SBAA499A July 2021 – April 2022 PCM3120-Q1 , PCM5120-Q1 , PCM6120-Q1 , TLV320ADC3120 , TLV320ADC5120 , TLV320ADC6120
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Common-mode in a differential amplifier is formally defined as the average voltage of the two input pins. Difference amplifiers are designed to amplify the differential signal and reject signals common to both input pins, such as a DC bias, noise picked up in the system, or a shifting reference. How well the amplifier does this is called the common-mode rejection ratio, or CMRR. CMRR is formally expressed as the ratio of the amplifier's differential gain to common-mode gain. In practice though, CMRR is typically measured by changing the input common-mode and observing the change in output. This measured change can be referred to the input and considered an offset voltage at the amplifier's inputs.
The TLV320ADCx120 and PCMx120-Q1 family of devices has an input architecture that allows it to support a wide range of input configurations while maintaining high performance. For optimal performance the common-mode variation at the device input should be limited to less than 100 mVpp for AC coupled settings. For applications that cannot avoid large common-mode fluctuations, the TLV320ADCx120 and PCMx120-Q1 family offers multiple modes to configure the device for higher common-mode tolerance as shown in Table 2-1
Increasing the common-mode tolerance of the input does require a performance trade-off. This application note details the typical performance for each of the input modes.
The TLV320ADCx120 and PCMx120-Q1 family supports three different modes for common-mode tolerance that should be selected based on the maximum expected common-mode variation. Since wider common-mode tolerance does degrade other performance parameters, it is recommended to select the lowest tolerance mode possible.
P0_R58_D[7:6] : CH1_INP_CM_TOL_CFG[1:0] | CHANNEL 1 INPUT COMMON-MODE TOLERANCE |
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00 (default) | Channel 1 input common-mode tolerance of: AC-coupled input = 100 mVPP, DC-coupled input = 2.82 VPP. |
01 | Channel 1 input common-mode tolerance of: AC/DC-coupled input = 1 VPP. |
10 (high CMRR mode) | Channel 1 input common-mode tolerance of: AC/DC-coupled input = 0-AVDD (supported only with an input impedance of 10 kΩ and 20 kΩ). For input impedance of 2.5 kΩ, the input common-mode tolerance is 0.4 V to 2.6 V. |
11 | Reserved (do not use this setting) |
It is important to keep in mind that in all modes, the full-scale range of the device is still 2-Vrms. This is especially important for large common-mode signals as they will limit the effective input range. Mode 2, for example, can support a common-mode range of 0-V to AVDD, but there would be no room left for a differential signal to be applied to the input pins at either of these extremes. The PGA gain can be used to amplify the differential signal as needed but the attenuated common-mode signal will be amplified as well.