LPC4370 low impedance HSADC on the first 10 samples

[vitaliylivnov]

Hello. We are writing a waveform acquisition program using LPC4370 HSADC and GPDMA. We tuned the ADC to a sampling frequency of 80 MHz, applied a DC bias to the inverting input, set up a table of descriptors as follows:

Chip_HSADC_SetupDescEntry(LPC_ADCHS, 0, 0, (HSADC_DESC_CH(0) | HSADC_DESC_MATCH(0x00) | HSADC_DESC_BRANCH_FIRST |
 HSADC_DESC_THRESH_NONE | HSADC_DESC_RESET_TIMER));
Chip_HSADC_SetupDescEntry(LPC_ADCHS, 0, 1, (HSADC_DESC_CH(0) | HSADC_DESC_MATCH(0x30) | HSADC_DESC_BRANCH_FIRST |
 HSADC_DESC_THRESH_NONE | HSADC_DESC_RESET_TIMER));
Chip_HSADC_SetupDescEntry(LPC_ADCHS, 1, 0, (HSADC_DESC_CH(0) | HSADC_DESC_MATCH(0x00) | HSADC_DESC_HALT |
 HSADC_DESC_THRESH_NONE | HSADC_DESC_RESET_TIMER));‍‍‍‍‍‍‍‍‍‍‍‍‍‍‍‍‍‍‍‍‍

Sampling starts on request from the client with the following function:

void scopeStart(void)
{
 Chip_HSADC_FlushFIFO(LPC_ADCHS);
 Chip_HSADC_SetActiveDescriptor(LPC_ADCHS, 0, 1);
 Chip_HSADC_EnablePower(LPC_ADCHS);

 DmaHsadcInit();

 Chip_HSADC_SWTrigger(LPC_ADCHS);

 Board_LED_Set(0, 1);
}‍‍‍‍‍‍‍‍‍‍‍‍‍‍‍‍‍‍‍‍‍‍‍‍

Sampling stops with the following function:

void scopeStop(void)
{
 LPC_GPDMA->C0CONFIG |= (1 << 18); //halt further requests
 while (LPC_GPDMA->C0CONFIG & (1<<17)) {}; // wait for the current dma transaction to complete
 Chip_GPDMA_Stop(LPC_GPDMA, HSADC_DMA_CH);

 //switch to halt-descriptor
 Chip_HSADC_SetActiveDescriptor(LPC_ADCHS, 1, 0);
 Chip_HSADC_DisablePower(LPC_ADCHS);

 Board_LED_Set(0, 0);
}‍‍‍‍‍‍‍‍‍‍‍‍

We use the LPC-LINK2 board, to which a 12 MHz sine wave generator with an output resistance of 100 Ohms is connected. The problem is that after the start of sampling, the first 10 samples show a slight increase in amplitude, which then gradually decreases:

When you connect an oscilloscope to the input of LPC-LINK2, you can see the following picture:

It can be seen that immediately after the launch, the ADC has a low input impedance, which causes a jump in the amplitude of the signal upwards with each sample. After a dozen of samples, the picture is normalized.

Please tell us how to fix this problem.

[xiangjun_rong · Accepted solution]

Hi, Vitaliy,

Regarding your question that the external analog signal is distorted when the ADC converter is launched, as you know that the ADC converter includes a Sample/Hold circuit, which is a capacitor, when the ADC is launched, at the sample phase,  the external circuit charge the capacitor, because of high conversion rate(80MSPS), the sample time is very short, which behaves like a low impedance, so the analog signal is distorted.

If you sample a periodic signal, I suppose you can truncate a initial samples when you process the data.

Hope it can help you

BR

XiangJun Rong

[vitaliylivnov]

Hi, xiangjun.rong‌

Thanks for your answer. I understand what you mean. Probably, excluding the first 10 samples will be the easiest option.