How to reduce noise in VCC line...

[alank2]

Hi,

I've got a temperature sensor MCP9701A in today.  I noticed that it is very sensitive to noise.  If I hook it to my bench power supply the output is correct around 0.9V, but if I power it from my frequency counter breadboard with a 150 MHz oscillator being measured, it pushes the voltage higher, 1.2V or so.

I've tried a low pass filter with a 270 ohm resistor and various capacitors and this wasn't very helpful.  What other ideas should I try?  This is mostly a learning exercise...

The data sheet does say this:

4.3 Layout Considerations
The MCP9700/9700A and MCP9701/9701A family
does not require any additional components to operate.
However, it is recommended that a decoupling
capacitor of 0.1 ?F to 1 ?F be used between the VDD
and GND pins. In high-noise applications, connect the
power supply voltage to the VDD pin using a 200?
resistor with a 1 ?F decoupling capacitor. A high
frequency ceramic capacitor is recommended. It is
necessary for the capacitor to be located as close as
possible to the VDD and GND pins in order to provide
effective noise protection. In addition, avoid tracing
digital lines in close proximity to the sensor.

[alm]

Solderless breadboards with their long wires aren't exactly the most favorable for EMI. Making wires shorter, reducing coupling between wires (no large loops or parallel runs), twisting positive and negative wires and increasing the distance between sections might help. Note that there is about 10pF of capacitance between the rows in a breadboard that might capacitively couple the signals. A 10 pF capacitor has an impedance of only 100 ohm at 150 MHz. You can try grounding rows in between the high frequency signals and power rails. 150 MHz is much higher than I would ever go with solderless breadboards, since building circuits with 10 pF capacitors spread around the circuit gets quite tricky at those frequencies.

Try veroboard or even better soldering the components involving high frequencies to a piece of copperclad (that acts as a ground plane) to reduce the noise emitted from the frequency counter.

If it's just noise conducted via Vcc, then you can try cleaning it up with an inductor or ferrite bead that's effective at those frequencies.