Correlation between 2xADC channels at LPC4370

[Radist]

Hi,

we are using LPC4370 and capturing differential signal at 2x ADC channels. MCU is configured for maximum speed - 204MHz. ADc is configured for 80 MHz.

We found, that we have correlation between signals, see picture below:

As you can see - that is not an issue of analog front end for sure. We are using DMA, so i dont think that it is an issue with SW. Also we have found, that in case, when ADC is configured for 10MHz - issue is absent:

Does anybody knows, why we have such kind of issue and how to solve it?

We have tried to connect 1k resistor in parallel with ADC input - to discharge ADC - no result.

[xiangjun_rong]

Hi, Sergii,

This is only my guess, as you know that the ADC analog channel is connected to a S/H(sample/hold), which is a capacitor actually.  In the sampling phase, the sampling route is connected, the external signal source charges the capacitor of S/H, while the holding route is disconnected. In the Holding phase of S/H component, the sampling route is disconnected, the holding route is connected, the ADC converter samples the voltage of the capacitor of S/H.

I suppose that the your external signal source has not enough driving capacitor, in other words, the external signal source can not  provide enough current to charge the capacitor, which leads to the voltage drop. I suggest you use a signal buffer with OP amplifier, for example, connect the tested signal to the non-inverter of OP AMP, connect the inverter pin of AMP to the output pin of AMP, connect the output pin of AMP to ADC analog channel. The AMP output has low impedance and high driving capability.

Pls have a try.

BR

Xiangjun Rong

[Radist]

Hi,

we have made our own library, that works OK with 80 MSps. As we can see - that issue is a purely HW issue, due to wrong connection of OPAMP to ADC input. Nothing with SW.

Does anybody have any application notes for this MCU, how to connect OPAMP correctly? What kind of RC network is needed?

--

Regards,

Sergii 

[a_bet]

Ok, I would ask if it is possible to have a look, but I bet it's not.
I know nothing about HW set-up, sorry.

[a_bet]

Hi,
pay attention to LPC4370+ADCHS, you can find a lot of posts describing a lot of issues ( see1, see2, see3, see4).
What I can say is that I never managed to get 80msps bandwith with ADCHS+DMA. But the system is working fine for 40msps.

[Alice_Yang]

Hello Sergii Makovetskyi ,

Sorry I have some confused about the picture, how to get this ? Do you mean ADC result data make up

the curve?

How about the HSADC demo under LPCopen ?

LPCOpen Software for LPC43XX | NXP         

Have a great day,
TIC

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