7.2.1 Design Requirements

Table 7-1 lists the parameters for this typical application.

Table 7-1. Design Requirements

PARAMETER

VALUE

Nominal DC link voltage

48V

Linear voltage sensing range (VFSR)

60V

Voltage drop across the sensing resistor (R2) for a linear response

1V

Maximum current through the resistive divider, ICROSS, at linear full-scale

voltage

100µA

7.2.2 Detailed Design Procedure

The 100μA cross-current requirement at the maximum system voltage (60V) determines that the total

impedance of the resistive divider is 600kΩ. The impedance of the resistive divider is dominated by R1, thus

neglecting the voltage drop across R2 for a moment is acceptable. The closest value to 600kΩ from the E96

series is 604kΩ. This value is for R1.

The linear full-scale input voltage (VFSR) of the AMC0136 is 1V. R2 is sized to produce a 1V drop at the

maximum system voltage of 60V. R2 is calculated as R2 = VFSR / (VDC-link, max – VFSR) × R1. The resulting value

is 10.24kΩ and the closest value from the E96 series is 10.2kΩ.

Table 7-2 summarizes the design parameters for this application.

Table 7-2. Design Summary

PARAMETER

VALUE

R1 resistor

604kΩ

Sense resistor value (RSNS)

10.2kΩ

Resulting current through resistive divider (ICROSS at 60V)

97.7μA

Resulting full-scale voltage drop across sense resistor (RSNS)

996mV

Total power dissipated in resistive divider

5.9mW

7.2.3 Input Filter Design

Place a RC filter in front of the isolated amplifier to improve signal-to-noise performance of the signal path. Input

noise with a frequency close to the ΔΣ modulator sampling frequency (typically 10MHz) is folded back into the

low-frequency range by the modulator. The purpose of the RC filter at the input is to attenuate high-frequency

noise below the desired noise level of the measurment. In pactice, a cutoff frequency that is two orders of

magnitude lower than the modulator frequency yields good results.

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

scale down the input voltage. In this case, a single capacitor (as shown in Figure 7-2) is sufficient to filter the

input signal. Assuming a 10kΩ sensing resistor (R2), a 100pF filter capacitor yields in a 160kHz cutoff frequency.

AVDD

AGND

IN

AGND

DVDD

CLKIN

DOUT

DGND

R1

VAC

R2

100 pF

Figure 7-2. Input Filter

AMC0136

SBASAZ0 – AUGUST 2024

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