I've had a number of issues getting good stable analog readings from an RTD temperature sensor. The temperature readings have a tendency to bounce as much as +/- 5 Deg C. The circuit is very basic, a voltage divider is formed between a fixed resistor and an RTD connected to ground. The voltage at the RTD is then fed into a programmable gain amplifier set at 16 V/V. The analog signal output from the programmable gain amplifier is then fed into a 10-bit ADC on a PIC microprocessor. I've implemented a low pass filter in software to attempt to fix the problem, but the temperature readings are still very unstable. Does anyone have any suggestions about where I could look to find the source of the problem and correct it?
Electronic – Reading RTD Temperature Sensor
analogpicsensortemperature
Related Solutions
It sounds like you are missing pull up or pull down resistors on your inputs. If you leave your inputs floating you will get unreliable results.
http://en.wikipedia.org/wiki/Pull-up_resistor <== read this for more info
If your input is normally off you'd use a pull down resistor and if normally on you'd use a pull up resistor. This is a simplified way of thinking about it but it makes it easy to know what to expect when reading the input.
When you look at the little schematic it show an open switch and a pull up resistor. This means that when they switch is open Vin will be going thru the buffer to Vout. Vout = Vin here. When the switch is close Vin will go thru the pull up resistor to thru the switch and to ground. The input of the logic gate would have 0 volts and would be directly connected to ground. Thus Vout = Ground.
Increase R5 and R10 to 50K and add a 1uF ceramic from U3 +input to ground and from U2 +input to a solid analog ground (keep the analog grounds together). Split R8 into two series 50K and add a 1uF ceramic capacitor to ground from the mid point. Lose the two Schottky diodes (too leaky, especially with 50K) and R11 (useless).
Make sure your return wire from the RTD (you must have three wires running back to your circuit from the RTD, which must not be grounded at the chassis) is connected to the low side of R14 and thence to circuit ground as I said above about keeping them together.
If you don't have access to a 'scope you'll be fairly limited, but if you're still seeing 'noise' try measuring the voltage at each op-amp output (including the current source) using a multimeter on the minimum AC volts range. Many will block the DC and show you some indication of what is going on in the AC world.
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Best Answer
The first thing I'd look at is ripple in your voltage rail. Noisy voltages are killer with a voltage divider. A higher-quality measurement system might use a regulated constant current source to drive the RTD instead of a voltage divider. An LM317 can be used to do this - hook a resistor between the OUTPUT and ADJ terminal, and hook the RTD between ADJ and GND. The value of the resistor between output and adjust will set the current going through the RTD - use a precise resistor to be certain of the amount of current.
Otherwise, attempt to do the filtering in hardware if possible. You first have to figure out where the noise is coming from to make it effective. Determine what frequencies of noise you're seeing and then probe at the input to the gain stage, the output, and the input to the ADC. If there's noise everywhere, then it's in the source, otherwise it's being injected somewhere else. Make sure all of your IC's have bypass capacitors to start with. Then make sure that you don't have any long ground loops - make everything as direct (high current) of a connection to ground as possible. Don't daisy chain grounds - everything should get its own connection to ground that doesn't run through other chips.
If you're seeing noise at the source, chances are it's your voltage source for the divider. To combat this, you could put a capacitor in parallel with the RTD to make a simple filtering circuit. Just figure out what frequencies of noise you're seeing and match the capacitor to the resistance of the RTD and figure it out.