Datasheet AD7821 (Analog Devices) - 9

AD7821 INHERENT TRACK-AND-HOLD
A major benefit of the AD7821’s input structure is its ability to measure a variety of high speed signals without the help of an external track-and-hold. Any ADC which does not have a built-in track-and-hold, regardless of its speed, requires the analog input to remain stable to at least 1/2 LSB for the duration of the conver- sion to maintain full accuracy. This requires the use of a track-and-hold whenever the input is a high-speed signal. The AD7821’s sampled-data comparators, by nature of their input switching, inherently accomplish this track-and-hold function. Although the conversion time for the AD7821 is 660 ns (WR-RD mode, tWR + tRD + tACC1), the time for which VIN must be stable to 1/2 LSB is much smaller. The AD7821 tracks VIN between conversions only, and its value on the falling edge of WR or RD in the WR-RD or RD modes, respectively, is the measured value. Figure 6. AD7821 Equivalent Input Circuit The input capacitors must charge to the input voltage through the
SINUSOIDAL INPUTS
on resistance of the analog switches (about 2 kΩ to 5 kΩ). In The bandwidth of the built-in track-and-hold is 100 kHz max addition, about 12 pF of input stray capacitance must be charged. (150 kHz typ, 5 V p-p). This is limited by the analog bandwidth of the comparators and timing skew between the comparator The analog input can be modeled as an equivalent RC network switches. This means that the analog input frequency can be up as shown in Figure 7. As RS (source impedance) increases, the to 100 kHz without the aid of an external track-and-hold. The input capacitance takes longer to charge. Nyquist criterion requires that the sampling rate be at least The comparators track the analog input between conversions. twice the input frequency (i.e., ≥2 ⫻ 100 kHz). This requires an A minimum delay time (tP) of 350 ns is required between ideal antialiasing filter with an infinite roll-off. To ease the prob- conversions to allow for voltage source settling and comparator lem of antialiasing filter design, the sampling rate is usually set tracking time. This allows input time constants of 50 ns without much greater than the Nyquist criterion. The maximum sampling settling time problems. Typical total input capacitance values of rate (fMAX) for the AD7821 in the WR-RD mode, (tRD < tINTL) 55 pF allow RS to be 0.9 kΩ without lengthening tP to give VIN can be calculated as follows: more time to settle. 1 f = MAX t + t + t + t WR RD RI P 1 f = MAX (0 25× − 10 6 ) + (0 25 × − 10 6 ) + (0 15 × − 10 6 ) + (0 35 × − 10 6 . ) tWR = Write Pulsewidth tRD = Delay Time between WR and RD Pulses tRI = RD to INT Delay Figure 7. RC Network Model tP = Delay Time between Conversions This permits a maximum sampling rate for the AD7821 of
INPUT TRANSIENTS
1 MHz, which is much greater than the Nyquist criterion for Transients on the analog input signal caused by charging current sampling a 100 kHz analog input signal. flowing into VIN will not normally degrade the ADC’s perfor- mance. In effect, the AD7821 does not “look” at the input when
DIGITAL SIGNAL PROCESSING APPLICATIONS
these transients occur. The comparators’ inputs track VIN and are In Digital Signal Processing (DSP) application areas such as voice not sampled until the falling edge of WR (WR-RD Mode) or recognition, echo cancellation, and adaptive filtering, the dynamic RD (RD Mode), so at least 350 ns (tP) is provided to charge the characteristics (Signal-to-Noise Ratio, Harmonic Distortion, ADC’s input capacitance. It is, therefore, not necessary to filter Intermodulation Distortion) of an ADC are critical. Since the out these transients with an external capacitor at the VIN terminal. AD7821 is a very fast ADC with a built-in track-and-hold function, it is specified dynamically as well as with standard dc specifications (Total Unadjusted Error, and so on). –8– REV. B Document Outline FEATURES GENERAL DESCRIPTION FUNCTIONAL BLOCK DIAGRAM PRODUCT HIGHLIGHTS SPECIFICATIONS Test Circuits ORDERING GUIDE ABSOLUTE MAXIMUM RATINGS PIN CONFIGURATIONS PIN FUNCTION DESCRIPTIONS TERMINOLOGY LEAST SIGNIFICANT BIT (LSB) TOTAL UNADJUSTED ERROR SLEW RATE TOTAL HARMONIC DISTORTION INTERMODULATION DISTORTION SIGNAL-TO-NOISE RATIO PEAK HARMONIC OR SPURIOUS NOISE Typical Performance Characteristics CIRCUIT INFORMATION BASIC DESCRIPTION OPERATING SEQUENCE REFERENCE AND INPUT INPUT CURRENT INPUT TRANSIENTS INHERENT TRACK-AND-HOLD SINUSOIDAL INPUTS DIGITAL SIGNAL PROCESSING APPLICATIONS SIGNAL-TO-NOISE RATIO AND DISTORTION EFFECTIVE NUMBER OF BITS INTERMODULATION DISTORTION HISTOGRAM PLOT DIGITAL INTERFACE RD Mode (MODE = 0) WR-RD Mode (MODE = 1) MICROPROCESSOR INTERFACING AD7821 – 68008 INTERFACE AD7821 – 8088 INTERFACE AD7821 – TMS32010 INTERFACE AD7821 – 8051 INTERFACE APPLYING THE AD7821 UNIPOLAR OPERATION BIPOLAR OPERATION 16-CHANNEL TELECOM A/D CONVERTER SIMULTANEOUS SAMPLING ADCS OUTLINE DIMENSIONS Revision History