View max6691 datasheet:

19-2304; Rev 0; 1/02 Four-Channel Thermistor Temperature-to-Pulse- Width Converter General Description Features The MAX6691 four-channel thermistor temperature-to- Simple Single-Wire Interface pulse-width converter measures the temperatures of up Measures Up to Four Thermistor Temperatures to four thermistors and converts them to a series of out- Low-Average Thermistor Current Minimizes Self- put pulses whose widths are related to the thermistors’ temperatures. Each of the four thermistors and an Heating Errors external fixed resistor (REXT) form a voltage-divider that Internal Voltage Reference Isolates Thermistor is driven by the MAX6691’s internal voltage reference from Power-Supply Noise (VREF). VREF and the voltage across REXT are mea- Accommodates Any Thermistor