7.3.2 Modulator

Figure 7-2 conceptualizes the second-order, switched-capacitor, feed-forward ΔΣ modulator implemented in the

AMC3336. The analog input voltage VIN and the output V5 of the 1-bit digital-to-analog converter (DAC) are

differentiated, providing an analog voltage V1 at the input of the first integrator stage. The output of the first

integrator feeds the input of the second integrator stage, resulting in an output voltage V3 that is differentiated

with the input signal VIN and the output of the first integrator V2. Depending on the polarity of the resulting

voltage V4, the output of the comparator is changed. In this case, the 1-bit DAC responds on the next clock

pulse by changing the associated analog output voltage V5, causing the integrators to progress in the opposite

direction and forcing the value of the integrator output to track the average value of the input.

VIN

Integrator 1

V1

V2

V4

V5

V3

fCLKIN

Σ

Integrator 2

Σ

+

–

DAC

0 V

DOUT

Figure 7-2. Block Diagram of a Second-Order Modulator

The modulator shifts the quantization noise to high frequencies, as depicted in Figure 7-1. Therefore, use a

low-pass digital filter at the output of the device to increase the overall performance. This filter is also used to

convert the 1-bit data stream at a high sampling rate into a higher-bit data word at a lower rate (decimation). TI's

C2000™ and Sitara™ microcontroller families offer a suitable programmable, hardwired filter structure termed

a sigma-delta filter module (SDFM) optimized for usage with the AMC3336. Alternatively, a field-programmable

gate array (FPGA) or complex programmable logic device (CPLD) can be used to implement the filter.

7.3.3 Isolation Channel Signal Transmission

The AMC3336 uses an on-off keying (OOK) modulation scheme, as shown in Figure 7-3, to transmit the

modulator output bitstream across the SiO2-based isolation barrier. The transmit driver (TX) illustrated in the

Functional Block Diagram transmits an internally generated, high-frequency carrier across the isolation barrier

to represent a digital one and does not send a signal to represent a digital zero. The nominal frequency of the

carrier used inside the AMC3336 is 480 MHz.

Figure 7-3 shows the concept of the on-off keying scheme.

Modulator Bitstream

on High-side

Clock

Signal Across Isolation Barrier

Recovered Sigal

on Low-side

Figure 7-3. OOK-Based Modulation Scheme

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