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Part Number: ADS1256
This is in regards with ADS1256.
History: The ADS1256 VDD is connected to 3.3VDC rather than 5VDC, this is an design error. Since we did not notice any abnormal operation on the ADC functionality and clarification from this forum we have not modified the circuit and continued.
We are using all the 8 Channel of the ADC for measurement of Temperature (1 Channel), pH (1 Channel), conductivity (4 Channels) and Turbidity (2 Channels). The ADC reference voltage is set to 1.6VDC
Test setup: 3.3V to the system is provided from the Launchpad MSP-EXP432E401Y.
System operation: On completion of AIN0 sampling and conversion, the channel is changed to the next channel until AIN7 is complete. The sampling rate for each channel is different at mentioned below.
AIN0 Sample rate 2.5SPS with 10 Samples
AIN1 Sample Rate 2.5SPS with 20 Samples
AIN2 Sample Rate 25SPS with 10 Samples
AIN3 Sample Rate 25SPS with 10 Samples
AIN4 Sample Rate 25SPS with 10 Samples
AIN5 Sample Rate 25SPS with 10 Samples
AIN6 Sample Rate 3750SPS with 10 samples
AIN7 Sample Rate 3750SPS with 10 samples
Observation 1: With this setup we the observed the ADC reading for AIN0 or AINx are different from the reading taken when sampling only one channel (collect 10 sample do required calculation and continue to collect samples in a loop). Basically if the sequence is followed we get 'x' value from the ADC for AIN0 and if only AIN0 is sampled continuously without channel change we get 'y'. The difference (x-y) is linear for all inputs on AIN0. We would like to understand why there should be a difference in the conversion data if the channel is changed. We are debugging the code to check if there is a firmware bug.
Observation 2: When the system supply is provided using TPS63020 (BUCK-Boost) with a battery input, we see that there is power drain (Voltage drops linearly 3.3V to 2.8V and rises linearly to 3.3V) for a period of 3ms on the 3.3V rail which is resetting the micro-controller (MSP432E4) when we are changing the channels of the ADC, it is quite random sometimes it is on the first channel change or second channel change.Which is not observed when 3.3V is provided from the Launchpad.
We have tested the TPS63020 separately by applying with sudden load upto 1A, the drop noted is 0.1V, but no micro reset (micro operating voltage is 2.9 to 3.5). The drop is from 3ms, we tried adding 1000uF cap to the 3.3V rail, but no change. TPS63020 power save mode is disabled by connecting it to VINA as recommended. We changed the inductor for high current capability, still there is no change in the power drain. Based on the testing we have done. We are confident the problem noticed is not related to the TPS63020.
When we insert a new battery, the power drain on the 3.3V is noted when the battery open voltage is 2.7V. We are testing the TPS63020 functionality and not observed any issues with it till now, no load or with load or when load is applied suddenly up-to battery voltage of 2.3V. (Load applied on 3.3V is 250mA)
Since the problem is noted only during the channel changes. We would like to know if there is any specific sequence to be followed before changing the channel. In debugging mode we are unable to note exact point of reset. We have only LED indication to provide the info on micro-reset.
Hi Muruga, Did you have additional questions regarding the AVDD voltage? I provided some guidance in a previous post (see: e2e.ti.com/.../751792), but perhaps I should have clarified that running AVDD at 3.3V is not recommended as it is outside of the datasheet specifications. Do you know what the typical input voltage are on each channel? Perhaps there is some relationship with the reduced AVDD voltage, but it is also possible that when switching channels the PGA inputs could be over-ranged (causing the PGA outputs to saturate) and drawing additional current. It might be a good practice to program the ADCON register (to reduce the PGA gain) first, and then update the MUX register when switching channels to avoid this possibility.
Best regards,Chris HallApplications Engineer | Precision ADCs
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In reply to Christopher Hall:
Thank you for the response.
We changed the AVDD of ADS1256 to 5V from 3.3VDC. All channel sampling is done with PGA set to 1. As the AVDD is set to 5V, the input voltage AINx should not be a problem. We have replaced the ADS1256 with new IC before testing with 5V AVDD.
We could share the complete schematics if required for your reference.
Following are testing done and observations:
In reply to Murugavelu Thangavelu1:
Hi Chris, Sorry for the delayed response. The scope capture was battery voltage and 3.3V rail We had probed the 5V rail and did not see the dip on the rail. Hence it is not the ADS1256 which is creating the problem or might be (because it happens when there is a channel change). Now we are looking at TPS63020 to check why the behavior is so. Currently have isolated the supply to Main board and TPS63020 by a diode, we are testing the configuration to check the performance. Regards, Muruga
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