Chandrashekar, A typical RTD ADC measurement is ratiometric and doesn't use a specific external reference. It simply uses an excitation current to pass current through the RTD and a precision reference resistor so that the output code from the ADC is proportional to the RTD resistance divided by the reference resistor. In one of your previous posts, I gave a link to TIPD120 . It shows how this is calculated (with an error analysis). Additionally, there is another application note: "Example Temperature Measurement Applications Using the ADS1247 and ADS1248 " which gives a couple of other examples. You can find it at the following link: www.ti.com/.../sbaa180.pdf. Going back to your question, Are you sure you need a reference with a better long term drift? From the graph, the typical long term drift is about 60ppm. This translates directly to gain error. Which would be the equivalent of 0.006%. Note that this is the drift and does not account for the initial accuracy of the reference. However, if you decide to use an external reference, I'll need to ask another group if there are any references with less of a long term drift. I'm not too familiar with entire product line. For question 2, the input mux is a 8:2 multiplexer. If four inputs are used for two differential input channels, then there are four more inputs. You could make 3 "single-ended" measurements, if one input is used as a common input that would be tied to the ADC AINN. Note that this AINN input cannot be tied to AVSS (which would be ground for a unipolar supply) because this input would not be within the common-mode input range of the PGA. For question 3, the same reference can be used to measure the other "single-ended" measurements. However, note that for the ADS1248 has several reference options. First, there is the internal reference that can be used to make measurements. Then there are two sets of reference pins that can be used to make measurements REFP0 and REFN0 or REFP1 and REFN1 which can be selected by the MUX1 register. For question 4, I'm not sure what you mean. The ADS1248 doesn't take a 4-20mA input directly. The accuracy depends on what you use to convert that current to a voltage that the ADC can measure. As I'd indicated in a previous post, I don't know of any application notes that explain how to make the conversion. However, if you do have a circuit or schematic that can do that, we can discuss the accuracy. Joseph Wu
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