When connecting the ISOTMP35 analog output to an ADC, it is important to use an RC filter on the output. Most ADCs have a sampled comparator input structure. When the sampling is active, a switch internal to the ADC will charge an internal capacitor (C_SAMPLE). The capacitor requires instantaneous charge from the analog output source (ISOTMP35), so this will lead to voltage drops on the ISOTMP35 analog output, which will appear 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 will store charge from the analog output that the ADC can pull from when sampling, so there will not be a 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 1000 pF, therefore the total capacitance on the analog output, including those in the ADC input, must not exceed 1000 pF.

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

ADCs often have customizable full scale ranges (FSR), either digitally or through reference voltages. The ISOTMP35 at 150°C will output a maximum voltage of 2017.5 mV. When choosing an ADC, there should be a full scale range option with at least that much range. TI recommends a FSR option of at least +3 V 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 will determine the measurement resolution.

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