AMC3330 datasheet(23/36 Pages) TI

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8.2.1 Design Requirements

Table 8-1 lists the parameters for this typical application.

Table 8-1. Design Requirements

PARAMETER

VALUE

Low-side supply voltage

3.3 V or 5 V

Voltage drop across the sensing resistor for a linear response

1 V (maximum)

Current through the resistive divider, ICROSS

100 µA (maximum)

8.2.2 Detailed Design Procedure

Use Ohm's Law to calculate the minimum total resistance of the resistive divider to limit the cross current to the

desired value ( RTOTAL = VLx / ICROSS ) and the required sense resistor value to be connected to the AMC3330

input: RSNS = VFSR / ICROSS.

Consider the following two restrictions to choose the proper value of the sense resistor RSNS:

• The voltage drop on RSNS caused by the nominal voltage range of the system must not exceed the

recommended input voltage range: VSNS ≤ VFSR

• The voltage drop on RSNS caused by the maximum allowed system overvoltage must not exceed the input

voltage that causes a clipping output: VSNS ≤ VClipping

Table 8-2 lists examples of nominal E96-series (1% accuracy) resistor values for systems using 120-V and

230-V AC line voltages.

Table 8-2. Resistor Value Examples

PARAMETER

120-VRMS LINE VOLTAGE 120-VRMS LINE VOLTAGE 230-VRMS LINE VOLTAGE

Peak voltage

170 V

325 V

Resistive divider resistors RL11, RL12

845 kΩ

1.62 MΩ

Sense resistor RSNS

10 kΩ

Current through resistive divider ICROSS

100 µA

Resulting voltage drop on sense resistor VSNS

1.00 V

8.2.2.1 Input Filter Design

TI recommends placing an RC filter in front of the isolated amplifier to improve signal-to-noise performance of

the signal path. 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

• The impedances measured from the analog inputs are equal

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

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

10 nF

INP

INN

AMC3330

HGND

Figure 8-2. Differential Input Filter

www.ti.com

AMC3330

SBASA34A – JUNE 2020 – REVISED OCTOBER 2020

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