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AD28MSP01 Datasheet(PDF) 16 Page - Analog Devices |
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AD28MSP01 Datasheet(HTML) 16 Page - Analog Devices |
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16 / 28 page  AD28msp01 REV. A –16– DESIGN CONSIDERATIONS Analog Input The analog input signal to the AD28msp01 must be ac coupled. Figure 12 shows the recommended input circuit for the AD28msp01’s analog input pin (VIN). The circuit of Figure 12 implements a first-order low-pass filter with a 3 dB point at 20 kHz; this is the only filter that must be implemented external to the AD28msp01 to prevent aliasing of the sampled signal. Since the AD28msp01’s ADC uses a highly oversampled ap- proach that transfers the bulk of the anti-aliasing filtering into the digital domain, the off-chip anti-aliasing filter need only be of low order. In the circuit shown in Figure 12, scaling of the analog input is achieved by the resistors RIN and RFB. The input signal gain, –RFB/RIN, can be adjusted by varying the values of these resis- tors. Total gain must be configured to ensure that a full-scale in- put signal (at CIN in Figure 12) produces a signal level at the input to the sigma-delta modulator of the ADC that does not exceed VINMAX, which is specified under “Analog Interface Elec- trical Characteristics.” If the total gain is increased above unity (i.e., gain >1), signal-to-noise (SNR + THD) performance may not meet the listed specifications. The dc offsetting of the analog input signal is accomplished with an on-chip voltage reference which nominally equals 2.5 V. The input signal must be ac coupled with an external coupling ca- pacitor (CIN). CIN and RIN should be chosen to ensure a cou- pling corner frequency of 30 Hz. CIN should be 0.1 µF or larger. VOLTAGE REFERENCE AD28msp01 CFB CIN INPUT SIGNAL RIN RFB VFB VIN Figure 12. Recommended Analog Input Circuit To select values for the components shown in Figure 12, use the following equations: Gain = – R FB R IN C IN = 1 60 π R IN C FB = 1 (2 π)(20 *10 3 ) R FB 10 k Ω ≤ R FB, RIN ≤ 50 kΩ 150 pF ≤ C FB ≤ 600 pF Figure 13 shows an example of a typical input circuit configured for 0 dB gain. The circuit’s diodes are used to prevent the input signal from exceeding maximum limits. INPUT SIGNAL 10k Ω 1.0 µF V CC 10k Ω 20k Ω 330pF GND A VOLTAGE REFERENCE AD28msp01 V FB V IN Figure 13. Typical Input Circuit (0 dB Gain) Analog Output The AD28msp01’s differential analog output (VOUTP, VOUTN) is produced by an on-chip differential amplifier. The differential amplifier can drive a minimum load of 2 k Ω (R L ≥ 2 kΩ) and has a maximum differential output voltage swing of 6.312 V peak-to-peak (3.17 dBm0). The differential output can be ac- coupled directly to a load or dc-coupled to an external amplifier. AD28msp01 V OUTP V OUTN C OUT C OUT R L Figure 14. Example Circuit for Differential Output with AC Coupling Figure 14 shows a simple circuit providing a differential output with ac coupling. The capacitor of this circuit (COUT) is op- tional; if used, its value can be chosen as follows: C OUT = 1 (60 π) R L The VOUTP–VOUTN outputs must be used as differential outputs; do not use either as a single-ended output. Figure 15 shows an example circuit which can he used to convert the differential output to a single-ended output. The circuit uses a differential- to-single-ended amplifier, the Analog Devices SSM2141. |
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