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The ADS1292, ADS1298, and ADS1299 devices in the ADS129x bio-potential ADC family offer multiple channel variants. Sometimes, the value read from the Device ID register may reflect the incorrect number of channels. Why does this happen?
Applications Engineer | Precision ADCs
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Most of the time, an incorrect Device ID is caused by an improper power-up sequence. After your system powers on, both the ADC's analog and digital supplies require some time to ramp to their final value. In addition, there are internal voltage nodes generated from the supplies that must ramp as well, such as the internal bandgap voltage (VBG). The internal bandgap is used to bias much of the device's internal circuitry and can be connected to an external bypass capacitor for filtering on the VCAP1 pin. It's important for VBG to fully ramp since it is needed to read the one-time programmable (OTP) bits that contain the channel count information and are set during the device's final test program. Depending on how fast the supplies ramp, these bits may be misread and default to another value.
An external /RESET pulse is strongly recommended after power-up to ensure the device initializes in a known good state. Check the Power-Up Sequencing section in the datasheet to determine when to toggle the /RESET pin and begin using the device. As an example, refer to the figure below from the ADS1298 datasheet:
FIGURE 1: ADS1298 Power-on Reset Timing
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This post applies to ADS1191, ADS1192, ADS1291, ADS1292, ADS1292R, ADS1294, ADS1294R, ADS1296, ADS1296R, ADS1298, ADS1298R, ADS1299-4, ADS1299-6, and ADS1299.
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