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ADC3643: How does scale of ADC code relate to reference voltage? What is the range an external reference voltage?

B Luey
Prodigy 80 points

Part Number: ADC3643

I don't see anything in the datasheet relating the ADC code to the reference voltage. For example, in a setup like what's shown in figure 8-6 (right) with a single-ended input and the output to be in binary mode,  then INP= Vcm = 0.95. If INM = 0.95, the voltage difference is 0V and believe  I should get midscale (8192 or 2^13). What voltage at INM would give full-scale (16384)? The datasheet says that the max range is 0.5625 above CM, would that voltage (0.5625 + 0.95) correspond to 16384? If so, where does this 0.5625 voltage come from? Is there a fixed divider from the 1.6V reference? If I input an external voltage reference of, say, 1.59V, would the voltage needed to get 16384 scale accordingly so 0.5625 goes to 0.559 and the max would move down to 1.509V?

Also, what is the acceptable input range for the external reference voltage? The datasheet says typical if 1.6V, but can I put in 1.5V? 1.0? 

Thank you,

  • 0
    TI__Genius 15321 points

    Hello,

    Pg. 28 of the datasheet mentions that "The analog inputs of ADC364x are intended to be driven differentially. Both AC coupling and DC coupling of the analog inputs is supported. The analog inputs are designed for an input common mode voltage of 0.95 V which must be provided externally on each input pin. DC-coupled input signals must have a common mode voltage that meets the device input common mode voltage range."

    Section 8.3.1.2.2 describes how the front end can be configured for single ended input, only operating with the positive signal input. "This operating mode must be enabled via SPI register write (address 0xE). The single ended signal is connected to the negative ADC input and both the positive and negative input need to be biased to VCM as shown in Figure 8-6." (datasheet pg. 29)

    This results in a 6 dB decrease in signal swing (and a correspond 3 dB decrease in SNR), where Vpp is 1.125V or swinging +/- 0.5625 V around Vcm. Vcm has a minimum of 0.9V, typ 0.95V and max of 1V. 

    Regards, Amy

  • 0 in reply to Amy
    Prodigy 80 points

    When configured in this way, (DC coupling, singled-ended) how does the reference voltage relate to the ADC counts? If I put in 0.95V on INP and 1.0V on NNM, what ADC code should I expect to read out?

    Also, if used this way or differentially, what is the allowed input voltage range for the reference voltage? 

  • 0 in reply to B Luey
    TI__Genius 15321 points

    Hi B,

    If you use single ended DC coupling you will not be able to reach full scale. Rather, the max code value will be reduced by half on either side of the mid-code value. Look at it this way – this is 14b ADC, so the max code value will be 2^14 or 16,384 with a midcode value of 2^13 = 8192. A single ended signal will swing up to a max code of 12,288 (2^13 + 2^12) and down to a min code of 4096 (2^13 - 2^12) if swinging 1.125 Vpp. A Vcm value of 0.95 is adjusted to mid-code, meaning that code 12,288 will correspond to 1.5125 V and code 4096 will correspond to 0.3875 V.

    In your example inputting 0.95V on INP and 1.0V on NNM, the ADC will see the signal voltage as INP – NNM = -0.05V. The code per voltage ratio is code 16384 / 2.25 Vpp = 7282 codes per volt, times -0.05V = 364. This is the number of codes that would be offset from the midcode value of 8192, or a 1V DC signal on NNM would be seen at code 7828.

    Hope this helps to clarify.

    Regards, Amy

  • 0 in reply to Amy
    Prodigy 80 points

    Actually, my question is about the reference voltage. Specifically:

    What is the input range for the external reference voltage? Can I input 1.5V? 1.0V? 1.8V?   Section 6.6 only puts 1.6V as 'typical' for Vref External, but not min/max bounds.

    How does the reference voltage affect the ADC code? With a differential signal, a 2.25V ADC output will be full scale. If I apply a 1.5V reference, does full-scale ADC code scale as I would expect to a 2.11V? 

    Thank you,

  • 0 in reply to B Luey
    TI__Genius 15321 points

    Hi B,

    The input common mode (VCM) is what needs to be biased for a singled ended signal (please refer to Figure 8-6 and section 8.3.1.2.2 Single Ended Input in the datasheet, pg. 29). This has a min / typical / max range of 0.9 / 0.95 / 1.0 V (datasheet pg. 7).

    Let me check on your question of the impact of the reference voltage (VREF) and ADC code and get back to you. 

    Regards, Amy

  • 0 in reply to Amy
    TI__Genius 15321 points

    Hi B, 

    Here is a quick update: 

    To answer your question on using an external VREF - you can only use a 1.6 V external voltage reference. Unfortunately, there is not a range for this device.

    To answer your second question, the voltage reference (VREF) is independent of the VCM pin. Therefore, the VCM will be 0.95 V.

    Regards, Amy