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Noise levels in ADC in STM32L4?

Question asked by Dave Jones on Sep 19, 2017
Latest reply on Sep 24, 2017 by Dave Jones

I have a custom board using an STM32L452. I'm doing some testing of the ADC and am finding pretty high levels of noise.

 

I have a rail-to-rail op amp driving the ADC pin with a 300 ohm resistor from the amp to the pin and a .047uF cap to ground at the pin. The board is laid out well, with lots of ground under and around the chip, and bypass caps on every power pin, right next to the pins. Plus a bulk 10uF ceramic on the Vdd. The Vaa has it's own .1uF and 10uF right next to the pin. The Vdd and the Vaa each have a ferrite separating them from the main board power source (and therefore from each other).

 

In addition, to try and reduce external noise during my tests I added several extra caps on top of the cap on the ADC pin. A .1uF, a 1uF, and a 10uF ceramic, all stacked on top of the original cap. I did the same series on top of the bypass cap for the Vaa. (these extra caps will be removed after the tests) Those caps are all X7R and progressively larger physical packages to overcome the DC Bias issue ceramic caps have.

 

The input signal is a constant DC biased in the middle of the input range.

 

In the DMA interrupt routine I look at each result from that one channel and compare it to a previously stored high and low value. If the new one is higher than the old high, it replaces it. If lower than the old low value, it replaces it. After 100,000 conversions I save that high and low value in another pair of variables and reset the pair used in the comparison. Those saved values are then read via I2C by an external device that displays them and the difference between them. So that LCD shows me the high and low value for the past 100,000 conversions plus the difference between them.

 

In an ideal ADC that difference would be zero. In my previous version of this board (same op amp driving the ADC) I had a PIC24 chip, and with 12 bit conversions (and no additional caps) I saw typical conversions that fluctuated by about 4-6 numbers. With this board and its STM32L452 I am seeing typical variations in the range of about 13 to 22 numbers. About 4 times more noise than the PIC.

 

I tried various settings for the ADC clock prescaler, both synchronous and asynchronous, divided by 1, 2, and 4. And  tried all the different sampling times. There were small changes at different settings, but not a large amount. The results ranged from 13-16 numbers different up to 19-22 numbers different. So varying from about 3.5 to 4.5 bits worth of noise instead of the 2 to 2.5 bits worth of noise I had with the PIC.

 

That means that at 12 bit conversions I'm really only seeing 7 to 8 bits of clean signal.

 

I tried the oversampling, and it does help. But I need conversions at a decent rate and it would take 256 times oversampling to recover about 4 bits. That would reduce the speed far too much for my application. This is going to process control voltages in the audio frequency range, not extremely slow sensors, where I could get by with excessive filtering in both the hardware and software.

 

Has anybody else done an extensive test of the ADC on an L4 (or other STM32)? And are you seeing similar amounts of noise in the raw conversions?

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