10 Ÿ

10 Ÿ

8.2 nF

INP

INN

AMC3306

HGND

21

AMC3306M25

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SBAS996 – MAY 2020

Product Folder Links: AMC3306M25

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8.2.1.2 Detailed Design Procedure

The AMC3306M25 requires a single 3.3-V or 5-V supply on its low-side. The high-side supply is internally

generated by an integrated DC/DC converter as explained in the Isolated DC/DC Converter section.

The floating ground reference (HGND) is derived from one of the ends of the shunt resistor that is connected to

the negative input of the AMC3306M25 (INN). If a four-pin shunt is used, the inputs of the device are connected

to the inner leads and HGND is connected to one of the outer shunt leads. To minimize offset and improve

accuracy, set the ground connection to a separate trace that connects directly to the shunt resistor rather than

shorting HGND to INN directly at the input to the device. See the section for more details.

8.2.1.2.1

Shunt Resistor Sizing

•

The voltage drop caused by the nominal current range must not exceed the recommended differential input

•

The voltage drop caused by the maximum allowed overcurrent must not exceed the input voltage that causes

8.2.1.2.2

Input Filter Design

TI recommends placing an RC filter in front of a ΔΣ modulator 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 of the

ΔΣ modulator

•

The DC impedance of the filter does not generate significant voltage drop in the analog input lines

•

The source impedance of the analog inputs are equal

For most applications the structure shown in Figure 16 achieves excellent performance.

Figure 16. Differential input filter

8.2.1.2.3

Bitstream Filtering

For modulator output bitstream filtering, a device from TI's TMS320F2807x family of low-cost microcontrollers

(MCUs) or TMS320F2837x family of dual-core MCUs is recommended. These families support up to eight

channels of dedicated hardwired filter structures that significantly simplify system level design by offering two

filtering paths per channel: one providing high-accuracy results for the control loop and one fast-response path

for overcurrent detection.

8.3 What To Do and What Not To Do

Do not leave the inputs of the AMC3306M25 unconnected (floating) when the device is powered up. If both

modulator inputs are left floating, the input bias current drives these inputs to the output common-mode of the

analog front-end of approximately 1.9 V. That voltage is above the specified input common-mode range, and the

modulator outputs a bitstream resembling a zero input differential voltage.