Datasheet AD7781 (Analog Devices) - 8

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AD7781 PIN CONFIGURATIONS AND FUNCTION DESCRIPTIONS NC 1 16 NC SCLK 1 14 FILTER SCLK 2 15 FILTER DOUT/RDY 2 13 PDRST DOUT/RDY 3 14 PDRST NC 3 12 DV AD7781 AD7781 DD NC 4 13 DV TOP VIEW DD GAIN 4 TOP VIEW 11 AVDD (Not to Scale) GAIN 5 (Not to Scale) 12 AVDD AIN(+) 5 10 GND AIN(+) 6 11 GND AIN(–) 6 9 BPDSW REFIN(+) 7 8 REFIN(–) AIN(–) 7 10 BPDSW
06 -0 62
REFIN(+) 8 9 REFIN(–) NC = NO CONNECT
81 7 0 00 2-
NC = NO CONNECT
16 08 Figure 6. SOIC Pin Configuration Figure 7. TSSOP Pin Configuration
Table 6. Pin Function Descriptions Pin No. SOIC TSSOP Mnemonic Description
1 2 SCLK Serial Clock Input. This serial clock input is for data transfers from the ADC. The SCLK pin has a Schmitt- triggered input. The serial clock can be active only when transferring data from the AD7781. The data from the AD7781 can be read as a continuous 32-bit word. Alternatively, SCLK can be noncontinuous during the data transfer, with the information being transmitted from the ADC in smaller data batches. 2 3 DOUT/RDY Serial Data Output/Data Ready Output. DOUT/RDY serves a dual purpose: as a data ready pin, going low to indicate the completion of a conversion, and as a serial data output pin to access the data register of the ADC. Eight status bits accompany each data read (see Figure 22). The DOUT/RDY falling edge can be used as an interrupt to a processor, indicating that new data is available. If the data is not read after the conversion, the pin goes high before the next update occurs. The serial interface is reset each time that a conversion is available. Therefore, the user must ensure that any conversions being transmitted are completed before the next conversion is available. 3 1, 4, 16 NC No Connect. This pin can be left floating. 4 5 GAIN Gain Select Pin. When GAIN is low, the gain is set to 128. When GAIN is high, the gain is set to 1. 5 6 AIN(+) Analog Input. AIN(+) is the positive terminal of the differential analog input pair, AIN(+)/AIN(−). 6 7 AIN(−) Analog Input. AIN(−) is the negative terminal of the differential analog input pair, AIN(+)/AIN(−). 7 8 REFIN(+) Positive Reference Input. An external reference can be applied between REFIN(+) and REFIN(−). The nomi- nal reference voltage (REFIN(+) − REFIN(−)) is 5 V, but the part can function with a reference of 0.5 V to AVDD. 8 9 REFIN(−) Negative Reference Input. 9 10 BPDSW Bridge Power-Down Switch to GND. When PDRST is high, the bridge power-down switch is closed. When PDRST is low, the switch is opened. 10 11 GND Ground Reference Point. 11 12 AVDD Supply Voltage, 2.7 V to 5.25 V. 12 13 DVDD Digital Interface Supply Voltage. The logic levels for the serial interface pins and the digital control pins are related to this supply, which is between 2.7 V and 5.25 V. The DVDD voltage is independent of the voltage on AVDD; therefore, AVDD can equal 5 V with DVDD at 3 V or vice versa. 13 14 PDRST Power-Down/Reset. When this pin is low, the ADC is placed in power-down mode, and the low-side power switch is opened. All the logic on the chip is reset, and the DOUT/RDY pin is tristated. When PDRST is high, the ADC is taken out of power-down mode. The on-chip clock powers up and settles, and the ADC contin- uously converts. In addition, the low-side power switch is closed. The internal clock requires approximately 1 ms to power up. 14 15 FILTER Filter Select Pin. When FILTER is low, the fast settling filter is selected. The update rate is set to 16.7 Hz, which gives a filter settling time of 120 ms. When FILTER is high, the high rejection filter is selected. The update rate is set to 10 Hz, which gives a filter settling time of 300 ms. With this filter, the stop-band (higher than fADC) attenuation is better than −45 dB. Rev. 0 | Page 7 of 16 Document Outline FEATURES APPLICATIONS FUNCTIONAL BLOCK DIAGRAM GENERAL DESCRIPTION TABLE OF CONTENTS REVISION HISTORY SPECIFICATIONS TIMING CHARACTERISTICS Circuit and Timing Diagrams ABSOLUTE MAXIMUM RATINGS THERMAL RESISTANCE ESD CAUTION PIN CONFIGURATIONS AND FUNCTION DESCRIPTIONS TYPICAL PERFORMANCE CHARACTERISTICS OUTPUT NOISE AND RESOLUTION THEORY OF OPERATION FILTER, DATA RATE, AND SETTLING TIME GAIN POWER-DOWN/RESET (PDRST) ANALOG INPUT CHANNEL BIPOLAR CONFIGURATION DATA OUTPUT CODING REFERENCE BRIDGE POWER-DOWN SWITCH DIGITAL INTERFACE APPLICATIONS INFORMATION WEIGH SCALES AD7781 PERFORMANCE IN A WEIGH SCALE SYSTEM EMI RECOMMENDATIONS GROUNDING AND LAYOUT OUTLINE DIMENSIONS ORDERING GUIDE