PPM (Parts Per Million)

Parts per million (ppm) is the unit engineers reach for whenever percentages fail. 1 ppm = 0.0001 % = 1×10⁻⁶. A resistor with ±25 ppm/°C TCR drifts 0.0025 % per degree; over a 100 °C swing that is 0.25 % — well below the ±1 % a typical thick-film part allows. Without ppm we would write 0.000025 %/°C, which is unwieldy.

In resistor datasheets, ppm appears in four main places. (1) TCR, in ppm/K or ppm/°C — the smaller, the more temperature-stable. Foil resistors reach ±0.05 ppm/K; wirewound ±5 to ±50 ppm/K; thick film ±100 to ±200 ppm/K. (2) Voltage coefficient (VCR), in ppm/V — usually <0.1 ppm/V for wirewound, up to 100 ppm/V for thick film at high voltage. (3) Long-term stability, ppm per 1000 hours at rated load. (4) Tolerance — tight precision parts are labelled in ppm rather than percent, e.g. ±100 ppm = ±0.01 %.

When interpreting ppm, always check the reference quantity: ppm of nominal resistance, of full-scale voltage, of temperature, etc. For a 1 kΩ shunt sensing 100 mV, a 25 ppm/°C drift over 50 °C = 1250 ppm = 0.125 Ω = 0.125 mV measurement error — enough to wreck a 0.1 % accuracy spec.