stm32f302 ADC capture influences input signal

[YDong.11]

I'm testing stm32f302 ADC capture with DMA transfer, but get some strange issues.

I set the ADC1 to capture single channel AIN2(PA1) in continuous and DMA mode at 72MHz and 1.5 sample cycles.

Set the DAC1 output a fixed voltage of 2048 with VDDA = 3.3V at PA4.

If I connect the PA4 to PA1 using a jumper, during the ADC capture, there will be a huge disturbance in PA4, about 248mVpp with 5.14MHz frequency.

If stop the ADC capture or remove the jumper connection, there will be no these disturbances.

1. Connect the PA4 to PA1 and start the ADC capture.

2. Disconnection PA4 and PA1 or stop the ADC capture.

Anybody has same issue or there are some bugs of ADC?

[YDong.11]

The test board is NUCLEO-F302R8

[waclawek.jan]

The ADC works as a capacitor with a few pF capacity (value specified in the datasheet), charged to half of VREF, which is at the beginning of the sampling phase connected to the input.

The DAC has a significant output impedance (depending on whether its output buffer is switched on or not). Again, for the particular value resort to the datasheet.

These two factors determine the "dip" in the input signal at the beginning of the ADC sampling phase.

JW

[YDong.11]

The output buffer is enabled. But the captured ADC data also has a huge noise about 83 code(peak-peak).

This doesn't make sense.

[waclawek.jan]

Try a different power supply.

Try a "known good" signal source, e.g. a common 1.5V AA battery.

JW

[Ozone]

A sample time of 1,5 cycles is extremely short. Is your input circuitry really up to that ?

> The output buffer is enabled.

With the output buffer enabled, the DAC has up to 200mV offset.

Check the datasheet.

[YDong.11]

Yes, I need 4-5MSPS to capture a 600kHz sine wave. And I try to increase the sample time to 181.5, this issue still exists.

Additionally, I add a extra OPAMP1 in Follower mode as a buffer. But the result is same.

I have never seen this issue in Cypress, TI MSP430 ADC.

I don't care the offset. The offset is a fixed value which can be calibrated.

[Ozone]

> Yes, I need 4-5MSPS to capture a 600kHz sine wave.

That's fine, just wanted to mention that. The design/selection of an appropriate input buffer for 5MSps is a non-trivial task.

For the DAC part, did you see the bandwidth specs ?

This device is IMHO not up to the task.

I would try a proper source for the 600kHz sine signal.