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ADS7844NG4 Datasheet(PDF) 10 Page - Texas Instruments |
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ADS7844NG4 Datasheet(HTML) 10 Page - Texas Instruments |
10 / 22 page ADS7844 10 SBAS100A www.ti.com cally 25pF). After the capacitor has been fully charged, there is no further input current. The rate of charge transfer from the analog source to the converter is a function of conversion rate. REFERENCE INPUT The external reference sets the analog input range. The ADS7844 will operate with a reference in the range of 100mV to +VCC. Keep in mind that the analog input is the difference between the +IN input and the –IN input as shown in Figure 2. For example, in the single-ended mode, a 1.25V reference, and with the COM pin grounded, the selected input channel (CH0 - CH7) will properly digitize a signal in the range of 0V to 1.25V. If the COM pin is connected to 0.5V, the input range on the selected channel is 0.5V to 1.75V. There are several critical items concerning the reference input and its wide voltage range. As the reference voltage is re- duced, the analog voltage weight of each digital output code is also reduced. This is often referred to as the LSB (least significant bit) size and is equal to the reference voltage divided by 4096. Any offset or gain error inherent in the A/D converter will appear to increase, in terms of LSB size, as the reference voltage is reduced. For example, if the offset of a given converter is 2 LSBs with a 2.5V reference, then it will typically be 10 LSBs with a 0.5V reference. In each case, the actual offset of the device is the same, 1.22mV. Likewise, the noise or uncertainty of the digitized output will increase with lower LSB size. With a reference voltage of 100mV, the LSB size is 24 µV. This level is below the internal noise of the device. As a result, the digital output code will not be stable and vary around a mean value by a number of LSBs. The distribution of output codes will be gaussian and the noise can be reduced by simply averaging consecutive conversion results or applying a digital filter. With a lower reference voltage, care should be taken to provide a clean layout including adequate bypassing, a clean (low noise, low ripple) power supply, a low-noise reference, and a low-noise input signal. Because the LSB size is lower, the converter will also be more sensitive to nearby digital signals and electromagnetic interference. The voltage into the VREF input is not buffered and directly drives the capacitor digital-to-analog converter (CDAC) portion of the ADS7844. Typically, the input current is 13 µA with a 2.5V reference. This value will vary by microamps depending on the result of the conversion. The reference current diminishes directly with both conversion rate and reference voltage. As the current from the reference is drawn on each bit decision, clocking the converter more quickly during a given conversion period will not reduce overall current drain from the reference. DIGITAL INTERFACE Figure 3 shows the typical operation of the ADS7844’s digital interface. This diagram assumes that the source of the digital signals is a microcontroller or digital signal processor with a basic serial interface (note that the digital inputs are over-voltage tolerant up to 5.5V, regardless of +VCC). Each communication between the processor and the converter consists of eight clock cycles. One complete conversion can be accomplished with three serial communications, for a total of 24 clock cycles on the DCLK input. The first eight clock cycles are used to provide the control byte via the DIN pin. When the converter has enough information about the following conversion to set the input multiplexer appropriately, it enters the acquisition (sample) mode. After three more clock cycles, the control byte is complete and the converter enters the conversion mode. At this point, the input sample/hold goes into the hold mode. The next twelve clock cycles accomplish the actual analog- to-digital conversion. A thirteenth clock cycle is needed for the last bit of the conversion result. Three more clock cycles are needed to complete the last byte (DOUT will be LOW). These will be ignored by the converter. Control Byte Also shown in Figure 3 is the placement and order of the control bits within the control byte. Tables III and IV give detailed information about these bits. The first bit, the ‘S’ bit, must always be HIGH and indicates the start of the control byte. The ADS7844 will ignore inputs on the DIN pin until the start bit is detected. The next three bits (A2 - A0) select the active input channel or channels of the input multiplexer (see Tables I and II and Figure 2). FIGURE 2. Simplified Diagram of the Analog Input. Converter +IN −IN CH0 CH1 CH2 CH3 A2-A0 (shown 000 B) (1) SGL/DIF (shown HIGH) CH4 CH5 CH6 CH7 COM NOTE: (1) See Truth Tables, Table 1, and Table 2 for address coding. |
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