OSR

1

10

100

1000

10000

0

2

4

6

8

10

12

14

16

D034

sinc3

sinc2

sinc1

10 Ÿ

10 Ÿ

8.2 nF

AINP

AINN

AMC1336-Q1

AGND

28

AMC1336-Q1

SBASA51 – MAY 2020

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Copyright © 2020, Texas Instruments Incorporated

Use a power supply with a nominal voltage of 3.3 V for DVDD to directly connect all modulators to the

microcontroller.

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 MCU families support up to eight

channels of dedicated hardwired filter structures called sigma-delta filter modules (SDFMs) that significantly

simplify system level design by offering two filtering paths per channel: one providing high accuracy results for

the control loop and one that offers a fast response path for overcurrent detection. Use one of the pulse-width

modulation (PWM) sources inside the MCU to generate the clock for the modulators and for easy

synchronization of all feedback signals and the switching control of the gate drivers.

8.2.3 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 52 achieves excellent performance.

Figure 52. Differential Input Filter

8.2.4 Application Curve

The effective number of bits (ENOB) is often used to compare the performance of ADCs and ΔΣ modulators.

Figure 53 shows the ENOB of the AMC1336-Q1 with different oversampling ratios. In this document, this number

is calculated from the SINAD by using following equation: SINAD = 1.76 dB + 6.02 × ENOB.

Figure 53. Measured Effective Number of Bits versus Oversampling Ratio

8.2.5 What to Do and What Not to Do

Do not leave the inputs of the AMC1336-Q1 unconnected (floating) when the device is powered up. If either

modulator input is left floating, the input bias current can drive this input beyond the specified common-mode

input voltage range. If both inputs are beyond that range, the gain of the front-end diminishes and the output

bitstream is not valid.