When connecting the ISOTMP35-Q1 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-Q1), so this lead to voltage drops on the ISOTMP35-Q1 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-Q1 output. Users can also add R_FILTER to filter out noise on the analog output.

Consider the maximum load capacitance. The ISOTMP35-Q1 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-Q1. ADCs often have customizable sampling rates, so the settling time of the ISOTMP35-Q1 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-Q1 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-Q1, 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-Q1 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-Q1 analog output range is necessary.