Table 8-2 summarizes the design parameters for this application.

Table 8-2. Design Summary

PARAMETER

VALUE

Unit resistor value, RX

665 kΩ

Number of unit resistors

6

Sense resistor values (RSNS1 and RSNS2)

10 kΩ

Resulting current through resistive divider (ICROSS at 400 V)

99.8 μA

Resulting full-scale voltage drop across sense resistor (RSNS1 + RSNS2)

1.995 V

Total power dissipated in resistive divider

39.9 mW

Power dissipated in unit resistor RX

6.6 mW

AMC23C11 reference resistor value, RREF

11 kΩ

Resulting overvoltage detection threshold

443 V

8.2.2.1 Input Filter Design

Placing an RC filter in front of the isolated amplifier improves signal-to-noise performance of the signal path. In

practice, however, the impedance of the resistor divider is high and only a small-value filter capacitor can be

used to not limit the signal bandwidth to an unacceptable low value. Design the input filter such that:

• The cutoff frequency of the filter is at least one order of magnitude lower than the sampling frequency

(20 MHz) of the internal ΔΣ modulator.

• The input bias current does not generate significant voltage drop across the DC impedance of the input

filter. Because the device has an input bias current in the low nanoampere range, IR drops across the filter

impedance are usually not a concern.

Most voltage-sensing applications use high-impedance resistive dividers in front of the isolated amplifier to scale

down the input voltage. In this case, a single capacitor (as shown in Figure 8-2) is sufficient to filter the input

signal.

100 pF

DCDC_OUT

DCDC_HGND

HLDO_IN

HLDO_OUT

HGND

IN

HGND

NC

AMC3311

DCDC_IN

LDO_OUT

VDD

OUTP

OUTN

GND

DIAG

DCDC_GND

Figure 8-2. Input Filter

AMC3311

SBASAQ5A – JUNE 2023 – REVISED AUGUST 2023

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