8.2.2.1 Input Filter Design

Place an RC filter in front of the device 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 is to attenuate high-frequency noise below

the desired noise level of the measurement. In practice, 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 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. For (R1 + R2) >> RSNS,

the cutoff frequency of the input filter is 1 / (2 × π × RSNS × C5). For example, RSNS = 10kΩ and C5 = 100pF

results in a cutoff frequency of 160kHz.

C5

RSNS

R2

R1

VIN

VDD1

GND1

INP

SNSN

Figure 8-2. Input Filter

8.2.2.2 Connecting the REFIN Pin

The reference input has an internal, 90kΩ impedance connected to GND2. Consider this impedance when

driving the REFIN pin from a high-impedance source. Connect a 100nF capacitor from REFIN to GND2 to filter

out high-frequency noise at the reference input. Figure 8-3 shows different options for connecting the REFIN pin.

VDD2

OUT

REFIN

GND2

90k

100nF

Reference voltage

RSOURCE

VDD2

OUT

REFIN

GND2

100nF

90k

R1

R2

VDD2 supply

(3.3V or 5V)

VDD2

OUT

REFIN

GND2

100nF

90k

VDD2 supply

(3.3V or 5V)

Figure 8-3. Connecting the REFIN Pin

In the first example, REFIN is shorted to VDD2. In the second example, VREFIN is derived from VDD2 through

a resistive divider consisting of R1 and the internal 90kΩ input impedance. In the third example, an external

voltage source drives the reference input pin.

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