MCP9700 Readings flucuate

[den2k]

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.

[Dwalt]

@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.

[den2k]

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.

[Dwalt]

@den2k I was referring to the MCP9700 data sheet.

[den2k]

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.

[den2k]

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.