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ADS1251U2K5G4 Arkusz danych(PDF) 7 Page - Texas Instruments |
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ADS1251U2K5G4 Arkusz danych(HTML) 7 Page - Texas Instruments |
7 / 23 page ADS1251 7 SBAS184D www.ti.com THEORY OF OPERATION The ADS1251 is a precision, high-dynamic range, 24-bit, delta-sigma, A/D converter capable of achieving very high-resolution digital results at high data rates. The analog input signal is sampled at a rate determined by the frequency of the system clock (CLK). The sampled analog input is modulated by the delta-sigma A/D modulator, which is fol- lowed by a digital filter. A Sinc5 digital low-pass filter processes the output of the delta-sigma modulator and writes the result into the data-output register. The DOUT/DRDY pin is pulled LOW, indicating that new data are available to be read by the external microcontroller/microprocessor. As shown in the block diagram on the front page, the main functional blocks of the ADS1251 are the 4th-order delta-sigma modulator, a digital filter, control logic, and a serial interface. Each of these functional blocks is described in the following sections. ANALOG INPUT The ADS1251 contains a fully differential analog input. In order to provide low system noise, common-mode rejection of 98dB, and excellent power-supply rejection, the design topology is based on a fully differential switched-capacitor architecture. The bipolar input voltage range is from –4.096 to +4.096V, when the reference input voltage equals +4.096V. The bipolar range is with respect to –VIN, and not with respect to GND. The differential input impedance of the analog input changes with the ADS1251 system clock frequency (CLK). The rela- tionship is: Impedance ( Ω) = (8MHz/CLK) • 210,000 See application note Understanding the ADS1251, ADS1253, and ADS1254 Input Circuitry (SBAA086), available for down- load from TI’s web site www.ti.com. With regard to the analog-input signal, the overall analog performance of the device is affected by three items. First, the input impedance can affect accuracy. If the source impedance of the input signal is significant, or if there is passive filtering prior to the ADS1251, a significant portion of the signal can be lost across this external impedance. The magnitude of the effect is dependent on the desired system performance. Second, the current into or out of the analog inputs must be limited. Under no conditions should the current into or out of the analog inputs exceed 10mA. Third, to prevent aliasing of the input signal, the bandwidth of the analog-input signal must be band-limited; the bandwidth is a function of the system clock frequency. With a system clock frequency of 8MHz, the data output rate is 20.8kHz with a –3dB frequency of 4.24kHz. The –3dB frequency scales with the system clock frequency. To ensure the best linearity of the ADS1251, and to maxi- mize the elimination of even-harmonic noise errors, a fully differential signal is recommended. For more information about the ADS1251 input structure, refer to application note SBAA086 found at www.ti.com. BIPOLAR INPUT Each of the differential inputs of the ADS1251 must stay between –0.3V and VDD. With a reference voltage at less than half of VDD, one input can be tied to the reference voltage, and the other input can range from 0V to 2 • VREF. By using a three op amp circuit featuring a single amplifier and four external resistors, the ADS1251 can be configured to accept bipolar inputs referenced to ground. The conventional ±2.5V, ±5V, and ±10V input ranges can be interfaced to the ADS1251 using the resistor values shown in Figure 1. FIGURE 1. Level-Shift Circuit for Bipolar Input Ranges. 10k Ω 20k Ω R 1 OPA4350 OPA4350 OPA4350 +IN –IN V REF ADS1251 R2 Bipolar Input REF 2.5V BIPOLAR INPUT R1 R2 ±10V 2.5k Ω 5k Ω ±5V 5k Ω 10k Ω ±2.5V 10k Ω 20k Ω |
Podobny numer części - ADS1251U2K5G4 |
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Podobny opis - ADS1251U2K5G4 |
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