LAUNCHXL2-RM57L: ADC induction 3-channel Problem

D Elisei

Part Number: LAUNCHXL2-RM57L
Other Parts Discussed in Thread: HALCOGEN, LM324

Hi,

I have the version of the pcb hercules Launch XL2-RM57 (B) Rev (A1).

I am using 3 channels AD1 [0], AD1 [7] and AD1 [18]. In ADC1_0 I am entering a triangular signal of 50hz, by ADC1_7 I am entering a sinusoidal signal of 50Hz and by ADC1_18 I am entering a continuous signal of 2V.

The result of the ADC channels is a mixture of the 3 signals.

I Check that by entering only one signal and measuring with an oscilloscope on the input pins of the other two ADCs, I can see the entered signal on the oscilloscope screen.

As if the pins of the ADCs were electrically connected.

Thanks for your help .
Best regards

Diego Elisei

over 7 years ago

0 NeilBerry_at_Parker over 7 years ago

Expert 1995 points

Try selecting [] Enable Sampling Capacitor Discharge on that page in HalCoGen; and retest your results.

0 D Elisei over 7 years ago in reply to NeilBerry_at_Parker

Prodigy 215 points

Dear, thank you for your response. I Try clicking on "enable sampling capacitor discharge" and the problem is decreased

but still continuous.
 I ask you to indicate how I should set the times to eliminate the problem. Thanks for your help. Best regards.

0 NeilBerry_at_Parker over 7 years ago in reply to D Elisei

Expert 1995 points

I do not have an answer to your follow-up question. I do not have Launchpad. I have an HDK, and a 40ns discharge time is more than I ever needed to use.
You might try going to the ADC1 General Tab; and extending the cycle time.
You might also be seeing a voltage source issue which is not due to the Launchpad hardware.

0 D Elisei over 7 years ago in reply to NeilBerry_at_Parker

Prodigy 215 points

I probe using a sample time of 100 ns and it has improved. Because of this problem I have placed 3 buffer op amp with LM324 to test if this eliminates the problem. Do you recommend that i put some other external circuit to the ADC input like a resistance to ground?

0 NeilBerry_at_Parker over 7 years ago in reply to D Elisei

Expert 1995 points

I am not informed enough to answer those questions. It sounds like you are making progress, and may have been helped by my suggestions.

0 Martin Valencia over 7 years ago

Guru 15175 points

Hello D Elisei, maybe a friend can help us!
Jan Cumps and Anthony F. Seely .

Best Regards

0 Anthony F. Seely over 7 years ago in reply to Martin Valencia

TI__Guru 68920 points

Hi Diego,

Sounds like you may have a source with high output resistance driving the ADC inputs.

You have to charge the internal sample/hold capacitor of the ADC before converting and the amount of time this takes in part is related to the output resistance of your signal source. That is why the sample time is adjustable, if you have a high source resistance you can increase the sample time to allow more time for the voltage on the ADC's Sample/Hold capacitor to settle before beginning the conversion process.

There is only a single Sample/Hold capacitor and the different inputs are just switches that open to allow a particular pin to connect to this S/H.
So if you are getting a mix of values of different channels you likely are not giving enough sample time.

If you cannot simply increase the sample time (time before conversion starts) to account for this then you can alternatively buffer the input say w. an opamp to get a lower source resistance.

There is an equivalent circuit for the ADC in the datasheet that you can use to estimate either on paper or w. a SPICE program how long your external circuit requires to drive the internal S/H to a settling point that is within the desired accuracy range of your application.

0 D Elisei over 7 years ago in reply to Anthony F. Seely

Prodigy 215 points

Thanks Anthony for your answer. I believe that the problem comes because the capacitor does not finish discharging. The ADC inputs are connected to an op amp as buffer. With which the adc have a low impedance input.
Can you pass me the link or the name of the datasheet of the ADC that you say?.
What would be the recommended external circuit to work with a sample of about 100ns?
Thanks for your help

0 Anthony F. Seely over 7 years ago in reply to D Elisei

TI__Guru 68920 points

Hi the ADC equivalent circuit is in the datasheet, figure 7-10. link to online datasheet section is www.ti.com/.../peripheral_information_and_electrical_specifications

0 Anthony F. Seely over 7 years ago in reply to Anthony F. Seely

TI__Guru 68920 points

This appnote is good too - use the GS60/F021 information in the appnote for this particular product.
www.ti.com/.../spna118b.pdf

0 D Elisei over 7 years ago in reply to Anthony F. Seely

Prodigy 215 points

THANKS!!!!!

0 D Elisei over 7 years ago in reply to Anthony F. Seely

Prodigy 215 points

Thanks Anthony!!!!!

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LAUNCHXL2-RM57L: ADC induction 3-channel Problem