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ADS1298: Inconsistent SPI readout of test signal from ADC

Part Number: ADS1298
Other Parts Discussed in Thread: ADS1296, ADS1294, , ADS1296R, , ADS1294R

Hi there,

Background
Following some changes made in this thread: https://e2e.ti.com/support/data-converters-group/data-converters/f/data-converters-forum/1018492/ads1298-cannot-establish-spi-communications, we've made some progress with the SPI comms.

The voltage lines are stable now after tieing RESV1 to DGND, and we scoped the MOSI and MISO signals and they look ok.

However, we are seeing discrepancies in values for test signal on our design as compared to the dev kit.

The issue

We are trying to baseline the ADSx98 FE Rev C deg kit against our design which is almost identical with a few deviations.

We are attempting to perform an internally generated signal to read values from the internal registers. Below is a header file with more details of the register settings:

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#ifndef ADS1298_H
#define ADS1298_H
#ifdef __cplusplus
extern "C" {
#endif
#include "../common.h"
typedef enum {
// system commands
ADS_CMND_WAKEUP = 0x02, // wake-up from standby mode
ADS_CMND_STANDBY = 0x04, // enter standby mode
ADS_CMND_RESET = 0x06, // reset the device registers
ADS_CMND_START = 0x08, // start/restart (synchronize) conversions
ADS_CMND_STOP = 0x0A, // stop conversion
// data read commands
ADS_CMND_RDATAC = 0x10, // enable Read Data Continuous mode.
// - this mode is the default mode at power-up.
XXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXX

The issue is that there is a deviation in voltage measurements between the two setups in general. Below is a summary table of voltages measured between the two setups:

Channel No. ADS1298 DevKit & GUI SW (mV) Custom Design (mV)
1 -1.588392 -1.237154154
2 -0.723791122 -1.197433614
3 -1.223898 -1.365900203
4 -1.398134 -1.233053354
5 -1.174879 -0.781917665
6 -1.245832 350.5889834
7 -1.276207 -79.24800864
8 -0.989675522 -76.67232474

There also seems to be a large fluctuation in values read in our design when measuring the test data on channels 6, 7 and 8. All voltage measurements on all channels read on the dev kit via GUI are stable. Are there any hardware connection dependencies to perform this test?

Looking forward to some suggestions and/or troubleshooting steps.

Thank you in advance.

Best regards,

Eric

  • Hi Eric,

    There should not be hardware connection dependency for using the internal test signal. Can you verify all the channels are operating normally with the input-short test by setting MUXn[2:0] to 0b001? What does the ADC output waveform look like for CH6 to CH8 when using the internal test signal?

    Thanks

    -TC