I have looked around and found that the ADC appears to be notoriously unstable on this device. I decided to do some checking/testing. I set the analog output to 50% and connected it directly to the ADC. Gues what? EXTREMELY stable. out of 1000 samples taken at 0.5 second intervals 56% were 0.499, 43% were 0.498, 1% were 0.497. This suggests there is a problem with using an external voltage source on the ADC.
So, I tried a pot to see what happened. The variation was incredible. The pot was set for 1.65V (50%) from a regulated 3.3V source ( as checked with a calibrated DMM) with a ripple in the noise region (< 1uV) as checked with a calibrated oscilloscope. The variation went anywhere from 0.9 to 0.1 with all sorts of steps in between. The reading was unstable around 0.5 with extensive variation from 0.6 to 0.4 occurring at almost every step and rarely landing on 0.5.
I tried using a snubber network (resistor capacitor) to smoothe out the signal - in spite of the fat that the pot was connected across a regulated dc/dc power supply with a verified insignificant ripple. No improvement was noticed. Now it's possible that there is alot of EMI/RFI noise in my area, so I decided try a shielded conductor. A coax cable was used with with shield tied to the mbed ground. This did not improve the problem.
Now it's also possible that the breadboard I am using is just too much of an antenna, so I decided to see if I could recreate the problem using the mben analog output. I installed a 3' (3 foot - yes) conductor (just regular 24 gauge hookup wire) between the analog out and the analog in - as I did in my original test. The reading showed the same level of stability as the original test.
So, what gives?
How am I supposed to be able to use the ADC with an external source and have any confidence in the reading I am receiving?
I know I'm not the only one who has seen this problem, but I haven't found a solution. Does anyone have an idea of what I might can try? I would like to be able to use this device, but this is just so far from acceptable. Any thoughts?
Pekka Saavalainen.
1 reply
I have looked around and found that the ADC appears to be notoriously unstable on this device. I decided to do some checking/testing. I set the analog output to 50% and connected it directly to the ADC. Gues what? EXTREMELY stable. out of 1000 samples taken at 0.5 second intervals 56% were 0.499, 43% were 0.498, 1% were 0.497. This suggests there is a problem with using an external voltage source on the ADC.
So, I tried a pot to see what happened. The variation was incredible. The pot was set for 1.65V (50%) from a regulated 3.3V source ( as checked with a calibrated DMM) with a ripple in the noise region (< 1uV) as checked with a calibrated oscilloscope. The variation went anywhere from 0.9 to 0.1 with all sorts of steps in between. The reading was unstable around 0.5 with extensive variation from 0.6 to 0.4 occurring at almost every step and rarely landing on 0.5.
I tried using a snubber network (resistor capacitor) to smoothe out the signal - in spite of the fat that the pot was connected across a regulated dc/dc power supply with a verified insignificant ripple. No improvement was noticed. Now it's possible that there is alot of EMI/RFI noise in my area, so I decided try a shielded conductor. A coax cable was used with with shield tied to the mbed ground. This did not improve the problem.
Now it's also possible that the breadboard I am using is just too much of an antenna, so I decided to see if I could recreate the problem using the mben analog output. I installed a 3' (3 foot - yes) conductor (just regular 24 gauge hookup wire) between the analog out and the analog in - as I did in my original test. The reading showed the same level of stability as the original test.
So, what gives?
How am I supposed to be able to use the ADC with an external source and have any confidence in the reading I am receiving?
I know I'm not the only one who has seen this problem, but I haven't found a solution. Does anyone have an idea of what I might can try? I would like to be able to use this device, but this is just so far from acceptable. Any thoughts?