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AD7893SQ-10 Datasheet(PDF) 6 Page - Analog Devices |
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AD7893SQ-10 Datasheet(HTML) 6 Page - Analog Devices |
6 / 12 page AD7893 REV. E –6– CONVERTER DETAILS The AD7893 is a fast, 12-bit single supply A/D converter. It provides the user with signal scaling (AD7893-10), track/hold, A/D converter and serial interface logic functions on a single chip. The A/D converter section of the AD7893 consists of a conventional successive-approximation converter based on an R-2R ladder structure. The signal scaling on the AD7893-10, AD7893-5 and AD7893-3 allows the part to handle ±10 V, 0 V to +5 V and ±2.5 V input signals, respectively, while operating from a single +5 V supply. The AD7893-2 accepts an analog in- put range of 0 V to +2.5 V. The part requires an external +2.5 V reference. The reference input to the part is buffered on-chip. A major advantage of the AD7893 is that it provides all of the above functions in an 8-pin package, either 8-pin mini-DIP or SOIC. This offers the user considerable space saving advantages over alternative solutions. The AD7893 typically consumes only 25 mW, making it ideal for battery-powered applications. Conversion is initiated on the AD7893 by pulsing the CONVST input. On the rising 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 oscillator circuit. Conversion time for the AD7893 is 6 µs, and the track/hold acquisition time is 1.5 µs. To obtain optimum performance from the part, the read operation should not occur during the conversion or during 600 ns prior to the next conversion. This allows the part to op- erate at throughput rates up to 117 kHz and to achieve data sheet specifications. The part can operate at higher throughput rates (up to 133 kHz) with slightly degraded performance (see Timing and Control section). CIRCUIT DESCRIPTION Analog Input Section The AD7893 is offered as four part types: the AD7893-10, which handles a ±10 V input voltage range; the AD7893-3, which handles a ±2.5 V input voltage range; the AD7893-5, which handles a 0 V to +5 V input range; and the AD7893-2, which handles a 0 V to +2.5 V input voltage range. Figure 2 shows the analog input section for the AD7893-10, AD7893-5 and AD7893-3. The analog input range of the AD7893-10 is ±10 V into an input resistance of typically 33 kΩ. The analog input range of the AD7893-3 is ±2.5 V into an input resistance of typically 12 k Ω. The input range on the AD7893-5 is 0 V to +5 V into an input resistance of typically 11 k Ω. This in- put is benign with no dynamic charging currents, as the resistor stage is followed by a high input impedance stage of the track/hold AGND AD7893-10/AD7893-5 VIN REF IN TRACK/ HOLD TO ADC REFERENCE CIRCUITRY TO INTERNAL COMPARATOR R3 R2 R1 Figure 2. AD7893-10/AD7893-3/AD7893-5 Analog Input Structure amplifier. For the AD7893-10, R1 = 30 k Ω; R2 = 7.5 kΩ and R3 = 10 k Ω. For the AD7893-3, R1 = R2 = 6.5 kΩ, and R3 is open circuit. For the AD7893-5, R1 and R3 = 5 k Ω while R2 is open-circuit. For the AD7893-10 and AD7893-3, the designed code transi- tions occur on successive integer LSB values (i.e., 1 LSB, 2 LSBs, 3 LSBs . . .). Output coding is twos complement binary with 1 LSB = FS/4096. The ideal input/output transfer function for the AD7893-10 and AD7893-3 is shown in Table I. Table I. Ideal Input/Output Code Table for the AD7893-10/ AD7893-3 Digital Output Analog Input 1 Code Transition +FSR/2 – 1 LSB 2 011 . . . 110 to 011 . . . 111 +FSR/2 – 2 LSBs 011 . . . 101 to 011 . . . 110 +FSR/2 – 3 LSBs 011 . . . 100 to 011 . . . 101 AGND + 1 LSB 000 . . . 000 to 000 . . . 001 AGND 111 . . . 111 to 000 . . . 000 AGND – 1 LSB 111 . . . 110 to 111 . . . 111 –FSR/2 + 3 LSBs 100 . . . 010 to 100 . . . 011 –FSR/2 + 2 LSBs 100 . . . 001 to 100 . . . 010 –FSR/2 + 1 LSB 100 . . . 000 to 100 . . . 001 NOTES 1FSR is full-scale range and is 20 V (AD7893-10) and = 5 V (AD7893-3) with REF IN = +2.5 V. 21 LSB = FSR/4096 = 4.883 mV (AD7893-10) and 1.22 mV (AD7893-3) with REF IN = +2.5 V. For the AD7893-5, the designed code transitions occur again on successive integer LSB values. Output coding is straight (natural) binary with 1 LSB = FS/4096 = 5 V/4096 = 1.22 mV. The ideal input/output transfer function for the AD7893-5 is shown in Table II. The analog input section for the AD7893-2 contains no biasing resistors, and the VIN pin drives the input directly to the track/ hold amplifier. The analog input range is 0 V to +2.5 V into a high impedance stage, with an input current of less than 500 nA. This input is benign, with no dynamic charging cur- rents. Once again, the designed code transitions occur on suc- cessive integer LSB values. Output coding is straight (natural) binary with 1 LSB = FS/4096 = 2.5 V/4096 = 0.61 mV. Table II also shows the ideal input/output transfer function for the AD7893-2. Table II. Ideal Input/Output Code Table for AD7893-2/AD7893-5 Digital Output Analog Input 1 Code Transition +FSR – 1 LSB2 111 . . . 110 to 111 . . . 111 +FSR – 2 LSB 111 . . . 101 to 111 . . . 110 +FSR – 3 LSB 111 . . . 100 to 111 . . . 101 AGND + 3 LSB 000 . . . 010 to 000 . . . 011 AGND + 2 LSB 000 . . . 001 to 000 . . . 010 AGND + 1 LSB 000 . . . 000 to 000 . . . 001 NOTES 1FSR is Full-Scale Range and is 5 V for AD7893-5 and 2.5 V for AD7893-2 with REF IN = +2.5 V. 21 LSB = FSR/4096 and is 1.22 mV for AD7893-5 and 0.61 mV for AD7893-2 with REF IN = +2.5 V. |
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