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FDC2114: About Gain and Offset

Tao_2199
Genius 5660 points
Part Number: FDC2114

Hello,

Regarding to Gain and Offset on FDC2114, my customer is asking some question.

(Question)

(1) According to datasheet page 18, gain example is described following.

“If the conversion result for a channel is 0x07A3, with OUTPUT_GAIN=0x0, the reported output code is

0x07A. If OUTPUT_GAIN is set to 0x3 in the same condition, then the reported output code is 0x7A3. The

original 4 MSBs (0x0) are no longer accessible.”

When OUTPUT_GAIN is 0x0, 0 bits is shifted. But on above description , the reported output code is 0x07A and it is 4 bits right shifted.

(The same goes for OUTPUT_GAIN= 0x3.).

I couldn't make it clear about Conversion Data Output Gain.

Could you please tell me about conversion data of Output Gain and purpose of this feature more detail?

 

(2) According to datasheet page 19, offset is described following.

“An offset value may be subtracted from each DATA value to compensate for a frequency offset or maximize the dynamic range of the sample data."

I couldn't make it clear about it.

Could you please tell me about purpose of this feature more detail?

 

Regards,

Tao_2199

  • 0
    TI__Genius 17440 points

    Hello Tao,

    The FDC2114 conversion data is 16 bits wide, but the data output registers are only 12 bits wide. The OUTPUT_GAIN setting selects which 12 bits of the 16 total bits will be stored in the DATA_CHx registers. When OUTPUT_GAIN = 0x0, you are selecting bits 15:4 of the original 16-bit conversion. If you want a different 12-bit section of the 16-bit conversion, then you select an OUTPUT_GAIN setting to left shift the 16-bit data within the 12-bit window. The OUTPUT_GAIN setting only controls the amount of left shift; it's not possible to right shift the data. I suggest looking again at Figure 16 in the datasheet so see where the 12-bit window is when OUTPUT_GAIN = 0x0. As OUTPUT_GAIN increases, the 16-bit data is left shifted from the initial window position. I think this may be the source of your customer's confusion. 

    Some sensor frequency shifts due to target interaction are small enough that they require an OUTPUT_GAIN setting of 0x3 in order to be visible in the output data. If the sensor frequency shift is less than ~10kHz, this would be the case. However, If the OUTPUT_GAIN setting is set to 0x3, then the maximum possible sensor frequency that the output data can display is ~2.5MHz. The maximum frequency that the FDC2114 can drive is 10MHz, so valid sensor frequencies between ~2.5MHz - 10MHz can't be expressed in the output data. This is the purpose of the CHx_OFFSET. It can add an offset frequency so that the dynamic range of the output data is maximized. For example, if your sensor frequency is 6MHz and your target interaction reduces the sensor frequency by 5kHz, then you could use a offset frequency of 5MHz and an OUTPUT_GAIN setting of 0x03.

    Best Regards,