What should be considered when designing a Analog Input Board for RT BOX 2?

Question

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Hi, Plexim teams! 

I am using a RT BOX 2 to sample voltages and currents of a converter, and output PWM. However, the sampling and conditioning circuit, which works well with TI DSP F28335, seems to be not suitable for RTBOX 2. 

The waveforms of sampled current are given below (scale = 20). When the converters are not running, the noise is acceptable (within 10mV). But when the converters are running, spikes can be observed (1.2A → 60mV).

1. What may cause these spikes? grounding?

FYI. The sampling circuit consists of: current transducer LAH50P → sensing resistor → conditioning 1:1 → RC filter → RTBOX 2.

2. I have read "rtbox usermanual" and "analog breakout board usermanual". I think my circuit meet the requirements mentioned in these two documentations. Are there any other requirements? or are there any parts should be carefully handled to avoid noises? e.g. CMTI?

3. can you provide any application examples or recommended circuits? 

Answer 1

TWY, this type of question is best answered through our support mailbox. Can you reach out to support@plexim.com?

In your response additional information would be helpful:

• A schematic of your converter and interface circuitry will be helpful including information regarding grounding.
• Is the noise on just the current sensors or on all channels?
• The noise seems periodic, both with and without switching. Can you comment on the period and the operating environment of the converter?

Comments

Thank you for your answer. I will send an email to support@plexim.com, including all the information you mentioned above .

Answer 2

Some general remarks on the ADC inputs:

• The ADC inputs are differential, but you need a ground connection nonetheless. The acceptable input range of each differential signal with respect to ground is +/- 11 Volts.
• To minimize noise, the differential signals should be routed in parallel as far as possible. For a single-ended measurement, connect the negative input to ground as close to the measurement point as possible.

Comments

Thank you for your advice. I will reach out to support@plexim.com for help.