SBOU024C august 2004 – july 2023 PGA309
The PGA309 has circuitry to detect when the power supply is applied to the PGA309, and reset the internal registers and circuitry to an initial state. This reset also occurs when the supply is detected to be invalid, so that the PGA309 is in a known state when the supply becomes valid again. The rising threshold for this circuit is typically 2.2V and the falling threshold is typically 1.7V. After the power supply becomes valid, the PGA309 waits for approximately 33ms and then attempts to read the configuration data from the external EEPROM device.
If the EEPROM has the proper flag set in address location 0 and 1, then the PGA309 continues reading the EEPROM; otherwise, the PGA309 waits for 1.3 seconds before trying again. If the PGA309 detects no response from the EEPROM, the PGA309 waits for 1.3 seconds and tries again; otherwise, the PGA309 tries to free the bus and waits for 33ms before trying to read the EEPROM again. If successful (including valid checksum data), the PGA309 triggers the Temp ADC to measure temperature. For 16-bit resolution results the converter takes approximately 125ms to complete a conversion. Once the conversion is complete, the PGA309 begins reading the Lookup Table information from the EEPROM to calculate the settings for the Gain DAC and Zero DAC. This process is detailed in the flowchart shown in Figure 3-1.
The PGA309 reads the entire Lookup Table so that it can determine if the checksum for the Lookup Table is correct. Each entry in the Lookup Table requires approximately 500μs to read from the EEPROM. Once the checksum is determined to be valid, the calculated values for the Gain and Zero DACs are updated into their respective registers, and the Output Amplifier is enabled. The PGA309 then begins looping through this entire procedure, starting with reading the EEPROM configuration registers, then starting a new conversion on the Temp ADC, which then triggers reading the Lookup Table data from the EEPROM. This loop continues indefinitely.