7.3.1 Analog Input

The differential analog input of the AMC1204 and AMC1204B is implemented with a switched-capacitor circuit.

The AMC1204 and AMC1204B measure the differential input signal V_IN = (VINP – VINN) against the internal reference of 2.5 V using internal capacitors that are continuously charged and discharged. Figure 44 shows the simplified schematic of the AMC1204 and AMC1204B input circuitry; the right side of Figure 44 illustrates the input circuitry with the capacitors and switches replaced by an equivalent circuit.

In Figure 44, the S₁ switches close during the input sampling phase. With the S₁ switches closed, C_DIFF charges to the voltage difference across VINP and VINN. For the discharge phase, both S₁ switches open first and then both S₂ switches close. C_DIFF discharges approximately to AGND + 0.8 V during this phase. This two-phase sample/discharge cycle repeats with a period of t_CLKIN = 1/f_CLKIN. f_CLKIN is the operating frequency of the modulator. The capacitors C_IP and C_IN are of parasitic nature and caused by bonding wires and the internal ESD protection structure.

Figure 44. Equivalent Analog Input Circuit

There are two restrictions on the analog input signals VINP and VINN. First, if the input voltage exceeds the range AGND – 0.5 V to AVDD + 0.3 V, the input current must be limited to 10 mA because the input protection diodes on the front end of the converter begin to turn on. In addition, the linearity and the noise performance of the device are ensured only when the differential analog input voltage remains within ±250 mV.