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ADS7816UBG4 Arkusz danych(PDF) 8 Page - Texas Instruments |
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ADS7816UBG4 Arkusz danych(HTML) 8 Page - Texas Instruments |
8 / 24 page 8 ® ADS7816 THEORY OF OPERATION The ADS7816 is a classic successive approximation register (SAR) analog-to-digital (A/D) converter. The architecture is based on capacitive redistribution which inherently includes a sample/hold function. The converter is fabricated on a 0.6 µ CMOS process. The architecture and process allow the ADS7816 to acquire and convert an analog signal at up to 200,000 conversions per second while consuming very little power. The ADS7816 requires an external reference, an external clock, and a single +5V power source. The external refer- ence can be any voltage between 100mV and VCC. The value of the reference voltage directly sets the range of the analog input. The reference input current depends on the conversion rate of the ADS7816. The external clock can vary between 10kHz (625Hz through- put) and 3.2MHz (200kHz throughput). The duty cycle of the clock is essentially unimportant as long as the minimum high and low times are at least 150ns. The minimum clock frequency is set by the leakage on the capacitors internal to the ADS7816. The analog input is provided to two input pins: +In and –In. When a conversion is initiated, the differential input on these pins is sampled on the internal capacitor array. While a conversion is in progress, both inputs are disconnected from any internal function. The digital result of the conversion is clocked out by the DCLOCK input and is provided serially, most significant bit first, on the DOUT pin. The digital data that is provided on the DOUT pin is for the conversion currently in progress—there is no pipeline delay. It is possible to continue to clock the ADS7816 after the conversion is complete and to obtain the serial data least significant bit first. See the Digital Interface section for more information. ANALOG INPUT The +In and –In input pins allow for a differential input signal. Unlike some converters of this type, the –In input is not re- sampled later in the conversion cycle. When the converter goes into the hold mode, the voltage difference between +In and –In is captured on the internal capacitor array. The range of the –In input is limited to ±200mV. Because of this, the differential input can be used to reject only small signals that are common to both inputs. Thus, the –In input is best used to sense a remote signal ground that may move slightly with respect to the local ground potential. The input current on the analog inputs depends on a number of factors: sample rate, input voltage, source impedance, and power down mode. Essentially, the current into the ADS7816 charges the internal capacitor array during the sample pe- riod. After this capacitance has been fully charged, there is no further input current. The source of the analog input voltage must be able to charge the input capacitance (25pF) to a 12-bit settling level within 1.5 clock cycles. When the converter goes into the hold mode or while it is in the power down mode, the input impedance is greater than 1G Ω. Care must be taken regarding the absolute analog input voltage. To maintain the linearity of the converter, the –In input should not exceed GND ±200mV. The +In input should always remain within the range of GND –200mV to VCC +200mV. Outside of these ranges, the converter’s lin- earity may not meet specifications. REFERENCE INPUT The external reference sets the analog input range. The ADS7816 will operate with a reference in the range of 100mV to VCC. There are several important implications of this. As the reference voltage is reduced, the analog voltage weight of each digital output code is reduced. This is often referred to as the LSB (least significant bit) size and is equal to the reference voltage divided by 4096. This means that 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. The typical performance curves of “Change in Offset vs Reference Voltage” and “Change in Gain vs Reference Voltage” provide more information. The noise inherent in the converter will also appear to increase with lower LSB size. With a 5V reference, the internal noise of the converter typically contributes only 0.16 LSB peak-to-peak of potential error to the output code. When the external reference is 100mV, the potential error contribution from the internal noise will be 50 times larger— 8 LSBs. The errors due to the internal noise are gaussian in nature and can be reduced by averaging consecutive conver- sion results. For more information regarding noise, consult the typical performance curves “Effective Number of Bits vs Reference Voltage” and “Peak-to-Peak Noise vs Reference Voltage.” The effective number of bits (ENOB) figure is calculated based on the converter’s signal-to-(noise + distortion) ratio with a 1kHz, 0dB input signal. SINAD is related to ENOB as follows: SINAD = 6.02 • ENOB +1.76. With lower reference voltages, extra care should be taken to provide a clean layout including adequate bypassing, a clean 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 external sources of error such as nearby digital signals and electromagnetic interference. The current that must be provided by the external reference will depend on the conversion result. The current is lowest at full-scale (FFFh) and is typically 25 µA at a 200kHz conversion rate (25 °C). For the same conditions, the current will increase as the input approaches zero, reaching 50 µA at an output result of 000h. The current does not increase linearly, but depends, to some degree, on the bit pattern of the digital output. |
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