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ADS7864 Datasheet(PDF) 8 Page - Burr-Brown (TI) |
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ADS7864 Datasheet(HTML) 8 Page - Burr-Brown (TI) |
8 / 16 page 8 ADS7864 ® 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 ADS7864. The ADS7864 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 ADS7864 is double-buffered. If the internal reference is used to drive an external load, a buffer is provided between the reference and the load ap- plied to pin 33 (the internal reference can typically source 2mA of current—load capacitance should not exceed 100pF). If an external reference is used, the second buffer provides isolation between the external reference and the CDAC. This buffer is also used to recharge all of the capacitors of both CDACs during conversion. ANALOG INPUT The analog input is bipolar and fully differential. There are two general methods of driving the analog input of the ADS7864: 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 ±1/2V REF 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 ADS7864 is a high speed, low power, dual 12-bit A/D converter that operates from a single +5V supply. The input channels are fully differential with a typical common-mode rejection of 80dB. The part contains dual 2 µs successive approximation ADCs, six differential sample-and-hold am- plifiers, an internal +2.5V reference with REFIN and REFOUT pins and a high speed parallel interface. There are six analog inputs that are grouped into three channels (A, B and C). Each A/D converter has three inputs (A0/A1, B0/B1 and C0/ C1) that can be sampled and converted simultaneously, thus preserving the relative phase information of the signals on both analog inputs. Each pair of channels has a hold signal (HOLDA, HOLDB, HOLDC) to allow simultaneous sam- pling on all 6 channels. 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). A conversion is initiated on the ADS7864 by bringing the HOLDX pin LOW for a minimum of 15ns. HOLDX LOW places both sample-and-hold amplifiers of the X channels in the hold state simultaneously and the conversion process is started on both channels. The BUSY output will then go LOW and remain LOW for the duration of the conversion cycle. The data can be read from the parallel output bus following the conversion by bringing both RD and CS LOW. Conversion time for the ADS7864 is 1.75 µs when an 8MHz external clock is used. The corresponding acquisition time is 0.25 µs. To achieve maximum output rate (500kHz), the read function can be performed during at the start of the next conversion. NOTE: This mode of operation is described in more detail in the Timing and Control section of this data sheet. SAMPLE-AND-HOLD SECTION The sample-and-hold amplifiers on the ADS7864 allow the ADCs to accurately convert an input sine wave of full-scale amplitude to 12-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 ADC even when the ADC is operated at its maximum throughput rate of 500kHz. The typical small-signal bandwidth of the sample- and-hold amplifiers is 40MHz. Typical aperture delay time or the time it takes for the ADS7864 to switch from the sample to the hold mode following the negative edge of HOLDX signal is 5ns. The average delta of repeated aperture delay values is typically 50ps (also known as aperture jitter). These specifications reflect the ability of the ADS7864 to capture AC input signals accurately at the exact same moment in time. FIGURE 1. Methods of Driving the ADS7864 Single-Ended or Differential. ADS7864 ADS7864 Single-Ended Input Common Voltage –V REF to +VREF peak-to-peak Differential Input Common Voltage V REF peak-to-peak V REF peak-to-peak |
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