The AMC1303 incorporates a front-end circuitry that contains a differential amplifier and sampling stage, followed by a ΔΣ modulator. The gain of the differential amplifier is set by internal precision resistors to a factor of 4 for devices with a specified input voltage range of ±250 mV (for the AMC1303x25x), or to a factor of 20 in devices with a ±50-mV input voltage range (for the AMC1303x05x), resulting in a differential input resistance of 4.9 kΩ (for the AMC1303x05x) or 22 kΩ (for the AMC1303x25x).
For reduced offset and offset drift, the differential amplifier is chopper-stabilized with the switching frequency set at fCLK / 32. Figure 42 shows that the switching frequency generates a spur.
|AMC1303xxx20, sinc3 filter, OSR = 2, fIN = 1 kHz|
Consider the input resistance of the AMC1303 in designs with high-impedance signal sources that can cause degradation of gain and offset specifications. The importance of this effect, however, depends on the desired system performance. Additionally, the input bias current caused by the internal common-mode voltage at the output of the differential amplifier causes an offset that is dependent on the actual amplitude of the input signal. See the Isolated Voltage Sensing section for more details on reducing these effects.
There are two restrictions on the analog input signals (AINP and AINN). First, if the input voltage exceeds the range AGND – 6 V to AVDD + 0.5 V, the input current must be limited to 10 mA because the device input electrostatic discharge (ESD) diodes turn on. In addition, the linearity and noise performance of the device are ensured only when the differential analog input voltage remains within the specified linear full-scale range (FSR), that is ±250 mV (for the AMC1303x25x) or ±50 mV (for the AMC1303x05x), and within the specified input common-mode voltage range.