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AD7707 and reference voltage

I want to use AD7707 for temperature measurement ("RTD.gif") and analog signals 0-5V and 0-10V ("Analog In.gif ").

What to use for reference voltage:
- AD780 ("REF.gif")
- resistor 6.25 kOm

If I use a resistor of 6.25 kOm, how can it affect the measurement of analog signals?

 

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  • Hi,

    The 6.25kohm resistor will be your reference resistor that would generate your reference voltage for your RTD measurement. Since you are in a ratiometric configuration, variation in excitation current is not an issue. However, the reference resistor 6.25k should be precise (e.g. 0.01% tol) and have a very low temperature coefficient to avoid errors in the reference voltage. ADC output code is directly proportional to the analog input voltage and inversely proportional to the voltage reference. So any errors in reference will results error in ADC measurements. May I know if I have answer your question? or If you have other concern, just let me know.

    Thanks,

    Jellenie

  • I understand all this. But this same reference voltage will also be used to measure the voltage at the input AIN3. If the current source changes, then the reference voltage will change. This measure of resistance (RTD) will not be affected. But I also measure analog voltage (AIN3). How will this reference voltage affect the measurement of voltage?

  • I want to use this circuit of a current source.

  • Hi,

    I would suggest to use a stable external reference voltage for your other inputs. So make sure that your current source is stable if you intended to use the same reference voltage for this. Any variation in reference voltage may results errors or variations in your measurements. So if you could provide a low noise, more stable voltage reference, it would be better. REF192 is a good choice for reference voltage. Do you really want to use AD7707? We have new products which have internal excitation current as well as internal reference voltage that may suite your application. You may want to consider using AD7124 and AD717x series.  

    Thanks,

    Jellenie

  • Hi Pifa,

    As Jellenie has mentioned you may want to consider newer ADCs which have integrated current sources and reference voltage. Overall, it would provide a simpler and smaller solution. If you want to use an external reference please let me know what your overall requirements are (TC, noise, power, size etc.) and I can make a recommendation.

    Regards,

    Rob

  • I looked at Jellenie's offer. But:
    1. I have power supply CPU - 5 V. Therefore, for ADCs 3.6 V it is necessary to coordinate the levels.
    2. The RTD is far from me and the wires can be around 380 V. Therefore, I use a higher current source (1 mA) to reduce the leaks.
    3. I have input signals of a constant voltage 0-5 V and 0-10 V. For new ADCs then you need to put an attenuator.
    4. Me processes of measurement are very slow (air temperature, humidity, humidity of wood), so the speed is not needed here. Power is also not important, and the size is desirable SOIC. I do not like very small cases.

  • This question has been assumed as answered either offline via email or with a multi-part answer. This question has now been closed out. If you have an inquiry related to this topic please post a new question in the applicable product forum.

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    EZ Admin