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AD7898AR-10REEL Scheda tecnica(PDF) 8 Page - Analog Devices |
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AD7898AR-10REEL Scheda tecnica(HTML) 8 Page - Analog Devices |
8 / 16 page AD7898 –8– REV. A INPUT FREQUENCY – kHz –50 –55 –65 –75 1000 100 10 –40 –45 –60 –70 VDD = VDRIVE = 5.25V VDD = 5.0V, VDRIVE = 3.0V VDD = VDRIVE = 4.75V TPC 5. SINAD vs. Input Frequency at 220 kSPS TPC 5 shows a graph of Signal to (Noise + Distortion) ratio versus Input Frequency for various supply voltages while sampling at 220 kSPS. The on-chip track-and-hold can accommodate frequencies up to 4.7 MHz for AD7898-3, and up to 3.6 MHz for AD7898-10, making the AD7898 ideal for subsampling applications. Noise In an A/D converter, noise exhibits itself as a code uncertainty in dc applications, and as the noise floor (in an FFT, for example) in ac applications. In a sampling A/D converter like the AD7898, all information about the analog input appears in the baseband, from dc to half the sampling frequency. The input bandwidth of the track/hold exceeds the Nyquist bandwidth and, therefore, an antialiasing filter should be used to remove unwanted signals above fS/2 in the input signal in applications where such signals exist. TPC 6 shows a histogram plot for 8192 conversions of a dc input using the AD7898. The analog input was set at the center of a code transition. It can be seen that almost all the codes appear in one output bin, indicating very good noise perfor- mance from the ADC. 5000 4000 2000 0 2050 6000 3000 1000 5500 4500 2500 500 3500 1500 2049 2048 2047 2046 2045 2044 6500 TPC 6. Histogram of 8192 Conversions of a DC Input CONVERTER DETAILS The AD7898 is a fast, 12-bit single supply A/D converter. It provides the user with signal scaling, track/hold, A/D converter, and serial interface logic functions on a single chip. The A/D converter section of the AD7898 consists of a conventional successive-approximation converter based around an R-2R ladder structure. The signal scaling on the AD7898-10 and AD7898-3 allows the part to handle ±10 V and ±2.5 V input signals, respectively, while operating from a single 5 V supply. The part requires an external 2.5 V reference. The reference input to the part is buffered on-chip. The AD7898 has two operating modes, an internal clocking mode using an on-chip oscillator and an external clocking mode using the SCLK as the master clock. The latter mode features a power-down mechanism. These modes are discussed in more detail in the Operating Modes section. A major advantage of the AD7898 is that it provides all of the above functions in an 8-lead SOIC package. This offers the user considerable spacing saving advantages over alternative solutions. The AD7898 consumes only 22.5 mW maximum, making it ideal for battery-powered applications. In Mode 0 operation, conversion is initiated on the AD7898 by pulsing the CONVST input. On the falling edge of CONVST, the on-chip track/hold goes from track to hold mode, and the conversion sequence is started. The conversion clock for the part is generated internally using a laser-trimmed clock oscilla- tor circuit. Conversion time for the AD7898 is 3.3 µs, and the quiet time is 0.1 µs. To obtain optimum performance from the part in Mode 0, the read operation should not occur during the conversion. In Mode 1 operation, conversion is initiated on the AD7898 by the falling edge of CS. Sixteen SCLK cycles are required to complete the conversion and access the conversion result, after which time CS may be brought high. The internal oscillator is not used as the conversion clock in this mode as the SCLK is used instead. The maximum SCLK frequency is 3.7 MHz in Mode 1 providing a minimum conversion time of 4.33 µs. As in Mode 0, another conversion should not be initiated during the quiet time after the end of conversion. Both of these modes of operation allow the part to operate at throughput rates up to 220 kHz and achieve data sheet specifications. CIRCUIT DESCRIPTION Analog Input Section The AD7898 is offered as two part types: the AD7898-10, which handles a ±10 V input voltage range; the AD7898-3, which handles input voltage range ±2.5 V. VIN AGND VREF TO INTERNAL COMPARATOR TRACK/HOLD TO ADC REFERENCE CIRCUITRY R2 R3 R1 AD7898-10/AD7898-3 Figure 2. Analog Input Structure |
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