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AD7730BRU Fiches technique(PDF) 41 Page - Analog Devices |
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AD7730BRU Fiches technique(HTML) 41 Page - Analog Devices |
41 / 53 page AD7730/AD7730L –41– Long lead lengths from the bridge to the AD7730 facilitate the pickup of mains frequency on the analog input, the reference input and the power supply. The analog inputs to the AD7730 are buffered, which allows the user to connect whatever noise reduction capacitors are necessary in the application. The AD7730 boasts excellent common-mode and normal- mode rejection of mains frequency on both the analog and reference inputs. In CHOP mode, care must be taken in choosing the output update rate so it does not result in reducing line frequency rejection (see DIGITAL FILTERING section). The input offset current on the AD7730 is 10 nA maximum which results in a maxi- mum, dc offset voltage of 1.75 mV in a 350 Ω bridge applica- tion. Care should taken with inserting large source impedances on the reference input pins as these inputs are not buffered and the source impedances can result in gain errors. In many load-cell applications, a portion of the dynamic range of the bridge output is consumed by a pan weight or tare weight. In such applications, the 6-bit TARE DAC of the AD7730 can be used to adjust out this tare weight as outlined previously. AC Excitation of Bridge AC excitation of the bridge addresses many of the concerns with thermocouple, offset and drift effects encountered in dc-excited applications. In ac-excitation, the polarity of the excitation volt- age to the bridge is reversed on alternate cycles. The result is the elimination of dc errors at the expense of a more complex sys- tem design. Figure 24 outlines the connections for an ac-excited bridge application based on the AD7730. The excitation voltage to the bridge must be switched on alternate cycles. Transistors T1 to T4 in Figure 24 perform the switching of the excitation voltage. These transistors can be discrete matched bipolar or MOS transistors, or a dedicated bridge driver chip such as the 4427 from Micrel can be used to perform the task. Since the analog input voltage and the reference voltage are reversed on alternate cycles, the AD7730 must be synchronized with this reversing of the excitation voltage. To allow the AD7730 to synchronize itself with this switching, it provides the logic control signals for the switching of the excitation voltage. These signals are the nonoverlapping CMOS outputs ACX and ACX. One of the problems encountered with ac-excitation is the set- tling time associated with the analog input signals after the excitation voltage is switched. This is particularly true in appli- cations where there are long lead lengths from the bridge to the AD7730. It means that the converter could encounter errors because it is processing signals which are not fully settled. The AD7730 addresses this problem by allowing the user to program a delay of up to 48.75 μs between the switching of the ACX signals and the processing of data at the analog inputs. This is achieved using the DL bits of the Filter Register. The AD7730 also scales the ACX switching frequency in accor- dance with the output update rate. This avoids situations where the bridge is switched at an unnecessarily faster rate than the system requires. The fact that the AD7730 can handle reference voltages which are the same as the excitation voltages is particularly useful in ac-excitation where resistor divider arrangements on the reference input add to the settling time associated with the switching. Figure 24. Typical Connections for AC-Excited Bridge Application SIGMA- DELTA MODULATOR AVDD DVDD AD7730 6-BIT DAC SERIAL INTERFACE AND CONTROL LOGIC REGISTER BANK CLOCK GENERATION PROGRAMMABLE DIGITAL FILTER SIGMA-DELTA A/D CONVERTER BUFFER PGA AIN2(+)/D1 AIN2(–)/D0 ACX ACX STANDBY SYNC MCLK IN MCLK OUT SCLK CS DIN DOUT RESET RDY POL DGND AGND MUX AC EXCITATION CLOCK CALIBRATION MICROCONTROLLER + IN+ OUT– IN– OUT+ REF IN(+) REF IN(–) AIN1(+) AIN1(–) EXCITATION VOLTAGE = +5V T4 T3 T2 T1 +/– REV. B |
Numéro de pièce similaire - AD7730BRU |
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Description similaire - AD7730BRU |
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