Hi Frank, let me help out here while Bob is enjoying his time off. I realized this answer got a little long. If you only worry about thermistor measurements (or 2-wire RTD measurements) I would actually recommend using a solution where the reference resistor is placed on the low-side of the thermistor. This will make things much easier. On the ADS122U04EVM we are using a high-side reference resistor (=R77). This implementation is often preferable to measure 3-wire RTDs because it yields slightly better performance. For 2-wire and 4-wire RTD, and thermistor measurements there is no real benefit in placing the reference resistor on the high side. One disadvantage of the implementation using the reference resistor on the high-side is, that you need a bias resistor on the low-side (R79) to level-shift the signal across the thermistor/RTD into the common-mode voltage range of the PGA. This will also make it more challenging to meet the headroom for the IDAC to work when using a 3.3V supply. As Bob mentioned before, the input currents into the ADC inputs are usually negligible. I would not worry about them for now. The potential errors they introduce can be calibrated out using an offset and gain calibration which you might have to do in any case depending on your temperature measurement accuracy requirements. So let's come to calculating the value you need to choose for the reference resistor. Let's start by deriving the formula for converting the ADC code into a thermistor resistance value. The ADC compares the differential input signal coming out of the PGA with its reference voltage and provides an output code which represents the ratio between the two: Code/2^24 = (V_IN * Gain) / (2 * V_REF). V_IN is the voltage drop across the thermistor: V_IN = I_IDAC * R_Thermistor V_REF is the voltage drop across the reference resistor (R_REF): V_REF = I_IDAC * R_REF With that: Code/2^24 = (R_Thermistor * Gain) / (2 * R_REF). You can use this relationship to calculate R_Thermistor from the measured ADC code. From the datasheet specifications you will see that V_IN <= V_REF / Gain, or in other words R_Thermistor <= R_REF / Gain. For thermistor measurements you usually use Gain = 1 actually. This means that your R_REF needs to be larger than the largest thermistor value you want to measure. In your case 248kOhm I guess. With that we would start by choosing R_REF = 250kOhm for example (or something close but at least larger than 248kOhm). With an I_IDAC = 10uA, this would yield a reference voltage of 2.5V. This will not work out with a 3.3V supply. Means we definitely need to switch to a 5V supply, but as we will see in a bit this will also not work. Using AVDD = 5V, I_IDAC = 10uA and R_REF = 250kOhm, the maximum differential voltage across the thermistor would be 2.48V. R_REF and R_Thermistor are in series (no matter if we use the low-side or high-side R_REF approach), means we will create a voltage drop of (2.5V + 2.48V) = 4.98V across the two resistors at T=-40°C. This will leave no headroom for the IDAC to operate anymore. The conclusion would be that a 10kOhm NTC measurement down to -40°C using current excitation is not possible with ADS1220 / ADS122U04 because we do not offer small enough IDAC values. You would therefore have to choose a voltage excitation method which most customers anyway do for thermistor measurements. There are multiple ways to do this. The ADS124S06 for example offers a very convenient way of doing it, because this ADC offers a buffered 2.5V voltage reference output which could be used to excite the thermistor. Below is how the circuit would look like (not showing any RC filters on the inputs). I typically use the following formula to find the best value for R_BIAS: R_BIAS^2 = R_Thermistor_Min * R_Thermistor_Max = 874Ohm x 248.277kOhm In your case that would mean setting R_BIAS = 15kOhm. The relationship to calculate the thermistor value from the output code would read the following: Code/2^24 = (V_IN * Gain) / (2 * V_REF). We would use Gain = 1 again here and for V_IN use the following resistor divider equation: V_IN = V_REF x R_Thermistor / (R_Thermistor + R_BIAS) This yields: Code/2^23 = R_Thermistor / (R_Thermistor + R_BIAS) The solution using ADS124S06 will probably offer the best performance. However you can also use cheaper solutions such as ADS1220 or ADS122U04 together with the analog supply (or an external voltage reference) to excite the thermistor. I am describing this solution in the ADS1219 datasheet application section for example. Regards,
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