MCP9700 Readings flucuate

  • Does anyone else have this similar issue?

    When using power through a USB FTDI dongle, reading are very stable (beginning part of chart showing roughly 72F), but when I plug in just the battery source at around 11PM, the readings swing dramatically as you can see. This is my hookup minus the DPT sensor, swapped in with a MCP9700 sensor.


    10-8-2015 6-08-44 AM.jpg

  • @den2k You are getting noise from the voltage regulator. From the data sheet:
    ... it is recommended that a decoupling
    capacitor of 0.1 µF to 1 µF be used between the VDD
    and GND pins. In high-noise applications, connect the
    power supply voltage to the VDD pin using a 200ohm
    resistor with a 1 µF decoupling capacitor. A high fre-
    quency ceramic capacitor is recommended. It is neces-
    sary that the capacitor is located as close as possible
    to the VDD and GND pins in order to provide effective
    noise protection.

  • Thanks for the input. Just to clarify, are you talking the stepup data sheet?

    But aren't the caps on the stepup module enough to dampen and stabilize the power. I shouldn't assume it does, but figure I'd ask.

  • @den2k I was referring to the MCP9700 data sheet.

  • This is the hookup I have for the MCP9700. Hmm. Any additional thoughts? I'm going to try adding a 1uF and 200 ohm resistor and see if anything happens.


  • Problem solved. For those interested. The issue was the 3.3V Step up board. When the radio turns on there is a large power draw which causes the voltage on the 3.3V rail to drop. This caused the temperature reading fluctuations I was seeing. I added a large cap between the Vout / GND of the step up. Problem solved.

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