TMS320F28377S: A consultation on 16-BIT ADC

Hi K.K,

You are correct.  The 16-bit mode ADC feature of the 28377S requires that the input signal to be differential mode and the following requirements need to be satisfied:

    - Need 2 ADC input pins to take the differential input.  The lower even channel is the positive input (VINP) and the higher numbered odd channel is the negative input  (VINM).  At any given time, the sum of the input voltages VINP and VINM should be equal to VREFHI value.  The common mode voltage is VREF/2, VREFCM.  Input signal variation should not exceed VREFCM +/-50mv.  Look under section 4.2 Signal Description of the 28377S datasheet  for ADC channel assignments of the specific 28377S device package you are using.

    - An example of a differential input pair is ADCINA2 and ADCINA3.  Say you chose VREF as 3V (VREFHI=3V and VREFLO=0v) and wanted to use the differential mode for 16-bit operation.  Input is an AC signal (3vpp - 3v max and 0v min, sine wave with a period of 2PI for illustration purposes, see figure below) you wanted to sample.  Feed ADCINA2 with the "positive" signal such that at time 0, VINP is 1.5v and ADCINA3 with the "negative" signal so at time 0, VINM is also 1.5v.  At time 0, VINP + VINM = 3V = VREFHI, which meets the criteria.  At time =.25*PI, VINP is at 2.25v and VINM is at 0.75v and so on.....  You should always meet the criteria that VINP+VINM is equal to VREFHI, with a tolerance of +/-50mv, to ensure that the ADC will operate within specifications.

This is basically how a differential input pair works.  It is not just simply biasing the ADC channels to 1.5V and feeding the signal to an ADC input.  There are amplifiers that convert a single ended signal to differential output so you will not have to worry about producing the complementary signal on the VINM input.  Hope this clarifies your question.

Best regards,

Joseph

Hi K.K

1) You can use a fully differential op-amp to turn a single-ended signal into a differential one with the correct common mode voltage.  For example, see the "Single-Ended Input to Differential Output Amplifier" circuit example in the THS4531 datasheet.  It is also possible to use 2 single-ended op-amps to do the conversion.  The best place to get help on this topic would be to create a new post in the Precision Amplifier forum.

2+3) These pins that fan-out to multiple ADCs allow you some flexibility in choosing what ADC samples the pin.  For example, pin ADCIN14 could be sampled as input 14 on any ADC.  You could also sample the same reference voltage on multiple ADCs at the same time to try and eliminate ADC-to-ADC gain or offset error.   

Hi K.K,

Please see my response below:

1.) Yes, TI certainly has several products that cater to this application. Search in the ti.com website and look for fully differential amplifiers. There are several of these ICs, depending on spec requirement and application. For instance THS4551 is typically chosen for single supply rail application (from 2.7v to 5.4v) or THS413x for +5/-5V rails. VREFCM (common mode) is an input you can provide to the chip so it can be versatile depending on the reference voltage that is chosen.

2.) Yes, ADCIN14 and 15 goes to all the ADC modules and you can use this as differential input pairs for all the ADCs (A,B,C or D).

3.) ADCINA0 pin input also goes to all the ADC modules (channel 12) and this is also the output of DACA. It was configured this way such that user has the option to sample the output of the DAC on any of the ADCs - some applications may require a common reference signal for all the ADCs and to be compared later on (through comparator modules CMPSS or through SW by inspecting converted values on the ADCs) from various signal sources feeding through the different ADC modules.

Let me know if this addresses your questions.

Best regards,
Joseph