SparkFun Qwiic Scale NAU7802 - Voltage

Sensors Force & Weight
1

Hi, I would like to convert a 0-5.5V analog signal to digital (then send the digital signal over an optical fiber to a Raspberry Pi). I am interested in the [SparkFun Qwiic Scale - NAU7802, and I would like to confirm which of the following pins we can apply up to 5.5V to:

  • E+ “Red”

  • A+ “Green”

  • AVDD

  • 3v3

    • The [schematic sheet states “VCC Power can be 2.7 to 5.5V but be careful not to allow more than 3.3V onto a Qwiic bus”. How does this translate to which pins can take up to 5.5V? Thanks.](https://cdn.sparkfun.com/assets/6/a/5/9/d/Qwiic_Scale.pdf)](https://www.sparkfun.com/products/15242)

    2

    The qwiic scale will not work for what you are wanting to do. What you need is an analog to digital converter.(ADC)

    Sparkfun probably doesn’t have one that can handle 5.5 volts but should have several that can handle 5 volts.

    With a voltage divider made with a few 1 megaohm resistors, you can drop that 5.5 volts down to 2.75 and then read that voltage with a ADC. Just remember to double the figure you calculate to get the original input voltage.

    You could even use just an arduino uno, it has six built-in ADCs so you wouldn’t even need an extra. As a bonus, the arduino can drive a LED to provide your digital signal for your optical fiber.

    3

    YellowDog:
    What you need is an analog to digital converter.(ADC)

    Sparkfun probably doesn’t have one that can handle 5.5 volts but should have several that can handle 5 volts.

    I only know since I had looked it up elsewhere on the boards here yesterday but the SF 4 chan ADC does go up to 6.1 volts in high range.

    https://www.sparkfun.com/products/15334

    4

    Thanks for the suggestion, but I need at least 14bits on my ADC.

    Why would the QWII Scale not work as a generic 24 bit ADC? [Its schematic seems nearly identical to the 16-pin Application Circuit [from the NAU7802’s datasheet.](https://www.nuvoton.com/resource-files/NAU7802%20Data%20Sheet%20V1.7.pdf)](https://cdn.sparkfun.com/assets/6/a/5/9/d/Qwiic_Scale.pdf)

    5

    Note: The NAU7802 is an ADC. The QWII Scale is a breakout for the NAU7802.

    6

    The NAU7802 is for raw sensor input. It has built in bias (the good kind) handling and signal processing and temperature compensation and amplification so you can better detect tiny changes in low DC signals, specifically from strain gauges used for weight or pressure or elongation or light intensity or temperature or other electro-mechanical transducers. It also integrates a power supply and sense reading to excite the gauges. Only after this processing and amplifying is the signal sampled and digitized. We see this called different things: signal conditioning, processing, preamplification, device name: transmitter. I don’t think the chip has any designed provision to bypass the processing prior to digitizing and it would probably take quite a hack to get it to do so.

    Your 5 volt signal would appear to have already had the ‘front end’ of the signal processing performed. It’s similar to the difference between the guitar or a phonograph (!) input vs a ‘line in.’ The guitar/record player are electrically passive and need the help of the preamps to get their signals suitable for playback, mixing, recording, etc. Your signal is already loud enough for what ever the next step is (like digitizing).

    Which gets to another point to be aware of: in case you were hoping to digitize audio or anything else that changes more than basically a few times a second, it’s not going to work.