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 so high that adding a filter capacitor on the IN pin

limits the signal bandwidth to an unacceptable low limit, such that the filter capacitor is omitted. When used,

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

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

down the input voltage. In that case, no additional resistor is needed and a single capacitor (as shown in Figure

8-2) is sufficient to filter the input signal.

1 nF

RSNS

VDD1

GND1

IN

GND1

AMC1351

VDD2

OUTP

OUTN

GND2

R2

R1

VDC

Figure 8-2. Input Filter

8.2.2.2 Differential to Single-Ended Output Conversion

Figure 8-3 shows an example of a TLV6001-based signal conversion and filter circuit for systems using single-

ended input ADCs to convert the analog output voltage into digital. With R1 = R2 = R3 = R4, the output voltage

equals (VOUTP – VOUTN) + VREF. Tailor the bandwidth of this filter stage to the bandwidth requirement of the

system and use NP0-type capacitors for best performance. For most applications, R1 = R2 = R3 = R4 = 3.3 kΩ

and C1 = C2 = 330 pF yields good performance.

VDD1

GND1

IN

GND1

AMC1351

VDD2

OUTP

OUTN

GND2

+

–

R4

R3

R1

R2

V

REF

ADC

To MCU

C1

C2

TLV6001

Figure 8-3. Connecting the AMC1351 Output to a Single-Ended Input ADC

For more information on the general procedure to design the filtering and driving stages of SAR ADCs, see

the 18-Bit, 1MSPS Data Acquisition Block (DAQ) Optimized for Lowest Distortion and Noise and 18-Bit Data

Acquisition Block (DAQ) Optimized for Lowest Power reference guides, available for download at www.ti.com.

AMC1351

SBASAA8 – DECEMBER 2021

www.ti.com

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