When connecting the ISOTMP35 analog output to an ADC,using an RC filter on the output is important. Most ADCs have a sampled comparator input structure. When the sampling is active, a switch internal to the ADC charges an internal capacitor (C_SAMPLE). The capacitor requires instantaneous charge from the analog output source (ISOTMP35), so this lead to voltage drops on the ISOTMP35 analog output, which appears as incorrect temperature reads. By placing a filter capacitor (C_FILTER) load on the ISOTMP35 analog output, the voltage drops are mitigated. This works because C_FILTER stores charge from the analog output that the ADC can pull from when sampling, so there is no voltage drop on the ISOTMP35 output. Users can also add R_FILTER to filter out noise on the analog output.

Consider the maximum load capacitance. The ISOTMP35 has a maximum load capacitance of 1000pF, therefore the total capacitance on the analog output, including those in the ADC input, must not exceed 1000pF.

When choosing the R and C filter values, the RC time constant changes the settling time of the ISOTMP35. ADCs often have customizable sampling rates, so the settling time of the ISOTMP35 must be less than the selected sampling time of the ADC. For example, an ADC with a data rate (DR) of 1ksps has a conversion time of 1ms, therefore any selected R and C filter values must be completely settled within 1ms (5 × R × C < 1/DR).

ADCs often have customizable full scale ranges (FSR), either digitally or through reference voltages. The ISOTMP35 at 150°C outputs a maximum voltage of 2017.5mV. When choosing an ADC, there must be a full scale range option with at least that much range. TI recommends a FSR option of at least 3V to avoid headroom concerns in this example. To determine the desired ADC resolution, the ADC LSB size must be known. For the ISOTMP35, the device does not have an LSB but rather the LSB of the ADC determines the measurement resolution.

• For example, a 12bit ADC with an FSR of 3.3V, has an LSB size of 806µV. This translates to 80m°C of temperature resolution. A 16bit ADC with an FSR of 3.3V, has an LSB size of 50µV, which gives 5m°C of temperature resolution. A 12bit ADC is sufficient for most applications.
• The analog output voltage from the ISOTMP35 must not exceed the V_DD being supplied to the ADC. Selecting a V_DD for the ADC that exceeds the chosen FSR required to fully capture the ISOTMP35 analog output range is necessary.