LC2MOS High Speed, P-Compatible 8-Bit ADC with Track/Hold Function AD7821
FEATURES Fast Conversion Time: 660 ns max 100 kHz Track-and-Hold Function 1 MHz Sample Rate Unipolar and Bipolar Input Ranges Ratiometric Reference Inputs No External Clock Extended Temperature Range Operation Skinny 20-Pin DlPs, SOIC and 20-Terminal Surface Mount Packages
FUNCTIONAL BLOCK DIAGRAM
The AD7821 is a high speed, 8-bit, sampling, analog-to-digital converter that offers improved performance over the popular AD7820. It offers a conversion time of 660 ns (vs. 1.36 µs for the AD7820) and 100 kHz signal bandwidth (vs. 6.4 kHz). The sampling instant is better defined and occurs on the falling edge of WR or RD. The provision of a VSS pin (Pin 19) allows the part to operate from ± 5 V supplies and to digitize bipolar input signals. Alternatively, for unipolar inputs, the VSS pin can be grounded and the AD7821 will operate from a single +5 V supply, like the AD7820.
1. Fast Conversion Time The halflash conversion technique, coupled with fabrication on Analog Devices’ LC2MOS process, enables a very fast conversion time. The conversion time for the WRD mode is 660 ns, with 700 ns for the RD mode.
The AD7821 has a built-in track-and-hold function capable of digitizing full-scale signals up to 100 kHz max. It also uses a halflash conversion technique that eliminates the need to generate a CLK signal for the ADC.
2. Built-In Track-and-Hold This allows input signals with slew rates up to 1.6 V/µs to be converted to 8-bits without an external track-and-hold. This corresponds to a 5 V peak-to-peak, 100 kHz sine wave signal. 3. Total Unadjusted Error The AD7821 features an excellent total unadjusted error figure of less than ± 1 LSB over the full operating temperature range.
The AD7821 is designed with standard microprocessor control signals (CS, RD, WR, RDY, INT) and latched, three-state data outputs capable of interfacing to high speed data buses. An overflow output (OFL) is also provided for cascading devices to achieve higher resolution.
4. Unipolar/Bipolar Input Ranges The AD7821 is specified for single supply (+5 V) operation with a unipolar full-scale range of 0 to +5 V, and for dual supply (± 5 V) operation with a bipolar input range of ± 2.5 V. Typical performance characteristics are given for other input ranges.
The AD7821 is fabricated in Linear-Compatible CMOS (LC2MOS), an advanced, mixed technology process combining precision bipolar circuits with low power CMOS logic. The part features a low power dissipation of 50 mW.
5. Dynamic Specifications for DSP Users In addition to the traditional ADC specifications, the AD7821 is specified for ac parameters, including signal-tonoise ratio, distortion and slew rate.
REV. A Information furnished by Analog Devices is believed to be accurate and reliable. However, no responsibility is assumed by Analog Devices for its use, nor for any infringements of patents or other rights of third parties which may result from its use. No license is granted by implication or otherwise under any patent or patent rights of Analog Devices.
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