω OPA685 T1 1:2 50 Ω Source Noise Figure 11.8dB V CM V CM 600 Ω 100 Ω 100 Ω 43.2 Ω Power-supply decoupling not shown. 43.2 Ω V I V O A/D Converter Input V O V I = 12V/V (21.6dB) DIS DIS FIGURE 6. Wideband Differential A/D Converter Driver. DESIRED FULL-SCALE RANGE, CONNECT SEL1 CONNECT SEL2 VOLTAGE AT VREF VOLTAGE AT REFT VOLTAGE AT REFB FSR (Differential) (Pin 33) (Pin 32) (Pin 34) (Pin 41) (Pin 39) 2Vp-p (+10dBm) GND GND +1.0V +3V +2V 1.5Vp-p (+7.5dBm) GND +VS +0.75V +2.875V +2.125V 1Vp-p VREF GND +0.5V +2.75V +2.25V External Reference —— > +3.5V +2.75V to +4.5V +0.5V to +2.25V TABLE I. Reference Pin Configurations and Corresponding Voltage on the Reference Pins. |
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