How accurate is the temperature sensor on the AD9122, AD9146 or AD9148 Dual and Quad DACs?
When the temperature sensor was described in the AD9122 and AD9125 data sheet there was a misunderstanding on how accurate the temperature sensor will be. The band gap variation (used to sense the temperature) can cause the temp reading to vary 30-40 degrees part to part due to band gap voltage variation with wafer fabrication process. The same temperature sensor is used in the ADD9122, AD9125, and AD9148 DACs. The termpature sensor is meant to be used to sence temperature change. The temperature sensor needs to be calibrated against a know temperature then it can be used to monitor tepmerature change to within +/- 5 degrees.
The temperature must be calibrated against a known temperature to remove the part-to-part variation on the band-gap circuit used to sense the temperature. The DACCLK must be running at a minimum of 100 MHz to obtain a reliable temperature measurement.
To monitor temperature change, the user must take a reading at a known ambient temperature for a single-point calibration of each DAC.
Tx = Tref + 7.7 × (Code_x − Code_ref)/1000 + 1
where:Code_x is the readback code at the unknown temperature Tx.Code_ref is the readback code at the calibrated temperature Tref.
AD9122 and AD9125 Register set up
To use the temperature sensor, it must be enabled by setting Register 0x01, Bit 4, to 0. In addition, to obtain accurate readings, the range control register (Register 0x48) should be set to 0x02.
AD9148 Register set up
To use the temperature sensor, it must be enabled by setting Bit 0, Register 0x5C to 0. Before the temperature sensor data can be read back, it must be latched by toggling Bit 1, Register 0x5C from 0 to 1. In addition, to get accurate readings, the die temperature control register (Register 0x5D) should be set to 0x02
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