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AMC1304M25-Q1: Delta-Sigma Modulator

Amy Luo
Expert 6895 points
Part Number: AMC1304M25-Q1
Other Parts Discussed in Thread: ADS1202, AMC1304M25

Hi team,

Digital filtering is currently being attempted using DFSDM in STM32F7, with filter inputs supporting SPI, Manchester encoding, PDM, and parallel inputs.

Customer has the following questions about AMC1304:

1. What should the digital filter interface after AMC1304 be, is it the same SPI used as the ADS1202? If so, I need to read the data on the rising or falling edge.

2. Is the digital filter followed by a sin3 OSR  256? Can the processed values be read directly?

3. The datasheet says that the voltage value is calculated based on the ratio of 0 and 1 of the data stream, then does the range of this data stream refer to data over 128 cycles? Does it mean that I need to acquire values over 128 cycles and compare the ratio of 0 and 1 values?

Best Regards,

Amy Luo

  • +1
    TI__Guru**** 174346 points

    Hi Amy,

    For Q1: The AMC1304 and ADS1202 (in Mode 0) both change data on the falling MCLK edge, so the filter should expect to read the bit-stream on the rising MCLK edge.  If your customer can read the bit-stream from the ADS1202, they should not have to change anything to read data from the AMC1304M25, but they will need to pay attention to the setup and hold timing requirements for the filter module as the modulators have different delay times to output valid data.

    For Q2: The choice of filter order and OSR is totally up to the customer.  Our modulator datasheet specifications are based on a Sinc3 filter with OSR = 256.  I am not familiar with the processor or peripheral you mentioned above, but my understanding is that the filter module outputs 8-, 12-, 16- or 24-bit 'words' depending on the shifting parameters used in the module.

    For Q3: Your understanding of how delta-sigma modulators work is correct, the 1's density increases with a positive increase in the applied voltage and decreases as the input swings towards the defined negative input level.  The range of the output data stream depends only on the defined input voltage levels.  When you ask about '128 cycles', I believe you are referring to the OSR setting of the filter.  If that is the case then with a filter OSR setting of 128, yes - the filter will calculate the magnitude of the applied input signal by looking at the ratio of 1's and 0's over 128 cycles. 

  • 0 in reply to Tom Hendrick
    TI__Expert 6895 points

    Hi Tom,

    I appreciated your support.

    Could you help me answer another question

    I'd like to know what the relationship is between the range of the data stream and the input voltage level. How to get the range of the data stream

    Thanks,

    Amy

  • +1 in reply to Amy Luo
    TI__Guru**** 174346 points

    Hi Amy,

    Both the ADS1202 and the AMC1304M25 have an input voltage range of +/-250mV, which is the linear input range of the respective device.  Using the AMC1304M25 as an example (see 8.3.3), it has a full-scale range of +/-312.5mV.  With +312.5mV on the input of the modulator, you have a bit stream that would be 100% ones.  The specified +250 linear voltage level would give you 90% one's density. Please see equation (1) on page 24 of the AMC1304M25 datasheet.