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ads1605evm dc offset

Simone Piccirilli
Prodigy 230 points
Other Parts Discussed in Thread: ADS1605

Hi at all,

I'm trying to measure a continuous signal from the sma input of the ads1605evm.

I've selected the jumpers on W1 and W5 so the inputs are directly connected to the analog input. Using a constant signal of 0 V at the input, in the digital output I read a negative signal.  So I've used a multimeter to measure the voltage on jumpers W1 and W5. With an input of 0 V on W5 the measure is 0 V, but on W1 there are about 1.5 V.

Is normal have a dc offset or is there something wrong in the configuration?

Thanks for any suggestion.

Ciao

Simone

  • TI__Guru**** 182201 points

    Hi Simone,

    The input to the ADS1605 is differential, but there is some common mode associated with it.  Try putting the AIN-  (W1) at a DC level of  approximately 2.048V and then swing the AIN+ (W5) from 0V - 4.046V 4.096 and let us know what you see.

  • in reply to Tom Hendrick
    Prodigy 230 points

    Hi Tom,

    many thanks for your help.

    So, on jumper W1 there is a voltage of about 2 V. Applying an input from 0 V to 4.65 V the response of the evm is like in this figure:

    I'm sorry for the quality of some figure, but is the best that I can have...

    Working with a signal generator, using a constant signal, I found an offset of about 1.73 V. Based on the digital output 0 in response of 1.73 V, I've scaled the binary data to obtain data and figures. The adc is quite linear until an input of 2.5 V. After that voltage there are negative peaks. For little ones, there is like a factor 2 but this isn't a problem.

    In the last figure the three points are X=2.7 Y=0.2581, X=2.71 Y=0.5212 and X=2.72 Y=1.04.

    For the biggest ones,they are on the level of negative full scale. This for a constant signal.

    Moreover, if I use a sinewave, until an amplitude of 500 mV PP, the response is linear. Over 600 mV it seems that the offset limits the range of the evm.

    With a constant signal I cannot see saturation, instead of the case with a sinewave.

    I hope that this figures can help you to understand how works the adc.

    In the end I've another question regarding the evm. Sometime happens that the evm doesn't work in normal mode (1x speed, 5 MSPS), but if I select the funcion 2x mode, it works without problems. Could it be an electrical problem or a problem with the switch?

    Thank you for your interest.

    Ciao

    Simone