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ADS1258: Noise/saw tooth signal on AIN pin while converting

Matt11
Prodigy 120 points
Part Number: ADS1258
Other Parts Discussed in Thread: LMV358, OPA320

I am using the ADS1258 in a design that measures a 0 - 5V signal. I have the ADS1258 configured to take a sample every 64 Hz using the pulse convert command.

(note: the voltage divider components on the AIN6 line. These have been removed/shorted for this test.) 

I noticed that when the ADC conversion is taking place, the AIN6 line has significantly more noise on it. The noise looks like a ramping signal.

I've taken some screen shots of my oscilloscope to show the issue.

1. ADC_START pin low, measuring 2V signal into AIN6 while the ADC is not converting.

2. ADC_START pin high, measuring 2V signal into AIN6 while the ADC is converting.

3. ADC_START pin low, measuring 4V signal into AIN6 while the ADC is not converting.

4. ADC_START pin high, measuring 4V signal into AIN6 while the ADC is converting.

It seems that as the input signal increases in amplitude the noise/ramping also increases.

Is this noise/saw tooth signal caused by the internal ADC caps charging and discharging while taking samples? Is this considered normal?

  • 0
    TI__Mastermind 44680 points
    Hi Matt,

    Yes, the ADC inputs are connected directly into a switched capacitor circuit and there may be some charging/settling associated with these inputs. I suspect that as you increase the series resistance between the LMV358 and the ADS1258 that you'll also see the amplitude of the saw tooth signal increase as the RC time constant increases.

    Typically, we would recommend a higher bandwidth opamp, such as the OPA320, to buffer the ADC inputs. I'm not sure what level of performance you'd be able to achieve with the LMV358.

    Are you seeing any large gain errors in the ADC output codes that might indicate that you're having a settling issue?

    Best regards,
    Chris