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AD7895AR-10 Datasheet(PDF) 6 Page - Analog Devices |
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AD7895AR-10 Datasheet(HTML) 6 Page - Analog Devices |
6 / 12 page AD7895 –6– REV. 0 CONVERTER DETAILS The AD7895 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 AD7895 consists of a conventional successive-approximation converter based around an R-2R ladder structure. The signal scaling on the AD7895-10 and AD7895-3 allows the part to handle ±10 V and ±2.5 V input signals, respectively, while operating from a single +5 V supply. The AD7895-2 accepts an analog input 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. The AD7895 has two operating modes, the high sampling mode and the auto sleep mode, where the part automatically goes into sleep after the end of conversion. These modes are discussed in more detail in the “Timing and Control” section. A major advantage of the AD7895 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 spacing saving advantages over alternative solutions. The AD7895 consumes only 20 mW maximum, making it ideal for battery-powered applications. Conversion is initiated on the AD7895 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 oscillator circuit. Conversion time for the AD7895 is 3.8 µs in the high sampling mode (9.8 µs for the auto sleep mode), and the track/hold acquisition time is 0.3 µs. To obtain optimum performance from the part, the read operation should not occur during the conversion or during 300 ns prior to the next conversion. This allows the part to operate at throughput rates up to 192 kHz and achieve data sheet specifications. CIRCUIT DESCRIPTION Analog Input Section The AD7895 is offered as three part types: the AD7895-10, which handles a ±10 V input voltage range; the AD7895-3, which handles input voltage range ±2.5 V; and the AD7895-2, which handles a 0 V to +2.5 V input voltage range. AGND AD7895-10/AD7895-3 VIN REF IN TRACK/ HOLD TO ADC REFERENCE CIRCUITRY TO INTERNAL COMPARATOR R3 R2 R1 Figure 2. AD7895-10/AD7895-3 Analog Input Structure Figure 2 shows the analog input section for the AD7895-10 and AD7895-3. The analog input range of the AD7895-10 is ±10 V into an input resistance of typically 33 k Ω. The analog input range of the AD7895-3 is ±2.5 V into an input resistance of typically 12 k Ω. This input is benign with no dynamic charging currents, as the resistor stage is followed by a high input imped- ance stage of the track/hold amplifier. For the AD7895-10, R1 = 30 k Ω, R2 = 7.5 kΩ and R3 = 10 kΩ. For the AD7895-3, R1 = R2 = 6.5 k Ω and R3 is open circuit. For the AD7895-10 and AD7895-3, the designed code transi- tions occur on successive integer LSB values (i.e., 1 LSB, 2 LSBs, 3 LSBs . . .). Output coding is 2s complement binary with 1 LSB = FS/4096. The ideal input/output transfer function for the AD7895-10 and AD7895-3 is shown in Table I. Table I. Ideal Input/Output Code Table for the AD7895-10/-3 Digital Output Analog Inputl 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 GND + 1 LSB 000 . . . 000 to 000 . . . 001 GND 111 . . . 111 to 000 . . . 000 GND – 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 = 20 V (AD7895-10) and = 5 V (AD7895-3) with REF IN = +2.5 V. 21 LSB = FSR/4096 = 4.883 mV (AD7895-10) and 1.22 mV (AD7895-3) with REF IN = +2.5 V. The analog input section for the AD7895-2 contains no biasing resistors, and the VIN pin drives the input to the track/hold amplifier directly. 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 succes- sive integer LSB values. Output coding is straight (natural) binary with 1 LSB = FS/4096 = 2.5 V/4096 = 0.61 mV. Table II shows the ideal input/output transfer function for the AD7895-2. Table II. Ideal Input/Output Code Table for AD7895-2 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 GND + 3 LSB 000 . . . 010 to 000 . . . 011 GND + 2 LSB 000 . . . 001 to 000 . . . 010 GND + 1 LSB 000 . . . 000 to 000 . . . 001 NOTES 1FSR is full-scale range and is 2.5 V for AD7895-2 with VREF = +2.5 V. 21 LSB = FSR/4096 and is 0.61 mV for AD7895-2 with VREF = +2.5 V. Track/Hold Section The track/hold amplifier on the analog input of the AD7895 allows the ADC to accurately convert an input sine wave of full- scale amplitude to 12-bit accuracy. The input bandwidth of the track/hold is greater than the Nyquist rate of the ADC even when the ADC is operated at its maximum throughput rate of 192 kHz (i.e., the track/hold can handle input frequencies in excess of 100 kHz). |
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