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ADS8361IRHBRG4 Datasheet(PDF) 10 Page - Texas Instruments |
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ADS8361IRHBRG4 Datasheet(HTML) 10 Page - Texas Instruments |
10 / 29 page ADS8361 10 SBAS230E www.ti.com REFERENCE Under normal operation, the REFOUT pin (pin 2) should be directly connected to the REFIN pin (pin 1) to provide an internal +2.5V reference to the ADS8361. The ADS8361 can operate, however, with an external reference in the range of 1.2V to 2.6V for a corresponding full-scale range of 2.4V to 5.2V. The internal reference of the ADS8361 is buffered. If the internal reference is used to drive an external load, a buffer is provided between the reference and the load applied to pin 2 (the internal reference can typically source 10μA of current— load capacitance should be 0.1μF and 10μF). If an external reference is used, the second buffer provides isolation be- tween the external reference and the Capacitve Digital-to- Analog Converter (CDAC). This buffer is also used to re- charge all of the capacitors of both CDACs during conver- sion. ANALOG INPUT The analog input is bipolar and fully differential. There are two general methods of driving the analog input of the ADS8361: single-ended or differential (see Figures 1 and 2). When the input is single-ended, the –IN input is held at the common-mode voltage. The +IN input swings around the same common voltage and the peak-to-peak amplitude is the (common-mode + VREF) and the (common-mode – VREF). The value of VREF determines the range over which the common-mode voltage may vary (see Figure 3). When the input is differential, the amplitude of the input is the difference between the +IN and –IN input, or (+IN) – (–IN). The peak-to-peak amplitude of each input is ±1/2 VREF around this common voltage. However, since the inputs are 180° out-of- phase, the peak-to-peak amplitude of the differential voltage is +VREF to –VREF. The value of VREF also determines the range of the voltage that may be common to both inputs (see Figure 4). INTRODUCTION The ADS8361 is a high-speed, low-power, dual, 16-bit A/D converter that operates from +3V/+5V supply. The input channels are fully differential with a typical common-mode rejection of 80dB. The part contains dual, 4μs successive approximation A/D converter, two differential sample-and- hold amplifiers, an internal +2.5V reference with REFIN and REFOUT pins, and a high-speed serial interface. The ADS8361 requires an external clock. In order to achieve the maximum throughput rate of 500kSPS, the master clock must be set at 10MHz. A minimum of 20 clock cycles are required for each 16-bit conversion. There are four analog inputs that are grouped into two chan- nels (A and B). Channel selection is controlled by the M0 (pin 14), M1 (pin 15), and A0 (pin 16) pins. Each channel has two inputs (A0, A1 and B0, B1) that are sampled and converted simultaneously, thus preserving the relative phase information of the signals on both analog inputs. The part accepts an analog input voltage in the range of –VREF to +VREF, centered around the internal +2.5V reference. The part will also accept bipolar input ranges when a level shift circuit is used at the front end (see Figure 7). All conversions are initiated on the ADS8361 by bringing the CONVST pin HIGH for a minimum of 15ns. CONVST HIGH places both sample-and-hold amplifiers in the hold state simultaneously and the conversion process is started on both channels. The RD pin (pin 18) can be connected to CONVST to simplify operation. Depending on the status of the M0, M1, and A0 pins, the ADS8361 will (a) operate in either two- channel or four-channel mode and (b) output data on both the Serial A and Serial B output or both channels can be transmitted on the A output only. NOTE: See the Timing and Control section of this data sheet for more information. SAMPLE-AND-HOLD SECTION The sample-and-hold amplifiers on the ADS8361 allow the A/D converter to accurately convert an input sine wave of full- scale amplitude to 16-bit accuracy. The input bandwidth of the sample-and-hold is greater than the Nyquist rate (Nyquist equals one-half of the sampling rate) of the A/D converter even when the A/D converter is operated at its maximum throughput rate of 500kSPS. Typical aperture delay time, or the time it takes for the ADS8361 to switch from the sample to the hold mode following the CONVST pulse, is 3.5ns. The average delta of repeated aperture delay values is typically 50ps (also known as aperture jitter). These specifications reflect the ability of the ADS8361 to capture AC input signals accurately at the exact same moment in time. ADS8361 ADS8361 Single-Ended Input Common Voltage –VREF to +VREF peak-to-peak Differential Input Common Voltage VREF peak-to-peak VREF peak-to-peak FIGURE 1. Methods of Driving the ADS8361 Single-Ended or Differential. |
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