1.235 V shunt reference — obsolete but still on BOMs
The Texas Instruments LM385BYMX-1.2 is a precision shunt voltage reference delivering a fixed 1.235 V output with ±1% initial tolerance. As a shunt (two-terminal) reference, it behaves like a Zener diode — it regulates voltage by shunting current through the cathode, requiring only a series resistor from the supply. The minimum cathode current is 15 µA, and it can sink up to 20 mA of output current.
50 ppm/°C drift and 60 µVrms noise — the precision angles
The temperature coefficient is 50 ppm/°C, which means the output voltage shifts by about 62 µV per °C from the 1.235 V nominal. Over the full 0°C to 70°C range, that adds up to roughly 4.3 mV of total drift — acceptable for 8-bit or low-end 10-bit ADC references, but not for higher-resolution systems without calibration. Low-frequency noise is specified at 60 µVrms over the 10 Hz to 10 kHz band. This is the integrated noise that appears at the reference output and couples into the ADC or comparator input — a key figure for precision measurement circuits.