7.3.1.1 Differential Analog Input Pins

The ADC14155QML-SP has one pair of analog signal input pins, V_IN+ and V_IN–, which form a differential input pair. The input signal, V_IN, is defined as

Figure 21 shows the expected input signal range. Note that the common mode input voltage, V_CM, should be 1.5 V. Using V_RM (pin 46 or 47) for V_CM will ensure the proper input common mode level for the analog input signal. The peaks of the individual input signals should each never exceed 2.6 V. Each analog input pin of the differential pair should have a peak-to-peak voltage equal to the reference voltage, V_REF, be 180° out of phase with each other and be centered around V_CM.The peak-to-peak voltage swing at each analog input pin should not exceed the value of the reference voltage or the output data will be clipped.

Figure 21. Expected Input Signal Range

For single frequency sine waves the full scale error, E_FS, in LSB can be described as approximately

Where dev is the angular difference in degrees between the two signals having a 180° relative phase relationship to each other (see Figure 22). For single frequency inputs, angular errors result in a reduction of the effective full scale input. For complex waveforms, however, angular errors will result in distortion.

Figure 22. Angular Errors Between The Two Input Signals Will Reduce The Output Level Or Cause Distortion

It is recommended to drive the analog inputs with a source impedance less than 100 Ω. Matching the source impedance for the differential inputs will improve even ordered harmonic performance (particularly second harmonic).

Table 2 indicates the input to output relationship of the ADC14155.