AD7730 setting NOREF bit in AC excitation

Hey guys!

I am using an AD7730 in AC excitation mode. For AC excitation of a Wheatstone bridge circuit, I connected the ACX ports of the AD to a MIC4427 as recommended in the datahseet. REF+ and REF- are connected to the IN+ and IN- of the bridge, respectively. The whole setup is exactly as depicted on page 41 of the datasheet.

For controlling the AD7730, it is connected to a Arduino Nano via SPI. Communication is running fine.

The AD7730 is performing the following operations:

1. Internal Full Scale Calibration (unipolar, 24bit, 5V ref, 0 to 80 mV)

2. Internal Zero Scale Calibration (unipolar, 24bit, 5V ref, 0 to 80 mV)

3. Setup of Filter Register (= turn on AC excitation)

4. Setup of Mode Register (continuous conversion, unipolar, 24 bit, 0 to 80 mV)

5. Continuous Reading of the Data Register on RDY signal

So far, everything seemed fine, if only I would get real conversion results other than only 'ones'.

I read the datasheet in detail and found that the Data register is filled with all 'ones', if the Reference Voltage (Vref+ - Vref-) is below 0.3V. Also, the NOREF bit in the Status register is set in this case. When reading the Status register, I found NOREF being set, so no wonder I am only getting only all 'one' results.

How do I fix this issue? Obviously, the reference voltage will be -5V at some point due to the AC excitation.

Best regards

Daniel

  • 0
    •  Analog Employees 
    on Dec 5, 2018 10:56 AM

    Hi, Daniel.

    Can you share your schematic diagram along with the complete register map settings? The AD7730 should accept an inverted reference voltage when AC excitation bit is 1. Have you used the delay between switching signals? This is achieved using the DL bits of the filter register. Have you also tried to monitor or probe your reference voltage to see if it's present?  

    Thanks,

    Jellenie

  • Dear Jellenie,

    thank you for your reply. I added the schematic. Please note that RV1 and RV4 are potentiometers which are 0 Ohms in our test setup and might only be used for current limitation in our application. Thus, there is no voltage drop on those parts.

    The registers are mapped as follows:

    FILTER: 01111111 11110000 00111111

    MODE: 00110001 10110000

    I double-checked it for correct setup. Do you need me to post the other registers as well?

    I noticed that the NOREF bit is already set after the execution of the calibration which is done on every startup. At that time, the AD7730 operates in DC mode. During the calibration Vref+ is 5V and Vref- is 0V.

    We tried using both the maximum and minimum delay. We also monitored the reference voltage with an oscilloscope. The ac excited voltage levels are correctly applied to the pins and do have the correct phase shift. With RV1 being 0 Ohms, the excitation voltage is directly connected to the reference pins. I did also check that connection.

    schematic_v3.pdf

  • 0
    •  Analog Employees 
    on Dec 14, 2018 4:23 AM in reply to dgraebner

    Hi,

    Apologies for the delay.You have mentioned that you continuously checking the reference pins and they are all in a valid  reference voltage level is that correct? This is something unusual because NOREF bit is only active if the differential voltage reference is below 0.3V or either of the REF pin inputs is open. May I know also if you have checked or probe your analog inputs? Are they within the input specification limit? If you performed a reset and then check the NOREF bit after performing a reset does it flagged already?

    Thanks,

    Jellenie

  • Dear Jellenie,

    we did indeed check all pins for correct voltage levels. All voltages are within the correct range.

    We found that after reset the NOREF bit is not set. However, after executing the internal full scale or zero scale calibration, the NOREF bit is set. Please find the calibration code below:

    send(0b00000010); //write to mode register
    delay(5); //wait 5 ms

    send(0b10110001); //internal full scale calibration, unipolar, 24bit
    send(0b10110000); // 5 V ref, 0 to 80 mV
    while (digitalRead(RDY1) == HIGH) { };

    send(0b00000010); //write to mode register
    delay(5); //wait 5 ms

    send(0b10010001); //internal zero scale calibration, unipolar, 24bit
    send(0b10110000); // 5 V ref, 0 to 80 mV
    while (digitalRead(RDY1) == HIGH) { };

    Kind regards

    Daniel

  • 0
    •  Analog Employees 
    on Jan 28, 2019 11:32 AM in reply to dgraebner

    Hi,

    If NOREF bit becomes active upon performing either an offset or gain calibration then loading/updating calibration register is inhibited. And also If NOREF is active on completion of a conversion, the Data Register is loaded with all 1s. I noticed on the schematic that you have REF_IN1+ connected to REF_IN- and vice versa, is that correct? I think this should be in reversed position. Can you check this please.

    Thanks,

    Jellenie