# Conductance Noise and Drift Parent: [[Analog In-Memory Computing]] Every analog memory cell stores its weight as a physical conductance — a resistance level programmed into a material. That conductance is never exact. It fluctuates moment to moment (noise) and shifts slowly over time and temperature (drift). Both are physical facts that no amount of clever engineering fully eliminates. Noise comes from shot noise, thermal noise, and read disturb — the small changes caused by each act of reading the cell. Drift comes from atomic relaxation in the resistive material (severe in PCM, less so in ReRAM), from device aging, and from ambient temperature shifts. The combined error can push an effective bit-precision of 8 bits down to 4-5 bits within hours of programming, and lower after years. The honest way to characterise an analog accelerator is to state three numbers together: the bit-precision at the moment of programming, the precision after a specified retention time, and the precision over the operating temperature range. A claim of "8-bit accuracy" without retention or temperature qualifiers is marketing, not engineering. The standard mitigations are error-correcting codes applied to groups of cells, periodic re-programming (refresh), and — most importantly — training the network to tolerate noise in the first place. A model trained with noise injection can retain most of its accuracy under drift that would destroy a noise-naive baseline. This is why analog-aware training is not optional for any serious deployment. ## Related - [[Crossbar Arrays]] - [[Analog-Aware Training]] --- Tags: #hardware #analog #kp