Do I have a problem when ADC0OVR in ADCSTA is activated on a ADuC7060

In my actual design I switch between several RTD`s connected with three wires. I use both current sources and reference via 5k6 resistor for IEX0. It is connected like this:

IEX0 & Ref+---------[5k6]----------- Ref-  & Analog in0 ------------------> RTD+

IEX1 & Analog in 1 ---------------------------------------------------------------> RTD-

Ground -------------------------------------------------------------------------------> RTD GND

RTD- and RTD GND are connected with each other at the RTD. Because of simplicity the C-mos switched ADG713 are not mentioned here.

I use excitation current of 200µA for each source. By doing this I cancel the influence of the cable resistance.

The circuit and my source code is doing fine, but the ADC0OVR is alway set as soon as I connect my RTD. Does that mean my measurement is unstable?   Are there any code examples for 3 wire RTD applications?

Thank you!

  • 0
    •  Analog Employees 
    on May 9, 2012 1:12 PM

    Please find attached a simple project for Keil MDK µVision 4 which does convert RTD on ADuC7060 Evaluation Board, but no optimized linearization. It should show on a terminal like below all info about a PT100.

    Also have look on to the Circuit Note CN0075

    "USB Based Temperature Monitor Using the ADuC7061 Precision Analog Microcontroller and an External RTD"
  • 0
    •  Analog Employees 
    on May 23, 2012 8:59 AM

    This example was just updated recently for µV4. As the products has been develop, µV4 was not available. But a update of all our examples should only require some tool setting changes.

    Regarding your problem - you should have a look into the datasheet for input voltage specifications on page 6 & 7.

  • Hi MMA,

    thank you for your answer. The example is excellent and it´s first time that an ADI example is really µV4 compatible. ADI people should provide more µV4 examples in future...

    Regarding my problem it really depends where the shunt resistor is located. When I connect the resistor and the ref- Pin to GND everything works great and precise. If I move the resistor to IEXC0 out and Ref+ to IEXC0 out as shown in my first post then my results are corrupted. I don´t understand that situation, because the differential measurements should measure the same drop in voltage, because the currents are the same. Bit12 [HIGHEXTREF0] of the ADC0CON register just changes the result a little bit....

  • Hey MMA,

    thank you once again for your short reply. I read the data sheet again but I don´t see my mistake.

    External Reference Input Range can go from 0,1 to AVDD Volts. If I connect the resistor as shown in figure attached I have to set ADC0CON Bit 12, but the temperature values still differs from the method you have recommended. Figure is taken from data sheet from AD7711.

    Is it possible to use that circuit with ADuC706x anyhow?

    Thank you