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ADS1258: ADS1258

Indumathi Meenakshi
Prodigy 40 points
Part Number: ADS1258

Hi,


We are using ADS1258 in our design. Able to write and read register ADS1258 registers (00h-09h).

ADC channels from AIN0 to AIN15 need to operated as single ended channels. We have configured the ADS1258 registers as below.

CONFIGO -0X3A.

CONFIG1 -0X83.

MUXSCH -0XFF.

Reference voltage applied is 5V.

Required hardware signals START and DRDY are taken care. After the above ADC register configuration, START signal (18 micro sec) is being applied from host and after 36 micro secs DRDY goes low, then Channel data read command 0x30 has sent to ADS1258 and received 4 bytes (including status byte) of data from ADC.

We are facing the below issues.

1. What ever the voltage applied at channel input (0v or 5v), Channel data is same and it is 0x7FFFFF.

2. How to know that, received to belongs to which channel?

Thanks,

  • 0
    Prodigy 40 points

    Hi,

    We have one more observation that is, we modified the MUXSCH as 0xF0. In this case, channel data is different for 0V and 5V.

    Channel input voltage is 0V – Channel data is 0x7FFFFF.

    Channel input voltage is 5V – Channel data is 0x800000.

    We have tried different voltages (like 0.5 V, 1V, 2V, 3V, 4V) at channel input and expected is different channel data, but channel data is always switches between two values.

    if the channel input voltage is less than 4V – channel data is 0x7FFFFF and above 4V -- channel data is 0x800000.


    Here the query is that, why the channel data is not giving intermediate values for the voltages like 0.5 V, 1V, 2V, 3V, 4V?

    Thanks,

  • 0 in reply to Indumathi Meenakshi
    TI__Mastermind 44550 points

    Hi Indumathi,

    Welcome to the TI E2E Forums!

    Which ADC input pins are you connecting your input signal to? 

    Also, do you have a buffer circuit between the MUXOUT and ADCIN pins?

    In your current register configurations, you are setting the MUXMOD bit of the CONFIG0 register high, which operates the ADS1258 in fixed-channel mode. In fixed-channel mode, the only way to know which channel you are measuring is to recall or read the value of the MUXSCH register.

    Make sure that your input signal is applied to the same pins as the MUX channel that you select in the MUXSCH register. If MUXSCH = 0xF0, then the ADS1258 will measure the differential voltage between AIN15 (positive input) and AIN0 (negative input).

    Also, with the BYPAS bit of the CONFIG0 register high, the ADS1258 requires that you provide an external buffer or other signal conditioning circuit between the MUX output and the ADC input pins of the device. If you don't have these pins connected, then set BYPASS to 0.

  • 0 in reply to Christopher Hall
    Prodigy 40 points

    Hi Chris Hall,

    There were some wrong connections to Reference voltages, we have fixed that.

    We have configured ADC to auto scan mode (CONFIG0[MUXMOD] = 0) and selected Channel 0 (MUXSG0 = 0x01), Now channel data is varying for different input voltages.

    We are trying to operate ADC in fixed channel mode and like to select channel 0 (single ended channel). So we have configured ADC as (CONFIG0[MUXMOD] = 1) and (MUXSCH = 0x00). In this case, What ever the voltage applied, converted channel data is same.

    Here my query is that: What is the ADC configuration to operate any channel in “single ended channel” mode?

    Thanks,

  • 0 in reply to Indumathi Meenakshi
    TI__Mastermind 44550 points

    Hi Indumathi,

    In fixed-channel mode, the MUXSCH register is used to select the analog inputs. The upper nibble of this register selects the positive input and the lower nibble selects the negative input.

    Setting MUXSCH = 0x00 selects AIN0 for both the positive AND negative inputs (this is effectively a short).

    You'll likely want to set MUXSCH = 0x01, for example, to select AIN0 as the positive input and AIN1 as the negative input.

    One thing to be aware of in fixed channel mode, is that AINCOM is not selectable. Therefore, you may need to use another analog input as the common-reference for single ended signals. If you plan on switching between auto-scan fix channel modes, then I would recommend shorting AINCOM and AIN15, so that you can use the single-ended input channels in auto-scan mode, and use AIN15 as the common-reference in fixed channel mode.