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AD7863ARS-10REEL Scheda tecnica(PDF) 9 Page - Analog Devices |
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AD7863ARS-10REEL Scheda tecnica(HTML) 9 Page - Analog Devices |
9 / 24 page AD7863 Rev. B | Page 9 of 24 CONVERTER DETAILS The AD7863 is a high speed, low power, dual 14-bit analog-to- digital converter that operates from a single 5 V supply. The part contains two 5.2 μs successive approximation ADCs, two track-and-hold amplifiers, an internal 2.5 V reference, and a high speed parallel interface. Four analog inputs are grouped into two channels (A and B) selected by the A0 input. Each channel has two inputs (VA1 and VA2 or VB1 and VB2) that can be sampled and converted simultaneously, thus preserving the relative phase information of the signals on both analog inputs. The part accepts an analog input range of ±10 V (AD7863-10), ±2.5 V (AD7863-3), and 0 V to 2.5 V (AD7863-2). Overvoltage protection on the analog inputs for the part allows the input voltage to go to ±17 V, ±7 V, or +7 V, respectively, without causing damage. The AD7863 has two operating modes, the high sampling mode and the auto sleep mode, where the part auto- matically goes into sleep after the end of conversion. These modes are discussed in more detail in the Timing and Control section. Conversion is initiated on the AD7863 by pulsing the CONVST input. On the falling edge of CONVST, both on-chip track-and- holds are simultaneously placed into hold and the conversion sequence is started on both channels. The conversion clock for the part is generated internally using a laser-trimmed clock oscillator circuit. The BUSY signal indicates the end of conversion and at this time the conversion results for both channels are available to be read. The first read after a conver- sion accesses the result from VA1 or VB1, and the second read accesses the result from VA2 or VB2, depending on whether the multiplexer select A0 is low or high, respectively, before the conversion is initiated. Data is read from the part via a 14-bit parallel data bus with standard CS and RD signals. Conversion time for the AD7863 is 5.2 μs in the high sampling mode (10 μs for the auto sleep mode), and the track/hold acquisition time is 0.5 μs. To obtain optimum performance from the part, the read operation should not occur during the conversion or during the 400 ns prior to the next conversion. This allows the part to operate at throughput rates up to 175 kHz and achieve data sheet specifications. TRACK-AND-HOLD SECTION The track-and-hold amplifiers on the AD7863 allow the ADCs to accurately convert an input sine wave of full-scale amplitude to 14-bit accuracy. The input bandwidth of the track-and-hold is greater than the Nyquist rate of the ADC, even when the ADC is operated at its maximum throughput rate of 175 kHz (that is, the track-and hold can handle input frequencies in excess of 87.5 kHz). The track-and-hold amplifiers acquire input signals to 14-bit accuracy in less than 500 ns. The operation of the track-and- holds is essentially transparent to the user. The two track-and-hold amplifiers sample their respective input channels simultaneously, on the falling edge of CONVST. The aperture time for the track-and-holds (that is, the delay time between the external CONVST signal and the track-and-hold actually going into hold) is well-matched across the two track-and-holds on one device and also well-matched from device to device. This allows the relative phase information between different input channels to be accurately preserved. It also allows multiple AD7863s to simultaneously sample more than two channels. At the end of conversion, the part returns to its tracking mode. The acquisition time of the track-and-hold amplifiers begins at this point. REFERENCE SECTION The AD7863 contains a single reference pin, labeled VREF, that provides access to the part’s own 2.5 V reference. Alternatively, an external 2.5 V reference can be connected to this pin, thus providing the reference source for the part. The part is specified with a 2.5 V reference voltage. Errors in the reference source result in gain errors in the AD7863 transfer function and add to the specified full-scale errors on the part. On the AD7863-10 and AD7863-3, it also results in an offset error injected in the attenuator stage. The AD7863 contains an on-chip 2.5 V reference. To use this reference as the reference source for the AD7863, connect two 0.1 μF disc ceramic capacitors from the VREF pin to AGND. The voltage that appears at this pin is internally buffered before being applied to the ADC. If this reference is required for use external to the AD7863, it should be buffered because the part has a FET switch in series with the reference output resulting in a source impedance for this output of 5.5 kΩ nominal. The tolerance on the internal reference is ±10 mV at 25°C with a typical temperature coefficient of 25 ppm/°C and a maximum error over temperature of ±25 mV. If the application requires a reference with a tighter tolerance or the AD7863 needs to be used with a system reference, the user has the option of connecting an external reference to this VREF pin. The external reference effectively overdrives the internal reference and thus provides the reference source for the ADC. The reference input is buffered before being applied to the ADC with a maximum input current of ±100 μA. A suitable reference source for the AD7863 is the AD780 precision 2.5 V reference. |
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