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AD871JE Datasheet(PDF) 11 Page - Analog Devices |
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AD871JE Datasheet(HTML) 11 Page - Analog Devices |
11 / 16 page AD871 REV. A –11– The wide input bandwidth and superior dynamic performance of the input THA make the AD871 suitable for sampling inputs at frequencies up to the Nyquist Rate. The input THA is designed to recover rapidly from input overdrive conditions, returning from a 50% overdrive in less than 100 ns. Because of the THA’s exceptionally wide input bandwidth, some users may find the AD871 is sensitive to noise at frequencies from 10 MHz to 50 MHz that other converters are incapable of responding to. This sensitivity can be mitigated by careful use of the differential inputs (see previous paragraphs). Additionally, Figure 18 shows how a small capacitor (10 pF – 20 pF for 50 Ω terminated inputs) may be placed between VINA and VINB to help reduce high frequency noise in applications where limiting the input bandwidth is acceptable. 1 2 VINA VINB AD871 1V 10 OR 20pF Figure 18. Optional High Frequency Noise Reduction The AD871 will contribute its own wideband thermal noise. As a result of the integrated wideband noise (0.17 LSB rms, referred-to-input), applying a dc analog input may produce more than one code at the output. A histogram of the ADC output codes, for a dc input voltage, will be between 1 and 3 codes wide, depending on how well the input is centered on a given code and how many samples are taken. Figure 8 shows a typical AD871 code histogram, and Figure 9 illustrates the AD871’s transition noise. REFERENCE INPUT The nominal reference input should be 2.5 V, taken with respect to REFERENCE GROUND (REF GND). Figure 19 illustrates the equivalent model for the reference input: there is no clock or signal-dependent activity associated with the reference input cir- cuitry, therefore no “kickback” into the reference. 1 2 REF IN REF GND AD871 5k ( 20%) AVSS Figure 19. Equivalent Reference Input Circuit However, in order to realize the lowest noise performance of the AD871, care should be taken to minimize noise at the reference input. The AD871’s reference input impedance is equal to 5 k Ω (±20%), and its effective noise bandwidth is 10 MHz, with a referred- to-input noise gain of 0.8. For example, the internal reference, with an rms noise of 28 µV (using an external 1 µF capacitor), contributes 24 µV (0.05 LSB) of noise to the transfer function of the AD871. The full-scale peak-to-peak input voltage is a function of the ref- erence voltage, according to the equation: (VINA – VINB) Full Scale = 0.8 × (V REF – REF GND) Note that the AD871’s performance was optimized for a 2.5 V reference input: performance may degrade somewhat for other reference voltages. Figure 20 illustrates the S/(N+D) perfor- mance vs. reference voltage for a 1 MHz, –0.5 dB input signal. Note also that if the reference is changed during a conversion, all three conversions in the pipeline will be invalidated. 1.5 2 3.5 3 2.5 70 60 50 REFERENCE INPUT VOLTAGE – Volts Figure 20. S/(N+D) vs. Reference Input Voltage, fIN = 1 MHz, FS = 5 MHz Table II summarizes various 2.5 V references suitable for use with the AD871, including the onboard bandgap reference (see REFERENCE OUTPUT section). Table II. Suitable 2.5 V References Drift (PPM/ C) Initial Accuracy % REF-43B 6 (max) 0.2 AD680JN 10 (max) 0.4 Internal 30 (typ) 0.4 If an external reference is connected to REF IN, REF OUT must be connected to +5 V. This should lower the current in REF GND to less than 350 µA and eliminate the need for a 1 µF capacitor, although decoupling the reference for noise reduction purposes is recommended. Alternatively, Figure 21 shows how the AD871 may be driven from other references by use of an external resistor. The exter- nal resistor forms a resistor divider with the on-chip 5 k Ω resis- tor to realize 2.5 V at the reference input pin (REF IN). A trim potentiometer is needed to accommodate the tolerance of the AD871’s 5 k Ω resistor. |
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