Considerations for External Analog Inputs

Versal ACAP System Monitor Architecture Manual (AM006)

Document ID
AM006
Release Date
2022-05-17
Revision
1.3 English

The analog inputs are high-impedance differential inputs. The differential input scheme enables the rejection on common mode noise on any externally applied analog-input signal. The input AC impedance is typically determined by the sensor, the output impedance of the driving circuitry, or other external components because of the high impedance of each input (such as VP and VN). The following figure illustrates a simple resistor divider network that is used to monitor and reduce a higher voltage supply rail to be compatible with the ADC input voltage range in unipolar input mode. To ensure that noise coupled onto the analog inputs is common to both inputs (reduce differential noise), the impedance on each input must be matched. Analog-input traces on the PCB must also be routed as tightly coupled differential pairs.

Figure 1. Anti-Alias Filter and Voltage Attenuation

Also shown in the figure above is a low-pass filter network at the analog differential inputs. This filter network is commonly referred to as the anti-alias filter and must be placed as close as possible to the package pins. The sensor can be placed remotely from the package as long as the differential input traces are closely coupled. The anti-alias filter attenuates high-frequency signal components entering the ADC where they could be sampled and aliased, resulting in ADC measurement corruption. As shown in the figure above, resistors R1 and R2 can divide the 10V supply down to 0.5V to work with the SYSMON. R5 has been impedance matched to the parallel resistance of R1 and R2. See Driving the Xilinx Analog-to-Digital Converter (XAPP795) for additional details. A discussion of aliasing in sampled systems is beyond the scope of this document.